AECL-7523

ATOMIC ENERGY ^ ^ 3 L'ENERGIE ATOMIQUE

OF CANADA UMITED E & ^ W DU CANADA LIMITEE

EDDY CURRENT TESTINGMANUAL ON EDDY CURRENT METHOD

Essais par courant de FoucaultManuel des rne'thodes d'essai par courant de Foucault

Volume 1

V.S. CECCO, <3. VAN DRUNEN and F.L. SHARP

Chalk River Nuclear Laboratories Laboratoires nucl6aires de Chalk River

Chalk River, Ontario

November 1981 novembre

ATOMIC ENERGY OF CANADA LIMITED

EDDY CURRENT TESTING

VOLUME 1

MANUAL ON EDDY CURRENT METHOD

V.S. Cecco, G. Van Drunen and F.L. Sharp

Chalk River Nuclear LaboratoriesChalk River, Ontario KOJ 1J0

1981 NOVEMBER !

AECL-7523

L'ENERGIE ATOMIQUE DU CANADA, LIMITEE

EBBHIS par courant de Foucault

Volume 1

Manuel des méthodes d'essai par courant de Foucault

V.S. Cecco, G. Van Drunen et F.L. Sharp

Rësumë

Ce manuel de référence et d'instruction a pour but de fournira ceux qui font des essais par courant de Foucault les principesfondamentaux de la technique et les connaissances voulues pourinterpréter comme il faut les résultats souvent compliqués de cesessais. Une approche non rigoureuse est employée pour simplifierles complexes phénomènes physiques. L'accent est mis sur un choixapproprié de fréquences d'essai et sur l'interprétation des signaux.La détection et le diagnostic des défauts font l'objet d'une attentionparticulière. La conception et la réalisation des sondes sont traitéesde façon approfondie car les sondes jouent un rôle clé dans les essaispar courant de Foucault. Les avantages et les limitations des diverstypes de sondes sont indiqués.

La théorie électromagnétique, l'instrumentation, les méthodes Id'essai et les analyses de signaux sont décrites. Les réponses dessondes permettent d'avoir une compréhension fondamentale du comportementdes courants de Foucault, â condition d'avoir recours aux déductions 1simplifiées indiquées dans le manuel pour tester les paramètres. Les Isignaux des courants de Foucault sont présentés sur des diagrammes deplans d'impédance tout au long du manuel, car il s'agit là de l'Infor- tmation la plus commune affichée sur les instruments universels modernes. jL'emploi du "retard de phase" dans l'analyse des signaux est décrit endétail. Four compléter la théorie, des exemples pratiques sont donnés.Ces exemples ont pour but de rendre les inspections plus performanteset ils montrent comment les principes de base s'appliquent au diagnosticdes signaux réels.

Laboratoires nucléaires de Chalk RiverChalk River, Ontario KOJ 1J0

Novembre 19811

AECL-7523 \

ATOMIC ENERGY OF CANADA LIMITED

EDDY CURRENT TESTING

VOLUME 1

MANUAL ON EDDY CURRENT METHOD

V.S. Cecco, G. Van Drunen and F.L. Sharp

ABSTRACT

This training and reference manual was assembled to providethose involved in eddy current testing with both thefundamental principles of the technique as well as theknowledge to deal with often complicated test results. Anon-rigorous approach is used to simplify complex physicalphenomena. Emphasis is placed on proper choice of testfrequency and signal interpretation. Defect detection anddiagnosis receive particular attention. Design andconstruction of probes are covered extensively since probesplay a key role in eddy current testing. The advantages andlimitations of various probe types are discussed.

Electromagnetic theory, instrumentation, test methods andsignal analysis are covered. Simplified derivations of proberesponse to test parameters are presented to develop a basicunderstanding of eddy current behaviour. Eddy currentsignals are presented on impedance plane diagrams throughoutthe manual since this is the most common display on modern,general purpose instruments. The use of "phase lag" insignal analysis is covered in detail. To supplement theory,practical examples are presented to develop proficiency inperforming inspections, and to illustrate how basicprinciples are applied to diagnose real signals.

Chalk River Nuclear LaboratoriesChalk River, Ontario KOJ 1J0

1981 NOVEMBER

AECL-7523

-iii-

ACKNOWLEDGEMENTS

This manual is an accumulation of knowledge and experienceobtained by the NDT Development Branch (formerly QualityControl Branch) of CRNL through its 10 years of existence.

The authors are indebted to the other members of theNondestructive Testing Development Branch especiallyC.R. Bax, II.V. Ghent, J.R. Carter, G.A. Leakey andW. Fantermoller who assisted in collecting some of the datain the manual and made many constructive criticisms.

All rights reserved. No part of this report may bereproduced by any means, nor transmitted, nor translated intoa machine language without the written permission of AtomicEnergy of Canada Limited Research Company.

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TABLE OF CONTENTS

CHAPTER 1 - INTRODUCTION PAGE

1.1 EDDY CURRENT TESTING 11.2 PURPOSE OF THIS MANUAL 11.3 HISTORICAL PERSPECTIVE 2

CHAPTER 2 - EDDY CURRENT FUNDAMENTALS

2.1 BASIC EQUIPMENT 52.2 GENERATION OF EDDY CURRENTS 6

2.2.1 Introduction 62.2.2 Magnetic Field Around a Coil 62.2.3 Equations Governing Generation of Eddy

Currents 8

2.3 FUNDAMENTAL PROPERTIES OF EDDY CURRENT FLOW 102.4 SKIN EFFECT 11

2.4.1 Standard Depth of Penetration 122.4.2 Depth of Penetration in Finite Thickness

Samples 132.4.3 Standard Phase Lag 142.4.4 Phase Lag in Finite Thickness Samples 16

2.5 SUMMARY 172.6 WORKED EXAMPLES 18

2.6.1 Standard Depth of Penetration and Phase Lag 18

CHAPTER 3 - ELECTRICAL CIRCUITS AND PROBE IMPEDANCE

3.1 INTRODUCTION 193.2 IMPEDANCE EQUATIONS AND DEFINITIONS 193.3 SINUSOIDS, PHASORS AND ELECTRICAL CIRCUITS 213.4 MODEL OF PROBE IN PRESENCE OF TEST MATERIAL 233.5 SIMPLIFIED IMPEDANCE DIAGRAMS 25

3.5.1 Derivation of Probe Impedance for Probe/Sample Combination 25

3.5.2 Correlation Between Coil Impedance andSample Properties 28

3.6 SUMMARY 303.7 WORKED EXAMPLES 31

3.7.1 Probe Impedance in Air 313.7.2 Probe Impedance Adjacent to Sample 323.7.3 Voltage-Current Relationship 32

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CHAPTER 4 - INSTRUMENTATION

PAGE

4.1 INTRODUCTION 334.2 BRIDGE CIRCUITS 34

4.2.1 Simple Bridge Circuit 344.2.2 Typical Bridge Circuit in Eddy Current

Instruments 364.2.3 Bridge Circuit in Crack Detectors 37

4.3 RESONANCE CIRCUIT AND EQUATIONS 384.4 EDDY CURRENT INSTRUMENTS 40

4.4.1 General Purpose Instrument (Impedance Method) 404.4.2 Crack Detectors 424.4.3 Material Sorting and Conductivity

Instruments 44

4.5 SEND-RECEIVE EDDY CURRENT SYSTEMS 45

4.5.1 Hall-Effect Detector 464.5.2 Send-Receive Coils and Lift-Off Compensation 47

4.6 MULTIFREQUENCY EQUIPMENT 484.7 PULSED EDDY CURRENT EQUIPMENT 494.8 SPECIAL TECHNIQUES 504.9 RECORDING EQUIPMENT 51

4.9.1 Frequency Response 53

4 . 1 0 SUMMARY 534 . 1 1 WORKED EXAMPLES 54

4.11.1 Impedance at Resonance 54

CHAPTER 5 - TESTING WITH SURFACE PROBES

PAGE

5.1 INTRODUCTION5.2 SURFACE PROBES

5.2.1 Probe Types

5.2.2 Directional Properties

5.2.2.1 Sensitivity at Centre of a Coil

5.2.3 Probe Inductance

5.3 PARAMETERS AFFECTING SENSITIVITY TO DEFECTS

5.3.1 Sensitivity with Lift-off and Defect Depth5.3.2 Effect of Defect Length

5.4 COMPARISON BETWEEN SURFACE AND THROUGH-WALL INSPECTION5.5 IMPEDANCE GRAPH DISPLAY

5.5.1 Effect of Resistivity5.5.2 Effect of Permeability5.5.3 Effect of Thickness5.5.4 Effect of Frequency5.5.5 Effect of Probe Diameter 735.5.6 Comparison of Experimental and Computer

Impedance Diagrams 73

5555

5659

60

61

65

6566

6769

72727272

f'\!;/I

1II11

5.6 CHARACTERISTIC PARAMETER 745.7 DEFINITION OF "PHASE" TERMINOLOGY 775.8 SELECTION OF TEST FREQUENCY 78

5.8.1 Inspecting for Defects 78 j5.8.2 Measuring Resistivity 80 J5.8.3 Measuring Thickness 835.8.4 Measuring Thickness of a Non-conducting Layer

on a Conductor 84 •5.8.5 Measuring Thickness of a Conducting Layer on

a Conductor - 84

5 . 9 PROBE-CABLE RESONANCE 855 . 1 0 SUMMARY 865.11 WORKED EXAMPLES 88

5.11.1 Effective Probe Diameter 885.11.2 Characteristic Parameter 88

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CHAPTER 6 - SURFACE PROSE SIGNAL ANALYSISPAGE

6.1 INTRODUCTION 896.2 EDDY CURRENT SIGNAL CHARACTERISTICS 89

6.2.1 Defect Signal Amplitude 896.2.2 Defect Signal Phase 91

6.3 EFFECT OF MATERIAL VARIATIONS AND DEFECTS IN A FINITETHICKNESS 93

6.4 COIL IMPEDANCE CHANGES WITH DEFECTS 97

6.4.1 Surface Defect Measurement 976.4.2 Subsurface Defect Measurement 97

6.5 COIL IMPEDANCE CHANGES WITH OTHER VARIABLES 98

6.5.1 Ferromagnetic Indications 986.5.2 Electrical Resistivity 1006.5.3 Signals from Changes in Surface Geometry 100

6.6 CALIBRATION DEFECTS 1016.7 SUMMARY 104

CHAPTER 7 - TESTING OF TUBES AND CYLINDRICAL COMPONENTS

7.1 INTRODUCTION 1057.2 PROBES FOR TUBES AND CYLINDRICAL COMPONENTS 105

7.2.1 Probe Types 1057.2.2 Comparing Differential and Absolute Probes 1077.2.3 Directional Properties 1097.2.4 Probe Inductance 1107.2.5 Probe-Cable Resonance 112

7.3 IMPEDANCE PLANE DIAGRAMS 113

7.3.1 Solid Cylinders 1157.3.1.1 Sensitivity in Centre of a Cylinder 116

7.3.2 Tubes 1187.3.3 Characteristic Frequency for Tubes 120

7.3.4 Computer Generated Impedance Diagrams 122

7.4 CHOICE OF TEST FREQUENCY 123

7.4.1 Test Frequency for Solid Cylinders 123

7.4.2 Test Frequency for Tubes 1247.5 PROBES FOR DETECTING CIRCUMFERENTIAL CRACKS 1257.6 SUMMARY 1287.7 WORKED EXAMPLES 129

7.7.1 Calculate f/fg to operate at knee location,for a cylinder 129

7.7.2 (a) Calculate optimum test frequency for tubeinspection 129

(b) Determine operating point for above frequency 130(c) Calculate frequency to discriminate ferro-

magnetic indications 130

I-viii-

CHAPTER 8 - TUBE TESTING - SIGNAL ANALYSISPAGE

8.1 INTRODUCTION 1318.2 EDDY CURRENT SIGNALS 131

8.2.1 Defect Signal Characteristics 1318.2.2 Effect of Test Frequency 1358.2.3 Calibration Tubes and Simple Defects 1388.2.4 Vectorlal Addition and Defects at Baffle Plates 1428.2.5 Tube Inspection at Tubesheets 1468.2.6 Testing Tubes with Internal Surface Probes 147

8.3 ANOMALOUS EDDY CURRENT SIGNALS

8.3.1 Ferromagnetic Inclusions and Deposits

8.3.2 Conducting Deposits

8.4 MULTIFREQUENCY EDDY CURRENT TESTING

8.4.1 Background8.4.2 Multifrequency Testing of Dented Tubes

8.5 S UMMARY

CHAPTER 9 - METALLURGICAL PROPERTIES AND TESTING FERRO-MAGNETIC MATERIALS

9.1 INTRODUCTION9.2 ELECTRICAL CONDUCTIVITY

9.2.1 Factors Affectirg Resistivity

9.2.2 Material Sorting by Resistivity

9.3 MAGNETIC PROPERTIES

9.3.1 Magnetic Hysteresis9.3.2 Magnetic Permeability

9.3.3 Factors Affecting Magnetic Permeability

9.4 TESTING MAGNETIC MATERIALS

9.4.1 Simplified Impedance Diagrams9.4.2 Impedance Diagrams9.4.3 Material Sorting by Magnetic Permeability9.4.4 Testing for Defects in Magnetic Materials

9.5 S UMMARY

9.6 WORKED EXAMPLES

9.6.1 Calculate Conductivity9.6.2 Calculate Magnetic Permeability

9.6.3 Calculate Standard Depth of Penetration

CHAPTER 10 - DEFINITIONS, REFERENCES AND INDEX

10.1 DEFINITIONS10.2 REFERENCES10.3 INDEX

I!I

149153

155

155158

162

163163

163166

168

169170172

174

174176178178

184

185

185185186

187194195

1I1I1IEEGFKp

P

-ix-

NOMENCLATURE

SYMBOL

Ar1twDBCf

c8frH

IJLNP_RCRLv LXXLXcZ6

VPa$n3to

e

QUANTITY

Cross-Sectional areaRadiusLengthThicknessWidthDiameterMagnetic flux densityCapacitanceTest frequencyOptimum tube testing frequencyCharacteristic or LimitfrequencyResonant frequencyMagnetic field Intensity(Magnetizing force)CurrentCurrent densitySelf InductanceNumber of turns (Windings)Charac*-.' jistic ParameterResistanceResistive loadElectric potentialDepth below the surfaceInductive ReactanceCapacitive ReactanceImpedanceStandard Depthof PenetrationPermeabilityResistivityConductivityMagnetic fluxFill FactorPhase LagAngular frequencyAngle between Z & R

SI UNIT

2metremetremetremetremetremetre 2weber/meter or 1faradshertzhertz

hertzhertzamperes/meter orlenzeamperes 2amperes/meterhenrydimensionlessdimensionlessohmohmvoltmetreohmohmohm

metrehenry/metermicrohm-centimetreSiemens/meterweberdimensiop.lessradiansradians/seconddegrees

CHAPTER 1 - INTRODUCTION

1.1 EDDY CURRENT TESTING

Eddy current testing (ET) is a nondestructive test techniquebased on inducing electrical currents in the material beinginspected and observing the interaction between thosecurrents and the material. Eddy currents are generated byelectromagnetic coils in the test probe, and monitoredsimultaneously by measuring probe electrical impedance.Since it's an electromagnetic induction process, directelectrical contact with the sample is not required; however,the sample material has to be conductive.

Eddy current testing is a versatile technique. It's mainlyused for thin materials; in thick materials, penetrationconstraints limit the inspected volume to thin surfacelayers. In addition to flaw inspection, ET can be used toindirectly measure mechanical and metallurgicalcharacteristics which correlate with electrical and magneticproperties. Also, geometric effects such as thickness,curvature and probe-to-material spacing influence eddycurrent flow and can be measured.

The large number of potentially significant variables In ETis both a strength and a weakness of the technique sinceeffects of otherwise trivial parameters can mask Importantinformation or be misinterpreted. Virtually everything thataffects eddy current flow or otherwise influences probeimpedance has to be taken into account to obtain reliableresults. Thus, credible eddy current testing requires a highlevel of operator training and awareness.

1.2 PURPOSE OF THIS MANUAL

The purpose of this manual is to promote the development anduse of eddy current testing by providing a scientificallysound training and reference manual. The selection ofmaterial presented is based on the premise that a soundunderstanding of basic principles is essential to obtainingvalid data and interpreting it correctly. A non-rigorousapproach has been used to present complex physical phenomenain a document oriented towards application of eddy currenttechniques, especially for defect detection and diagnosis.

The presentation moves from theory (including a review ofbasic electrical concepts) to test methods and signalanalysis. Simplified derivations of probe response to testparameters are presented to develop a basic understanding ofeddy current test principles. Thus, eddy current signals are

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consistently illustrated on impedance plane diagrams (thedisplay used in modern eddy current test instruments) and toaid explanation, the parameter "eddy current phase lag" isintroduced*

Since probes play a key role in eddy current testing,technical aspects of probe design are introduced withpertinent electrical impedance calculations. While knowledgeof basic electrical circuits is required for a completeunderstanding of eddy current test principles, a goodtechnical base for inspection can still be ob*-?..ned ifsections of this manual requiring such a background areskipped.

From an applications point of view, the material in thismanual provides an inspector with the necessary background todecide:1) what probe(a) to use,2) what test frequencies are suitable,3) what calibration defects or standards are required for

signal calibration and/or simulation,4) what tests are required to differentiate between

significant signals and false indications.5) how to estimate depth of real defects.

To supplement theory, practical examples are presented todevelop proficiency in performing inspections, and toillustrate how basic principles are applied to diagnose realsignals.

A number of laboratory demonstrations, practical tests andmultiple choice questions are included in Volume 2, "EddyCurrent Course Supplement". They are divided into groupscorresponding to the chapters in this manual. Thedemonstrations are Intended for use in eddy current coursesto help clarify some of the more difficult concepts. Thepractical tests are to give students practice In usingequipment and performing typical tests. The multiple choicaquestions are intended to check students1 understanding ofthe course material and prepare for certification exams.

1.3 HISTORICAL PERSPECTIVE

Electromagnetic testing — the interaction of nagnetic fieldswith circulating electrical currents — had its origin In1831 when M. Faraday discovered electromagnetic Induction.He induced current flow in a secondary coll by switching abattery on and off. D.E. Hughes performed the first recordededdy current test in 1879. He was able to distinguishbetween different metals by noting a change in excitationfrequency resulting from effects of test material resistivityand magnetic permeability.

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THERE MUST BE DEFECTS /NTHESE TUBES SOMEWHERE —I SAW SQU/GGLES ON THE EDDYCURRENT SCREEN/ Y

Fig. 1.1: Misinterpreted Signals

Initially, the extreme sensitivity to many materialproperties and conditions made ET difficult and unreliable.Figure 1.1 illustrates this point. It took until 1926 beforethe first eddy current instrument was developed to measuresample thickness. By the end of World War II furtherresearch and improved electronics made industrial inspectionpossible, and many practical instruments were developed. Amajor breakthrough came in the 1950's when Forster developedinstruments with impedance plane signal displays. These madeit possible to discriminate between different parameters,though the procedure was still empirical. During the 1960'sprogress in theoretical and practical uses of eddy currenttesting advanced the technology from an empirical art to anaccepted engineering discipline.

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During that tine, other nondestructive test techniques such --as ultrasonics and radiography became well established and |eddy current testing played a secondary role, mainly in theaircraft industry. Recent requirements -- particularly forheat exchanger tube inspection in the nuclear Industry -- 'have contributed significantly to further development of ET Sas a fast, accurate and reproducible nondestructive testtechnique. ~,

Until recently, eddy current testing was a technology wherethe basic principles were known only to researchers, and a „"black box" approach to Inspection was often followed. The j.authors' objective in compiling this manualis to draw upon *research, laboratory and industrial inspection experience tobridge that gap and thereby permit the full potential of eddy ~fcurrent testing to be realised. \

i

I

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CHAPTER 2 - EDDY CURRENT FUNDAMENTALS

2.1 BASIC EQUIPMENT

Basic eddy current test equipment consists of an alternatingcurrent source (oscillator), a probe containing a coilconnected to the current source, and a voltmeter whichmeasures the voltage change across the coil, as shown inFigure 2.1.

OSCILLATOR VOLTMETER

CURRENT

PROBE

PROBE

MOVEMENTCRACK

TEST PLATE

Fig. 2.1; Eddy Current Test Equipment

The oscillator must be capable of generating a time varying(usually sinusoidal) current at frequencies ranging fromabout 1 kHz (1000 cycles per second) to about 2 MHz(2,000,000 cycles per second). Oscillators which operate athigher or lower frequencies, or with pulsed currents are usedfor specialized applications.

The coil within the probe is an insulated copper wire woundonto a suitable form. The wire diameter, the number of turnsand coil dimensions are all variables which must bedetermined in order to obtain the desired inspection results.Coil variables are discussed in later chapters.

— 6—

Depending upon the type of inspection, an eddy current probecan consist of a single test coil, an excitation coil with aseparate receive (sensing) coil, or an excitation coil with aHall-effect sensing detector, as shown in Figure 2.2.

VOLTMETER VOLTMETER VOLTMETER

TEST ARTICLE |

COIL

TEST ARTICLE

EXCITATIONCOIL

(A) SELF-INDUCTANCE (B) SEND-RECEIVE

, SENSING/ COIL

/ HALLDETECTOR

(C.) MAGNETIC REACTION

2.2

Fig. 2.2: Eddy Current Inspection Systems

The voltmeter measures changes in voltage across the coilwhich result from changes in the electrical conditions andproperties of the conducting material tested and/or changesin relative position between the coil and the materialtested. This voltage change consists of an amplitudevariation and a phase variation relative to the currentpassing through the coil. The reason for amplitude and phasechanges in this voltage is discussed in Chapter 3.

GENERATION OF EDDY CURRENTS

2.2.1 Introduction

In this section the topic of the magnetic field surrounding acoil carrying current is introduced together with themechanism by which eddy currents are induced and how they aremeasured.

2.2.2 Magnetic Field Around A Coil

Oersted discovered that whenever there is an electriccurrent, a magnetic field exists. Consider electric currentdirected along a wire, a magnetic field is created in such adirection that if your right-hand thumb points in thedirection of current, your curled fingers point in the

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direction of the magnetic field,rule".

This is the "right-hand

Associated with a magnetic field is Magnetic flux density.It has the same direction as the magnetic field and itsmagnitude depends upon position and current. It is thereforea field vector quantity and is given the symbol B. Its unitsin the SI system is the tesla (T) or webers per square metre(Wb/m2).

The B-field distribution around a long straight wire is shownin Figure 2.3(a). In Figure 2.3(b) the B-field in the axialdirection of a single turn is shown as a function of radius.As mere windings are added, each carrying the same current,the flux density rapidly increases and its associateddistribution is altered.

Ma.9ne.tlc Tiald

Straight WireFlouting into page

(b) Single Turn Coil

Fig. 2.3: Magnetic Flux Distribution

Flux density varies linearly with electric current in thecoil, i.e., if coil current doubles, flux density doubleseverywhere. The total magnetic flux, <j>p, contained withinthe loop is the product of B and area of the coil. The unitin the SI system for magnetic flux is the weber (Wb).

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2.2.3 Equations Governing Generation of Eddy Currents

In any electrical circuit, current flow is governed by Ohm'sLaw and is equal to the driving (primary circuit) voltagedivided by primary circuit impedance.

V /ZP P

(2-1)

The eddy current coil is part of the primary circuit. Thecurrent passing through the coil normally varies sinusoidallywith time and is given by:

I - I sin(fcit)P o

(2.2)

where Io is the peak current value In the circuit and u>(omega) is the frequency in radians/s (to equals 2irf when fis frequency in hertz).

From Oersted's discovery, a magnetic flux (<f>p) exists arounda coil carrying current (see Figure 2.4) proportional to thenumber of turns in the coil (Np) and the current (I p).

N IP P

(2.3)

PROBE

(primarycircuit)

SAMPLE

(secondarycircuit)

Fig. 2.4; Coil Carrying Alternating Current AdjacentTo a Test Sample

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Faraday's Law states a voltage (V 8) is created or induced ina region of space when there ia a changing magnetic field.When we apply this to our coil.

d<f>P J.

where ~^~ is the rate of change in v with time.Since coil current varies stnusoidally with time, totalmagnetic flux in the coil also varies sinusoidally,

<(> - $ sin(wt)

where 4>0 is the magnetic flux corresponding to I o.

The induced voltage as described by equation 2.4 results in

V - - N &)<(> cos Cut) (2.5)^ p c

which also varies periodically with time. If we bring thecoil close to a test sample, Ohm's Law states that if thereis a driving voltage (V s) and the sample's impedance isfinite, current will flow,

I B - V g/ Z g (2.6)

where <lg is current flowing through the sample, V s isinduced voltage and Z 8 is the sample's impedanceor opposition to the flow of current.

These induced currents are known as eddy currents because oftheir circulatory paths. They, in turn, generate their ownmagnetic field according to Lenz's Law, which opposes theprimary field,

and 4>E " *p " *B (2.8)

where $ E is t n e equilibrium magnetic flux surrounding thecoil in the presence of a test sample.

The flow of eddy currents results in resistive (Ohmic) lossesand a decrease in magnetic flux. This is reflected as adecrease in probe impedance. In equation form,

Z - <f>E (2.9)

and V - ZIp (2.10)

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Equation 2.9 indicates a coil's impedance is a function ofthe magnetic field surrounding it and in turn the magneticfield is governed by induced current in the specimen(equations 2.8 and 2.7). The relations between probeimpedance and sample properties will be derived in Chapter 3.

To summarize, flux is set up by passing alternating currentthrough the test coil. When this coil is brought close to aconductive sample, eddy currents are induced. In addition,the magnetic flux associated with the eddy currents oppose thecoil's magnetic flux, thereby decreasing net flux. Thisresults in a change in coil impedance and voltage drop. It isthe opposition between the primary (coil) and secondary (eddycurrent) fields that provides the basis for extractinginformation during eddy current testing.

It should be noted that if a sample is ferromagnetic,equation 2.9 still applies but the magnetic flux isstrengthened despite opposing eddy current effects. The highmagnetic permeability of ferromagnetic materialsdistinguishes them from non-ferromagnetic materials andstrongly influences eddy current test parameters.

Ferromagnetic specimen inspection is discussed in Chapter 9and unless specified the rest of the manual is restricted tonon-ferromagnetic materials.

2.3 FUNDAMENTAL PROPERTIES OF EDDY CURRENT FLOW

Eddy currents are closed loops of induced current circulatingin planes perpendicular to the magnetic flux. They normallytravel parallel to the coil's winding and parallel to thesurface. Eddy current flow is limited to the area of theinducing magnetic field.

Test frequency determines depth of penetration into thespecimen; as frequency is increased, penetration decreases andthe eddy current distribution becomes denser near thespecimen's surface. Test frequency also affects thesensitivity to changes in material properties and defects.

Figure 2.5(a) shows the algebraic relationships and Figure2.5(b) the oscilloscope display of eddy current and magneticfield distribution with depth into the specimen. Both theeddy currents and magnetic flux get weaker with depth becauseof "skin effect". In addition to this attenuation, the eddycurrents lag in phase with depth. Eddy currents' phase lagis the key parameter that makes eddy current testing a usefulNDT method. The parameters skin depth and phase lag arediscussed in the next section.

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COIL

\

(b)

Fig. 2.5; Eddy Current and Magnetic Flux DistributionWith Depth Into a Conductor

2.4 SKIN EFFECT

Eddy currents Induced by a changing magnetic field concentratenear the surface adjacent to the excitation coll. The depthof penetration decreases with test frequency and is a functionof electrical conductivity and magnetic permeability of thespecimen. This phenomenon Is known as the skin effect and Isanalogous to the situation in terrestrial heat conduction wheredaily surface temperature fluctuations are not appreciable belowthe earth's surface. Skin effect arises as follows: the eddycurrents flowing in the test object at any depth producemagnetic fields which oppose the primary field, thus reducingnet magnetic flux and causing a decrease in current flow asdepth increases. Alternatively, eddy currents near the surfacecan be viewed as shielding the coil's magnetic field therebyweakening the magnetic field at greater depths and reducinginduced currents.

The equation for flow of induced currents is

fJi IfV2J (2.11)

where J is current density, a is conductivity, y is magneticpermeability and V2 is a differential operatorof second order.

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For a semi-infinite (thick) conductor the solution to theabove equation is

Jx/J0 sin(u)t-B) (2.12a)

3

where Jx/J0 i s t h e r « t l 0 of eddy current density Jx at

depth x to the surface density J0,and e » 2.718 is the baseof natural logarithms. B is given by x/5 where 6 • (irfua)-the standard depth of penetration (see next section).

Equation 2.12(a) can be separated into two components:

Jx / Jo-x/6 (2.12b)

which describes the exponential decrease in eddy currentdensity with depth,and

Jx/Jo sin (u)t-x/8) (2.12c)

denoting the Increasing time or phase lag of the sinusoidalsignal with depth.

2.4.1 Standard Depth of Penetration

Figure 2.6 illustrates the change in eddy current density ina semi-infinite conductor. Eddy current density decreasesexponentially with depth.

<f>t sin

a

i

0.2 0.4 0.6 0.S 1.0

. m

Fig. 2.6; Eddy Current and Magnetic Flux DistributionWith Depth In a Thick Plate

"7f

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The depth at which eddy current density has decreased to 1/e or36.82 of the surface density is called the standard depth ofpenetration. The word 'standard' denotes plane waveelectromagnetic field excitation within the test sample(conditions which are rarely achieved in practice). Thestandard depth of penetration is given by

6 - 50/p/fUr , mm (2.13a)

or o " 2/p/fyr , inches (2.13b)

where p is electrical resistivity in microhm-centimetres,f is test frequency in hertz, and yr is relative magneticpermeability (dimensionless)*.

The skin depth equation is strictly true only for Infinitelythick material and planar magnetic fields. Using thestandard depth, 6, calculated from the above equationmakes it a material/test parameter rather than a truemeasure of penetration.

2.4.2 Depth of Penetration in Finite Thickness Samples

Sensitivity to defects depends on eddy current density atdefect location. Although eddy currents penetrate deeperthan one standard depth of penetration they decrease rapidlywith depth. At two standard depths of penetration (26), eddycurrent density has decreased to (1/e)"* or 13,5X of thesurface density. At three depths (36) the eddy current densityis down to only 5X of the surface density. i?csever, one shouldkeep in mind these values only apply to thick samples(thickness, t >56) and planar magnetic excitation fields.Planar field conditions require large diameter probes (diameter>10t) in plate testing or long coils (length >5t) in tubetesting. Real test coils will rarely meet these requirementssince they would possess low defect sensitivity. For thin plateor tube samples, current density drops off less than calculatedfrom equation 2.12(b) as shown in Figure 2.7. For solidcylinders the overriding factor is a decrease to zero at thecentre resulting from geometry effects as shown in Fig. 2.7(c)and discussed in Section 7.3.1.

One should also note, that the magnetic flux is attenuatedacross the sample, but not completely. Although the currentsare restricted to flow within specimen boundaries, themagnetic field extends into the air space beyond. Thisallows the inspection of multi-layer components separated byan air space.

*See Chapter 9 for a description of electrical and magneticproperties. u r » yA , incremental permeability, at zerobiasing magnetization flux.

-u-

( a ) PLATE (LARGE COIL, D > 1 O t )

EQUATION 2.12 ( b )

ACTUAL

( b ) TUBE (LONG ENCIRCLING COIL.4 > 5 t )

J o = EDDY CURRENT DENSITY AT SURFACE

J , OR J r = EDDY CURRENT DENSITY A; LOCATION

3 OR r BELOW THE SURFACE

(C) ROD (ENCIRC'.!:;c COIL, r0)

TUBE AND ROD GEOMETRY

( r , =0 FOR ROD)

Fig. 2.7: Eddy Current Distribution With Depth inVarious Samples

The sensitivity to a subsurface defect depends on the eddycurrent density at that depth, it is therefore important toknow the effective depth of penetration. The effective depthof penetration is arbitrarily defined aa the depth at whicheddy current de.isity decreases to 5% of the surface density.For large probes and thick samples, this depth is about threestandard depths of penetration. Unfortunately, for mostcomponents and practical probe sizes, this depth will be lessthan 3 5 , the eddy currents being attenuated more thenpredicted by the skin depth equation. The effect of probediameter on the decrease in eddy current density or defectsensitivity with depth is discussed in Section 5.3.1.

2.4.3 Standard Phase Lag

The signal produced by a flaw depends on both amplitude andphase of the currents being obstructed. A small surfacedefect and large internal defect can have a similar effecton the magnitude of test coil impedance. However, because ofthe increasing phase lag with depth, there will be acharacteristic difference in the test coil impedance vector.This effect allows location and extent of a defect to bedetermined. [;

-15-

Fhase lag Is derived from equation 2.12(c) for infinitelythick material. It represents a phase angle lag of x/<5radians between the sinusoidal eddy currents at the surfaceand those below the surface. It is denoted by the symbol 3(beta) and is given by:

3 - x/<5 radians (2.14a)

orx/6 x 57 degrees (2.14b)

where x is distance below the surface in the same units as

(DEGREES)

x 57, DEGREES

Fig. 2.8: Eddy Current Phase Lag Variation With Depthin Thick Samples

When x Is equal to one standard depth of penetration,phase lag is 57" or one radian. This means that the eddycurrents flowing below the surface, at one standard depth ofpenetration, lag the surface currents by 57°. At twostandard depths of penetration they lag the surface currentsby 114". This is illustrated in Figure 2.8.

-16-

2.4.4 Phase Lag in Finite Thickness Samples

For thin samples, eddy current phase decreases slightly lessrapidly with depth than stated above. See Figure 2.9(a), (b)and (c) for the plots of phase lag with depth for a plate,tube, and cylinder, respectively. The phase lag illustratedin these plotB does not change significantly with coildiameter or length. For thick samples and practical probesizes, equation 2.14 is sufficiently accurate.

_ = 0.7

2.0

0"

20*

40'

BO*

60'

00'

5̂\

r ri

*s**t5

NX ^ ^N X

\ sN \

0.B ^ ^

i I i

H = 1.4

.2 .4 1.0

( a ) PLATE ( b ) TUBE

.2 .4 .6 .8 1.0

rBi > t

J I

TUBE AND ROD ( r . - 0) GEOMETRV

0 ff = PHASE I I T H DEPTH x, OR r,RELATIVE

" ' r TO SURFACE CURRENTr0

( c ) ROD

Fie. 2.9; Eddy Current Phase Lag in Various Samples

ACTUAL CURVES

CALCULATED, EQUATION 2.14 ( b )

Phase lag can be visualized as a shift in time of thesinusoidally varying current flowing below the surface. Thiswas illustrated in Figure 2.5. Phase lag plays a key role inthe analysis of eddy current test signals. It will be usedthroughout the manual to link theory and observations. Itshould not be misinterpreted or confused with the phase anglebetween voltage and current in AC theory. Both the voltageand current (and magnetic field) have this phase shift or lagwith depth.

IIII

-17-

IIIIIIIII

II1I

2.5 SUMMARY

Eddy current testing is based on inducing electrical currentsin the material being inspected and observing the interactionbetween these currents and the material.

I This process occurs as follows: When a periodically varying

magnetic field intersects an electrical conductor, eddy cur-rents are induced according to Faraday's and Ohm's Laws. Theinduced currents (known as eddy currents because of theircirculatory paths) generate their own magnetic field whichopposes the excitation field. The equilibrium field is re-duced resulting in a change of coil impedance. By monitoringcoil impedance, the electrical, magnetic and geometric pro-perties of the sample can be measured. Eddy currents areclosed loops of induced current circulating in planes perpen-dicular to the magnetic flux. They normally travel parallelto the coil's winding and parallel to the surface. Eddy cur-rent flow is limited to the area of the inducing magneticfield.

Depth of penetration into a material depends on its electri-cal resistivity, magnetic permeability and on test frequency.The basic equation of ET is the standard depth of penetrationgiven by

mm (2.13a)

where p is electrical resistivity, microhm-centimetres;f is test frequency, hertz;

and yr is relative magnetic permeability, dimensionless.

It states that in thick materials eddy current densitydecreases to 37% of the surface density at a depth of onestandard depth of penetration. In most eddy current tests,especially with surface probes, the actual eddy currentdensity (at a depth equal to that calculated by equation2.13a) is much less than 37%.

Eddy currents also lag in phase with depth into the material.Phase lag depends on the same material properties thatgovern depth of penetration and is given by

0 = x/6 - x , radians (2.14a)5O/p/fUr

where x is distance below surface, mm.

Phase lag is the parameter that makes it possible to deter-mine the depth of a defect. It also allows discriminationbetween defect signals and false indications. It is the keyparameter in eddy current testing.

-18-

2.6 WORKED EXAMPLES

2.6.1 Standard Depth of Penetration and Phase Lag

PROBLEM: (a) Calculate the standard depth of penetrationin a thick 304 SS sample, at a test frequency of100 kHz.

(b) Determine the eddy current phase lag at adepth of 1.5 mm in 304 SS at 100 kHz.

SOLUTION: 304 SS properties: p - 72 microhm -centimetres

and p r - 1

(a) from equation 2.13(a),

50

f 100 x 103 x 1

- 50(7.2 xlO *) - 1.3 mm

Therefore the standard depth of penetration is 1.3 mm.

(b) from equation 2.14(b),

g - x/6 x 57

" T^f X 57 "" 64

Therefore the phase lag is 64 degrees.

-19-

CHAPTER 3 - ELECTRICAL CIRCUITS AND PROBE IMPEDANCE

3.1 INTRODUCTION

Eddy current testing consists of monitoring the flow anddistribution of eddy currents in test material. This isachieved indirectly by monitoring probe impedance during atest. An understanding of impedance and associatedelectrical quantities is therefore imperative for afundamental appreciation of eddy current behaviour.

The first two sections review the electrical quantitiesimportant in eddy current testing. This is followed bypresentation of a model of a test coil coupled to testmaterial and its equivalent electrical circuit. Theequivalent circuit approach permits derivation of simplifiedimpedance diagrams to show the effect of test and materialparameters on probe impedance in graphical form. Once thesimple impedance diagram concepts of this chapter areunderstood, the more complex diagrams of subsequent chaptersshould present little difficulty.

3.2 IMPEDANCE EQUATIONS AND DEFINITIONS

All information about a sample is obtained through changes inelectrical characteristics of the coil/sample combination.Therefore a basic understanding of electrical quantities isimportant for eddy current inspection.

RESISTANCE: (symbol: R, units: ohm, ft)

Opposition to the flow of electrical current iscalled resistance. It is constant for bothdirect and alternating current. The electricalcomponent is called a resistor.

V « IR Ohm's Law (3.1)

where, V is voltage drop across resistor (volt), andI is current through resistor (ampere)

INDUCTANCE: (symbol: L, units: henry, H)

The property of an electric circuit by virtue ofwhich a varying current induces an electromotiveforce in that circuit (self) or in aneighbouring circuit (mutual) is calledinductance. The electrical component is calledan inductor.

-20-

total flux linkagescurrent through coll (3.2a)

(3.2b)

- k-L (N*A/Jl) (3.3)

where, N is number of coil turns<|)p is magnetic flux (weber)I is current (ampere)k- is a geometric factor „A is coil's planar surface area (am )P. is coil's axial length (mm)

The self-inductance of a coil is proportional to coilwindings squared (N2)and planar surface area (A), andinversely proportional to coil length

INDUCTIVE REACTANCE: (symbol: XL, units: ohm, fi)

Opposition to changes in alternating currentflow through a coil is called inductiver e a c t a n c e . _,.

!'

XT - wL ( 3 . 4 a ) -

or XT * 2irfL ( 3 . 4 b )

where, f is frequency of alternating current

(hertz), and U) is angular frequency ~:(radians/second) I

CAPACITIVE REACTANCE (symbol: Xc, units: ohm, fi)

Opposition to changes in alternating voltage •*across a capacitor is called capacitivereactance. "'

Eddy current coil capacitive reactance isnormally negligible. However, capacitance canbe important when considering impedance of probecables (Sections 5.9 and 7.2.5).

xc " 2irfC (3.5)

where, C is capacitance (farad)

IMPEDANCE: (symbol: Z, units: ohm, ft)

The total opposition to alternating current flowis called IMPEDANCE. For a coil,

Z| «*R* + X' (3.6)

-21-

XLand 6 - Arctan -£• (3.7)

where |z| is magnitude of Z, and 6 is phase of Z(described in next section).

3.3 SINUSOIDS, PHASORS AND ELECTRICAL CIRCUITS

In a direct current (DC) circuit, such as a battery and lightbulb, current and voltage are described completely by theirrespective magnitudes, Figure 3.1(a). Analysis ofalternating current (AC) circuits is more complex. Sincevoltage and current amplitude vary with time, the phase (ortime delay) relationship between them must also be taken intoaccount. A typical AC circuit, an inductor in series with aresistor, is presented in Figure 3.1(b). This is asimplified model of a probe assembly: the inductor is thereactive part of the assembly (coil) while the resistormodels both coil wire and cable resistance. Figure 3.1(c)shows voltage across the inductor (VL) leads the current(I) by 90°, while voltage across the resistor (VR) is inphase with current. Since the current is common to bothinductor and resistor, it is possible to use current as apoint of reference. Hence, we deduce the voltage across theinductor leads the voltage across the resistor by 90°.

If one measures the voltage drop, VT, across both theinductor and resistor, we find Vj leads current (or VR)by an angle less than 90", as shown in Figure 3.1(d).

To evaluate the total voltage Vj, we add vectorially theseparate voltages VR and VL,

VT " VR + VL (3.8)

- I(R + jiuL) (3.9a)

where j is a mathematical operator (rotates a vectorCCW by 90°)

or VT - IR sin ( ait+O) + j IiOL sin ((Dt+ir/2) (3.9b)

Representing voltage waveforms as in Figure 3.1(d) orequation 3.9(b) can be both time consuming and confusing. Asimpler voltage representation is available by means ofphasor diagrams. In phasor diagrams the voltage isrepresented by its peak value (amplitude) and phase shift (6)relative to the current. The two terms in equation 3.9(b)both contain an amplitude and phase shift so they can be

-22-

D1RECT CURRENT

(IATTERY)

(>) DIRECT CURRENT CIRCUIT

V . IR

CURRENT AND VOLTAGE CAN BEDESCRIBED BY MAGNITUDE ONLY

ALTERNATING CURRENT

OJRRtHT HUS1 BE BESCKIMB V IAMPLITUDE AND PHASE

r(») ALTERNATING CURRENT CIRCUIT

VL LEADS I BY 90"

LEADS VR BY SO1

(O CRT DISPLAY OF VOLTAGE AND CURRFNT

CRT DISPLAY Of Vt, Vi. AND TOTAL VOLTASE DROP. YT

VBem>(<) VOLTAGE GRAPH DISPLAY OF PNASORS

RESISTANCE. R

(O IMPEDANCE GRAPH DISPLAY

Fig. 3.1; Representation of Direct Current andAlternating Current Circuit Parameters

-23-

represented by phasors. The first term's amplitude is IR andits phase shift is 0. The amplitude of the second term is IuLand its phase shift is ir/2 or 90°. Each phasor can berepresented by an arrow starting at the origin. The phasor'samplitude is indicated by the length of the arrow OP and thephase shift by the direction of the arrow, see Figure 3.1(e).Fhasors are displayed graphically with the resistivecomponent (VR), having a phase shift 6 = 0 , along thehorizontal axis. As 6 increases the phasor rotatescounter-clockwise. The reactive component (VL), having aphase shift 6 « 90°, will be represented along the verticalaxis.

Current is common to both voltage components and since V-IZ,the voltage graph of Figure 3.1(e) can be converted to animpedance graph display, as in Figure 3.1(f). If thisapproach is applied to eddy current testing, it is found thatany changes in resistance or inductive reactance will cause achange in the position of the end of the vector (point F)which represents the total impedance vector.

To obtain the reactive and resistive components from thisgraph requires knowledge of trigonometry.

Reactive component: XL » toL =• |z| sin 6 (3.10)

Resistive component: R - |z| cos 9 (3.11)

Amplitude of impedance: \z\ - «R 2 + XL2

Phase angle: 6 - Arctan X /R (3.7)

Note the x axis component represents pure resistance (phaseshift » 0°) while the y axis component represents pureinductive reactance (phase shift » + 90°). In thesecalculations it is assumed coil capacitance is negligible.

3.4 MODEL OF PROBE IN PRESENCE OF TEST MATERIAL

The test probe contains a coil which when placed on or closeto a test sample can be considered as the primary winding ofa transformer. The field created by alternating current inthe coil Induces eddy currents in the test sample which actsas a single turn secondary winding,Ng - 1, Figure 3.2(b).Eddy currents align to produce a magnetic field which tendsto weaken the surrounding net magnetic flux 4>p, accordingto Lenz's Law.

-24-

(a)

(b)

• P ' Sooo8

(c)

^ SECONDARY3' RECEIVE COIL3

Fig. 3.2; Model of a Coll with Test Object

There are two methods of sensing changes In the secondarycurrent, I8. The "Impedance method" of eddy currenttesting consists of monitoring the voltage drop across theprimary coil (V - Ipzp^* T h e impedance Zp isaltered by the load of the secondary of the transformer.Therefore, changes in secondary resistance, Rs, orinductance L8 can be measured as changes in V_.

The "send-receive" method of eddy current testing uses twocoils. Eddy current flow in the sample is altered by defectsand these variations are detected by monitoring the voltageacross a secondary receive coil, see Figure 3.2(c).

-25-

3.5 SIMPLIFIED IMPEDANCE DIAGRAMS

3.5.1 Derivation of Probe Impedance for Probe/Sample Combination

We now consider how changes in the test sample affect coilimpedance on the impedance graph display.

From the previous section the probe and test sample can bemodelled as a transformer with a multi-turn primary (coil)and single turn secondary (sample), Figure 3.3(a). Thiscircuit can be simplified to an equivalent circuit where thesecondary circuit load is reflected as a resistive load inparallel with the coil's inductive reactance, Figure 3.3(b).This circuit is an approximate model of a real coil adjacentto a conductor. It is assumed that all of the magnetic fluxfrom the primary coil links the test sample; the coupling isperfect (100%). It is also assumed that there is no skindepth attenuation or phase lag across the sample thickness.

•) MODEL OF A COIL AND TESTSAMPLE

«>) EQUIVALENT PARALLEL CIRCUIT

(c) EQUIVALENT SERIES CIRCUIT

Fig. 3.3 Equivalent Circuits

The equivalent circuit concept can be used to obtainsimplified impedance diagrams applicable to eddy currenttesting. These diagrams serve as an introduction to the moredetailed diagrams which include variations caused by the skineffect. The coil/sample circuit model can be transformedinto the simpler series circuit by the following mathematicalmanipulations. The load resistance R8 can be transferredfrom the secondary back to the primary winding by multiplyingit by the turns ratio squared, (N D/N a)

2, Figure 3.3(b).

-26-

The total impedance of this parallel circuit can be evaluated jand transformed into an equivalent series circuit as follows: •

Z,Z» i!Z -P Z, + Z2

2where Z^ - Np"RB

and Z2 - jX0,

where Xo» u)Lo, coil Inductive reactance in air.

jN2R XTherefore Z - • p a °- •p - • , - •

N R +jX \P s J o f.

which transforms to

N2R X2 (N2R )2X (3.12a) Iz » P s o +j p a' oP < NpV 2 + ( V 2 (NpR8)

2 + (x0)2 ii

This can be viewed as a series combination, in the primarycircuit, of resistance RL and Inductive reactance Xp or

Zn - R. + jXn (3.12b)p L p

The series circuit in Figure 3.3(c) is therefore fullyequivalent to the parallel one of Figure 3.3(b). Rp can beconsidered as coil wire and cable resistance whileZp-RL+jXp is the total impedance of the probe/samplecombination.

When the probe is far from the sample (probe in air), Rs isvery large and by substituting RB • « into equation 3.12aresults in

RL-0, Xp-X0 and Zp-X0

-27-

The above results can be obtained by removing componentN|RS from Figure 3.3(b), since Rs- « implies an opencircuit.

One last transformation in the equation is required beforeimpedance graphs can be obtained. Equation 3.12(a) can besimplified by setting

co " x o G

where G * l/N_Rg is equivalent circuit conductance.

Substitution in 3.12(a) yields

X

1 + C

Normalizing with respect to X o, the call's inductivereactance when far removed from the sample (coil in air)results in

Z_EXo

+ j1 +C 1 + C

(3.13)

By varying C o, in equation 3.13, from 0 to infinity theimpedance curve of Figure 3.4 is obtained. The Impedancelocus is that of a semi-circle with center at X p/X 0«%and R L/X 0 * 0; its radius .is \. With the help ofequation 3.13 and Figure 3.4, Impedance changes can berelated to changes in the sample characteristics.

NORMALIZEDINDUCTIVEREACTANCE

P (OPERATING POINT)

C n -«RL/St0

NORMALIZED RESISTBNCE

Fig. 3.4: Impedance Graph Display

-28-

3.5.2 Correlation Between Coil Impedance and Sample Properties

The effect of test parameter variations on probe impedancecan be derived from equation 3.13. Each parameter issubstituted in turn into C O » X O / N | R S ; if an increase inthe parameter results in an increase in Co> the operatingpoint (position on impedance diagram) moves DOWN the impedancecurve, if Co decreases, the operating point moves UP theimpedance curve. These correlations are useful in obtaining aqualitative appreciation of the effect of the various test _parameters. It is also useful to know that probe/sample fieffects can be derived from the simple equivalent parallel "circuit where the sample is treated as a resistor in parallelwith an inductor (coil). The complete effect can then be Itobtained by adding the effect of 'phase lag1, which will be ||treated in later chapters.

Study of equation 3.13 reveals the following:

1. An increase in Rs results in a decrease in Co«Therefore an increase in resistance to eddy current flowmoves the operating point, P, UP the impedance curve(along the semi-circle), see Figure 3.5(a).

IT2. R 8 - p«,/A II

where, p is electrical resistivity, £ is eddy current nflow distance and A is cross-sectional area to current ]lflow.

Therefore, p » constant x Rs l|

An increase in electrical resistivity will move theoperating point UP the impedance curve. The opposite is f?true for an increase in electrical conductivity. See ifFigure 3.5(a).

3. For thin wall tubes cr plates of thickness t, I

R8 « pil/A • pirD/tw

and for constant probe or tube diameter, D, and coil ''width, w,

ITR8 • constant/t • \\An increase in tube wall (or plate thickness) will move r-the operating point DOWN the impedance curve, see Figure ' j3.5(b).

-29-

4. Co - u>L0/NpRs - constant x w

for constant sample properties.

An increase in test frequency will move Che operatingpoint DOWN the impedance curve, see Figure 3.5(c).

5. I.o " constant x D2; probe inductance increases

proportional to probe or tube diameter squared.

6.

andAlso RB - pTD/tw - constant x D, for constantthickness, t, and coil width, w. Substituting LQ

Rs into Co - 0)Lo/NpR8 results in Co-constant x D.An increase in probe diameter or tube diameter willmove the operating point DOWN the impedance curve, seeFigura 3.5(d).

In the equivalent circuit derivation perfect coupling wasassumed for sake of simplification. However, it can beshown that when mutual coupling between coil and sampleis decreased, the impedance point traces smallerseal-circles as Co increases from 0 to infinity, seeFigure 3.5(e).

DTUBE

"SURFACE K

PROBE TF

(c)

. OECREASING FILL FACTOROR INCREASING LIFT-OFF

Fig. 3.5: Simplified Impedance Diagrams

-30-

3.6 SUMMARY

The impedance method of eddy current testing consists ofmonitoring the voltage drop across a test coil. TheImpedance has resistive and inductive components; theimpedance magnitude is calculated from the equation

|Z| - */R" + (wL)£ , ohms

and the impedance phase i s calculated from

e ArctanoiLR , degrees

(3.6)

(3.7)

The voltage across the test coil is V - IZ where I is thecurrent through the coil and Z is the impedance.

t

A sample's resistance to the flow of eddy currents isreflected as a resistive load and is equivalent to aresistance in parallel to the coll inductive reactance. Thisload results In a resistive and inductive impedance change inthe test coil. Coil impedance can be displayed on normalizedimpedance diagrams. These are two-dimensional plots with theinductive reactance displayed on the vertical axis andresistance on the horizontal axis as in Figure 3.6.

a»L0

NORMALIZEDINDUCTANCEREACTANCE

* OPERATING POINT

o-,t,f,D

NORMALIZED RESISTANCE,

Fig. 3.6; Impedance Graph Display

3.7

-31-

With this display we can analyze the effect of sample andtest parameters on coil impedance. The equivalent circuitderivation of coil impedance is useful for a qualitativeunderstanding of the effect of various test parameters. Itis valid only for non-ferromagnetic material and for thecondition of no skin depth attenuation or phase lag acrossthe sample. (Ferromagnetic materials will be covered inSection 9.4).

Note that all test parameters result in a semicircle displayas they increase or decrease. A resistance increase to theeddy current flow or increase of sample's electricalresistivity moves the operating point UP the impedancediagram, i.e., increase in coil inductance and a change incoil resistance.

An increase in a sample's electrical conductivity, thicknessor tube diameter, moves the operating point DOWN theimpedance curve. An increase in test frequency or probediameter also moves the operating point DOWN the impedancecurve. Although not shown in the above figure, a decrease infill-factor or increase in lift-off results in a decrease insemicircle radius and a smaller change in coil impedance.

In some test requirements it is advantageous to operate atspecific locations on the impedance diagram. By choosing theappropriate test parameters this is usually possible.

WORKED EXAMPLES

3.7.1 Probe Impedance in Air

PROBLEM: An eddy current test is carried out at a testfrequency of 50 kHz. Coil resistance is 15 ohmswhile its inductance is 60 microhenries.a) What is the inductive reactance of the test

coil?b) What is the impedance of the test coil?c) What is the angle, 6, between the total

impedance vector and the resistance vector?

SOLUTION:

a) XL - 2 irfL - (2 IT ) (50 x 103) (60 x 10~6 )XT * 18.8 ohms

b) (2 irfL)2

24.1 ohms( 1 8 . 8 ) 2

c ) Arctan 18.815 Arctan 1.253

-32-

3.7.2 Probe Impedance Adjacent to Sample

PROBLEM: An eddy current test is carried out on brass usinga surface probe at 50 kHz. Coil resistance in airis 15 ohms and its inductance in air is 60microhenries. Probe impedance with the probe onthe brass sample is measured as Z,and 6 « 35 degrees.

24.5 ohms

Calculate: a) X_, inductive reactance

SOLUTION!

and

a) X

b) R.

b) RL, resistive load

- Z sin6P P

« 24.5 sin 35° - 14.1 ohms

cos9 - RDC

= 24.5 cos 35 - 15.0 » 5.1 ohms

3.7.3 Voltage - Current Relationship

PROBLEM: For the above probe impedance problem calculatevoltage drop across the probe if test current is100 milliamperes.

SOLUTION: Probe impedance |z| - 24.5 ohms

Ohm's Law states that V - I |'z|

therefore, V - (0.10) (24.5) - 2.45 volts.

Voltage across the probe is 2.45 volts.

-33-

CHAPTER 4 - INSTRUMENTATION

4.1 INTRODUCTION

All the information about a test part is transmitted to thetest coil through the magnetic field surrounding it. Theimpedance eddy current method monitors voltage drop acrossthe primary coil, Vp •

Inzp» as coil impedance changesso will the voltage across the coil if current remains rea-sonably constant. The send-receive eddy current method moni-tors voltage developed across a sensing coil (or Hall effectdetector) placed close to the excitation coil, see Figure 2.2.

In most inspections, probe impedance (or voltage) changesonly slightly as the probe passes a defect, typically lessthan 1%. This small change is difficult to detect by measur-ing absolute impedance or voltage. Special instruments haVebeen developed incorporating various methods of detecting andamplifying small impedance changes.

The main functions of an eddy current instrument are illus-trated In the block diagram of Figure 4.1. A sine wave

I(Z,-Z2)

OSCILLATOR

AMPLIFIER

— BALANCE

PHASESENSITIVEAC TO DCCONVERTOR(PLUSFILTERING)

D.C. METER

PHASEROTATION

X-YMONITOR

TRANSFORMER

Fig. 4 . 1 : Block Diagram of Eddy Current Instrument

-34-

oscillator generates sinusoidal current, at a specified fre-quency, that passes through the test coils. Since the impe-dance of two coils is never exactly equal, balancing isrequired to eliminate the voltage difference between them.Most eddy current instruments achieve this through an ACbridge or by subtracting a voltage equal to the unbalancevoltage. In general they can tolerate an impedance mismatchof 5%. Once balanced, the presence of a defect in the vici-nity of one coil creates a small unbalance signal which isthen amplified.

Since the sinusoidal unbalance voltage signal is too diffi-cult and inefficient to analyze, it is converted to a directcurrent (DC) signal retaining the amplitude and phase charac-teristics of the AC signal. This is normally achieved byresolving the AC signal into quadrature components and thenrectifying them while retaining the approximate polarity. Ingeneral purpose instruments, these signals are normally dis-played on X-Y monitors. Simpler instruments, such as crackdetectors, however, have a meter to display only the changein voltage amplitude. To decrease electrical instrumentnoise, filtering ie used at the signal output; however, thisdecreases the frequency response and thereby restricts theinspection speed.

The most troublesome parameter in eddy current testing islift-off (probe-to-specimen spacing). A small change inlift-off creates a large output signal. The various methodsused to decrease this effect are discussed in the individualsections on specific eddy current instruments.

4.2 BRIDGE CIRCUITS

Most eddy current instruments use an AC bridge to sense theslight impedance changes between the coils or between asingle coil and reference impedance. In this section theimportant characteristics of bridge balance are discussed.

4.2.1 Simple Bridge Circuit

A common bridge circuit is shown in general form in Figure4.2, the arms being indicated as impedance of unspecifiedsorts. The detector is represented by a voltmeter. Balanceis secured by adjustments of one or more of the bridge arms.Balance is indicated by zero response of the detector, whichmeans that points A and C are at the same potential (have thesame instantaneous voltage). Current will flow through thedetector (voltmeter) if points A and C on the bridge arms areat different voltage levels (there is a difference in voltagedrop from B to A and B to C). Current may flow in eitherdirection, depending on whether A or C is at higherpotential.

-35-

Flg. 4.2; Common Bridge Circuit

If the bridge is made up of four impedance arms, havinginductive reactance and resistive components, the voltagefrom B to A must equal the voltage from B to C in bothamplitude and phase for the bridge to be balanced.

At balance,

and 1^3 » I2Z4

from which the following relationship is obtained:

(4.1)

Equation 4.1 states that the ratio of impedances of one pairof adjacent arms must equal the ratio of impedances of theother pair of adjacent arms for bridge balance.

If the bridge was made up of four resistance arms, bridgebalance would occur if the magnitude of the resistors satisfiesequation 4.1 (with Z± replaced with R]_, etc). However, if theimpedance components are eddy current probes consisting of bothinductive reactance and resistance, the magnitude and phase ofthe impedance vectors must satisfy equation 4.1.

-36-

In practice, this implies the ratio of inductive reactance ofone pair of adjacent arms must equal the ratio of inductivereactance of the other pair of adjacent arms; the same beingtrue for the resistive component of impedance.

Figure 4.2 and equation 4.1 can be used to illustrate thecharacteristic 'figure 8' signal of a differential probe. If

1 > _£* point C is at a higher potential than point A.Z2 4This implies that when Z-y increases (i.e., coil movingacross a defect) with Z2. Z3 & Z4 constant, the bridgevoltage unbalance increases,and the opposite happens whenZq increases* It is this bridge unbalance characteristicthat results in a plus-minus or 'figure 8' signal as thedifferential probe moves across a localized defect. Thissignal occurs independent of whether the two coils are woundin opposition or in addition.

4.2.2 Typical Bridge Circuit in Eddy Current Instruments

Figure 4.3 illustrates a typical AC bridge used in eddycurrent instruments. It is similar to the bridge in Figure4.2 except for two additional arms. In this bridge the probecoils are placed in parallel with variable resistors. Thebalancing, or matching of voltage vector phase and amplitude,is achieved by varying these resistors until a null isachieved. Potentiometer R2 balances the reactive componentof the coils to make the phase angle of each coil circuitequal. Potentiometer R^ balances the resultant voltagewith an equal voltage amplitude to null the instantaneousvoltage between R^ and R£>

TEST COIL

REFERENCECOIL

Fig. 4.3: Typical Bridge Circuit Used in EddyCurrent Instruments

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The Inductive voltage drop across each coll Is equalized bycontrolling the current passing through the colls. This Isdone by varying potentiometer R2« However, when the testcoll Inductance differs significantly from reference coilinductance, potentiometer R2 will have to be rotated to oneextremity. This means less <"-rrent passes through one collmaking it less sensitive than the other coil. When thisoccurs, a distorted (unsymmetrical) signal results if adifferential probe is used. In addition, the common cablelead carries the unbalanced current, resulting in cablenoise, especially if the cable is not properly shielded andgrounded.

In the Figure 4.3 circuit, the output voltage for large(>10%) off-null (off-balance) conditions is a nonlinear func-tion of the change in coil impedance. However, for defectdetection, close to balance, this discrepancy is small.

4.2.3 Bridge Circuit in Crack Detectors

Portable eddy current instruments are often used to inspectfor surface defects. A typical crack detector circuit isshown in Figure 4.4. An oscillator supplies an alternatingcurrent to an AC Bridge, containing a single eddy currentprobe coil as one arm of the bridge. A capacitor is connec-ted in parallel with the coil so the L-C (inductance-capacitance) circuit is near resonance. When the coil isplaced on a test sample, the bridge is unbalanced aid thepointer on the meter swings off-scale. The bridge can bebalanced by adjusting potentiometer R^.

Fig. 4.4: Simplified Circuit of Crack Detector

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4.3 RESONANCE CIRCUIT ASP EQUATIONS

Probe-cable resonance must be considered when operating athigh test frequencies and/or using long probe cables. Inaddition, crack detectors are purposely designed to operateclose to resonance. This section contains basic informationabout resonant (tuned) circuits.

If a capacitor is connected in parallel with the test coil(inductor), there is a unique frequency at which theinductance-capacitance (L-C) circuit resonates. At thisfrequency the circuit is said to be tuned. Undet thiscondition the output voltage, for a given measurement, ismaximum. A capacitor in parallel with the eddy current probeconverts the circuit of Figure 3.3(c) to that of Figure 4.5.

Fig. 4.5; Parallel LC Circuit

-39-

At resonance,

—2 2 ( 4 < 2 >

R +<x - x yp x chence Z • » when R « 0

If resistance, R, is negligible compared to Xp and Xcresonance occurs when

X « X or IUL • 1/toC (4.3a)P c

or u - l/v'Tc (4.3b)

Since h) " 2irf, resonant frequency i s

f - — i — (4.4a)27T/LC

where L is coil inductance in henries and C is cablecapacitance in farads.

When resistance, R, is significant,

(4.4b)

Xwhere Q » -£• , quality factor.

The resonant frequency of a practical parallel resonant circuit(R t 0) is the frequency at which the reactive power of theinductance' and capacitance are equal, or the total impedanceappears as pure resistance.

-40-

4.4 EDDY CURRENT INSTRUMENTS

General instrument functions were described using the blockdiagram of Figure 4.1. In this section specific instrumentsare covered. It answers the questions: What is the testfrequency? How is lift-off compensated for? How isbalancing achieved? What type of outputs do they have?

4.4.1 General Purpose Instrument (Impedance Method)

Figure 4.6 shows a typical eddy current instrument withvarious control functions. FREQUENCY control sets thedesired test frequency. Frequency is selected by continuouscontrol or in discrete steps from about 1 kHz to 2 MHz. Thecoils' impedances are normally balanced using an AC bridgecircuit. These bridges require two coils on adjacent bridgearms such as arms No. 2 and No. 4 in Figure 4.3. Coilimpedance must be compatible with internal bridge impedance.

FERRITE

CARBON STEEL.

'HONEL

S.S. TYPE 304

LEAD

BRASSALUMINIUM

COPPER STORAGE MONITOR

BALANCING

0

0

a0n

g 4.6; Typical Eddy Current Instrument WithStorage Monitor

Most bridges can tolerate a coil impedance between 10 and 200ohms. The BALANCING controls, labelled X and R in someinstruments, are potentiometers R^ and R£ in Figure 4.3.They match coil impedance to achieve a null when the probe isin a defect free location on the test sample. Someinstruments have automatic balancing.

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The bridge output signal amplitude is controlled by the GAINcontrol. In some instruments it is labelled as SENSITIVITY.It controls the amplifier of the bridge output signal, shownin Figure 4.1,and therefore does not affect current goingthrough the probe. However, some instruments controlamplification by varying current through the coils. This isundesirable because it could cause coil heating, and whentesting ferromagnetic materials the magnetization levelchanges, resulting in signal distortion and non-repeatablesignals.

Following amplification of the bridge unbalance signal, thesignal is converted to direct current signals. Since the ACsignal has both amplitude and phase it Is converted intoQUADRATURE X and Y components. The quadrature components ofthe bridge output are generated by sampling the sinusoidalsignal at two positions 90° apart (one-quarter wave) on thewaveform (or by using electronic multipliers). The DCvoltage values (amplitudes) constitute the X and Y quadraturecomponents. If phase is taken relative to the resistivevoltage component, then the X quadrature component is R^(or VR) and the Y component, XL (or V^i)>in equation3.12(b) or Figure 3.4. We now have an efficient way ofanalyzing bridge unbalance signals.

Eddy current instruments do not have a phase reference. Tocompensate for this, they have a PHASE SHIFT control (phase-discrimination control). Normal impedance diagram orientationwith inductive reactance displayed vertically (+Y) andresistive horizontally (-(-X) can be obtained experimentally.This is achieved by adjusting the PHASE control until thesignal from a probe approaching a ferrite sample (high yandvery high p) displays a vertical (+Y) signal indicating anincrease in probe inductive reactance, see Section 5.5.6 forexamples. PHASE control can also be used to minimize theeffect of extraneous signals such; as lift-off. The X-Ysignal pattern is rotated until Che lift-off signal ishorizontal (X). Thus any vertical (Y) channel signalindicates defects, thickness variations, etc., with littleeffect from probe wobble.

The output signal is normally filtered internally to decreaseinstrument or system noise. This decreases frequencyresponse of the instrument and reduces the maximum inspectionspeed; at faster inspection speeds signal distortion results.Instruments can have a frequency response of 30 to 1000 Hz,although 100 to 300 Hz is most common. At 300 Hz, themaximum attainable tube inspection speed, to detect an abruptdefect without signal distortion, is about 0.25 m/s.

Signals are commonly displayed on X-Y storage monitors withthe X component on the horizontal axis and the Y component onthe vertical axis. The writing speed or frequency responseis greater than 1 kHz for a storage CRT.

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Analysis of recorded signals is normally done visually. Thestorage monitor display in Figure 4.6 shows the change incoil impedance as a surface probe was placed on various testsamples illustrating the effects of resistivity, permeabilityand lift-off.

In the "impedance" method of eddy current testing, the flowof eddy currents is monitored by observing the effect oftheir associated electromagnetic fields on the electricalimpedance of the inspection coil(s). This impedance includescoil wire and cable resistance,

" V(BLCoil wire and cable resistance increase linearly withtemperature according to

R - RO(1-K*AT)

where a is temperature coefficient of resistance

and AT is change in temperature.

If the probe and/or cable experience a change in temperatureduring inspection, the output signal from the eddy currentinstrument changes; this is normally referred to as temperaturedrift.

4.4.2 Crack Detectors

A typical crack detector circuit was shown in Figure 4.4.Crack detector probes contain only one coil, with a fixedvalue capacitor in parallel with the coil to form a resonantcircuit. At this condition the output voltage, for a givenchange in coll impedance, is maximum. The coil's inductivereactance, X^> must be close to the capacitive reactance,Xc. In most crack detectors this is in the range of 20 to100 ohms.

Crack detectors that operate at or close to resonance do nothave selectable test frequencies. Crack detectors fornon-ferromagnetic, high electrical resistivity materials suchas Type 304 stainless steel typically operate between 1 and 3MHz; those for low resistivity materials (aluminum alloys,brasses) operate at lower frequency, normally in the 10 to100 kHz range. Some crack detectors for high resistivitymaterials can also be used to inspect ferromagneticmaterials, such as carbon steel, for surface defects.Normally a different probe is required; however, coilimpedance and test frequency change very little.

- 43 -

PROBE WITH L I F T - 0 F F = 0 . 1 mm

PRO8E WITH L I F T - O F F = 0 mm

SAMPLE WlTH DEFECT

0 . 8 0 . 9 1 . 0 1 . 1 1 . 2

OSCILLATOR FREQUENCY. _Lf

g. 4.7; Meter Output with Varying Oscillator Frequency

Crack detectors have a meter output and three basic controls:balance, lift-off, and sensitivity. BALANCING control isperformed by adjusting the potentiometer on the adjacentbridge arm, until bridge output is zero (or close to zero).GAIN control (sensitivity) adjustment occurs at the bridgeoutput. The signal is then rectified and displayed on aMETER. Because the signal is filtered, in addition to themechanical inertia of the pointer, the frequency response of ameter is very low (less than 10 Hz). LIFT-OFF CONTROL adjuststhe test frequency (by less than 25%) to operate slightly offresonance. In crack detectors the test frequency is chosen tominimize the effect of probe wobble (lift-off), not to changethe skin depth or phase lag. The set-up to compensate forprobe wobble can be described with the help of Figure 4.7.Frequency is adjusted by trial-and-error to obtain the sameoutput signal on the meter with the probe touching the sampleand at some specified lift-off (normally 0.1 mm). At thisfrequency a deap surface defect will give a different readingon the meter, as shown in Figure 4.7.

However,the meter output is a complex function of signal phaseand amplitude, and cannot be used to reliably measure depth ofreal defects. Nor can they be used to distinguish betweenreal and false indications such as ferromagnetic inclusions.

-44-

4.4.3 Material Sorting and Conductivity Instruments

Material sorting,or conductivity instruments,have aprecalibrated meter output and have a unique way ofcompensating for lift-off.. Instruments for sorting of highresistivity materials (Type 304 stainless steel) use a fixed,high test frequency, normally between 200 and 500 kHz,and thosefor low resistivity materials (aluminum alloys),a low testfrequency, between 20 and 100 kHz. They incorporate AC bridgesand normally have two coils (one as reference). Coll impedanceis in the range of 20 to 100 ohms. They either have bridgebalancing or a zeroing control, to keep the signal on scale.GAIN CONTROL or sensitivity adjustment occurs at the bridgeoutput. The signal is then rectified and displayed on a METER.

LIFT-OFF compensation is normally pre-aet. Figure 4.8explains how the probe-wobble (lift-off) Bignal iseliminated. The bridge is purposely unbalanced (by pre-setinternal adjustment)* such that the unbalance point, P, is atthe centre of curvature of the lift-off impedance locus, AB.The instrument meter reads a voltage proportional to thedistance, FB1 or PA1, from the chosen unbalance point to theimpedance curves. The amplitude of this voltage remainsconstant with probe wobble but changes significantly for wallthickness (and resistivity) variations. In fact any signalthat traces an impedance locus different from lift-off willchange meter output.

PRESET UNBALANCE

Fig. 4.8: Unbalanced Bridge Method Showing Selectionof Operating Point

*This is achieved by subtracting a signal equal to OP from thesignal OA.

-45-

With this type of instrument only the magnitude of theimpedance change is measured. This instrument is effectivefor conductivity and wall thickness measurement (and deepdefects) and is simple to operate. It has only two basiccontrols: balance and sensitivity.

4.5 SEND-RECEIVE EDDY CURRENT SYSTEMS

The "send-receive" eddy current method eliminates thetemperature drift sensed by general purpose instruments.The flow of eddy currents is monitored by observing theeffect of their associated electromagnetic fields on thevoltage induced in an independent coil(s), Figure 4.9. Theexcitation or primary coil is driven with a sinusoidalcurrent with constant peak-to-peak amplitude to obtain aconstant magnetomotive force,

s i n (2.3)

RECEIVE CO ILS

TEST ARTICLE

Fig. 4.9: Send-Receive Circuit

-46-

Thia makes the excitation magnetic flux $ independent ofprimary coil resistance. The secondary or receive coil(s) isconnected to a high input impedance amplifier, hence theinduced voltage V9 is not affected by receive coil resistance.

d*** N CO8 (lit (2.5)

The wire resistance of both the excitation and receive coilscan change, because of temperature, without affecting theoutput signals; temperature drift has thus been eliminated*Temperature independence makes this method useful formeasuring resistivity, wall thickness and spacing betweencomponents. It has no significant advantage over theimpedance method for defect detection, except in thethrough-wall transmission system discussed in Section 5.4.

4.5.1 Hall-Effect Detector

Most send-receive circuits consist of one excitation (ordriver) coil and one or more receive (or pick-up) coils.

However, the induced magnetic flux 4>s can be measuredwith a Hall-effect detector rather than by monitoring theinduced voltage VB across a pick-up coil, see Figures 2.2band 2.2c.

Fig. 4.10; Hall Detector Circuit

-47-

The induced voltage in a pick-up coil is proportional to thetime rate of change of the magnetic flux and therefore isproportional to the test frequency,

VPick-uP " f

The Hall detector instead responds to the instantaneousmagnitude of the magnetic flux, <j>0.

This means the output voltage is independent of testfrequency, making it useful for low frequency inspection(especially if the detector has to be small).

The Hall detector works as follows: When direct current ispassed through a Hall element, voltage (electric potential)is produced, perpendicular to current flow, see Figure 4.10.This voltage is proportional to the component of magneticflux perpendicular to the element and the element surfacearea. This voltage is NOT from a change in elementresistance. Hall elements as small as 1 mm square arecommercially available.

4.5.2 Send-Receive Coils and Lift-Off Compensation

General purpose "send-receive" instruments are similar to"impedance" instruments, as described in Section 4.4.1. Themain difference is the method of balancing because of thedifferent coil configuration. Most send-receive circuitsconsist of one excitation coil and two receive coilspositioned symmetrically inside or outside the excitationcoil. They can either be differential where both coils sensethe test specimen or absolute where only one coil senses thetest specimen, as shown in Figure 4.9. Although coilimpedance is not important in send-receive instruments, theinduced voltage is a function of number of windings and testfrequency. Therefore their inductive reactance tend to besimilar to coils used in impedance instruments.

The sensing coils are wound in opposition so the excitationfield induces no net voltage in the receive coils when theyboth sense the same material. In the presence of a defect,the voltage changes as each coil moves over it. Figure 4.9illustrates a surface reflection type probe where bothexcitation and pick-up coils are on the same side of the testsample. However, the excitation coil and pick-up coils canbe placed on opposite sides of the sample; this method isreferred to as through-wall transmission. The two methodsare compared in Section 5.4.

The output signals in most send-receive instruments are thequadrature components of the secondary voltage. However, insome special purpose instruments, one output signal isproportional to amplitude and the other to phase of thesecondary voltage (relative to primary voltage). They

-48-

compensate for LIFT-OFF as follows: if coil-to-sample spacingvaries there is a large change in amplitude of the secondaryvoltage but little change in phase. The phase shift betweenthe secondary and primary sinusoidal voltages Is measured ata voltage level Vo slightly larger than zero, Figure 4.11.At this voltage the sinusoidal voltages have the same phaseshift for aero lift-off as for maximum (perhaps 0.1 mm)lift-off. The voltage discriminator in these phase-shiftmeasuring eddy current instruments trigger on the Vo

voltage pointfand therefore»the output signal for lift-offbetween 0 and 0.1 mm is minimized. Measurement ofresistivity, wall thickness or deep defects can be madewithout lift-off noise.

V ( t )PROBE SIGNAL, L I F T - O F F = O

PROBE SIGNAL, L I FT - O F F = 0 . i mm

PROBE SIGNAL, DEFECT IN TEST ARTICLE

i

1

FIR. 4.11; Secondary Voltage Waveform forVarious Test Conditions

4.6 MDLTIFREQUENCY EQUIPMENT-

The eddy current NDT method is sensitive to many testparameters, making It very versatile. However, one isusually only interested in a single parameter such asdefects. Insignificant parameters such as changes inelectrical or magnetic properties, the presence of dents orsupport plates in tube inspection and lift-off in surfaceprobe inspection can mask defect signals. The multifrequencyeddy current method was developed to eliminate the effect ofundesirable parameters.

-49-

The response to various anomalies changes with testfrequency. This allows a means of discriminating againstunimportant changes. In multifrequency instruments, two ormore frequencies are used simultaneously (through the samecoil(s)). Coil current consists of two or more superimposedfrequencies, i.e., the coil(s) is excited with more than onetest frequency simultaneously. A three-frequencymultifrequency instrument acts the same way as three separate(single-frequency) eddy current instruments. Band-passfilters separate the signals at each frequency. Thediscrimination or elimination process is accomplished bycombining the output signals (DC signals) from individualfrequencies in a manner similar to simultaneous solution ofmultiple equations. The elimination of extraneous signals isachieved by matching the signal at two test frequencies andsubtracting. This process is continued for other unwantedsignals using other test frequencies until the final outputonly consists of only the defect signal. A discussion ofinspection results with multi-frequency is covered in Section8.4.

Multifrequency instruments have the same controls andfunctions as general purpose "impedance" type instruments,described in Section 4.4.1, with the addition of mixingmodules. These modules are used to combine or subtract theoutput signals from each combination of frequencies.

4.7 PULSED EDDY CURRENT EQUIPMENT

Faraday's Law states that eddy currents are induced in aconductor by a varying magnetic field. This magnetic fieldcan be generated by passing sinusoidally varying currentthrough a coil. However, the current can be of otherwaveforms such as a train of pulses. This method works onlyon the send-receive principle where the flow of eddy currentsis monitored by observing the effect of their associatedelectromagnetic fields on the induced voltage of the receivecoil(s). The voltage pulse is analyzed by observing itsamplitude with time, Figure 4.12.

To compensate for LIFT-OFF, the voltage is sampled at apreset time, tj. When the waveform is triggered (measured)at time tj_, the voltage for zero lift-off and maximumlift-off is the same, whereas the voltage waveform in thepresence of a defect is different. This method is quitesimilar to the send-receive method described in Section4.5.3. Therefore, by measuring the voltage at theappropriate crossing point, lift-off effect can bedrastically decreased.

-50-

V(t)DEFECT IN TEST ARTICLE

LIFT -OFF = 0.1 mi*

IFig. 4.12: Voltage Across a Pulsed Eddy Current Pick-UpColl as a Function of Time

The pulsed eddy current method offers another advantage. Thepulsed driving current produces an inherently widebandfrequency spectrum, permitting extraction of more selectiveinformation than can be determined from the test specimen bya single frequency method. Unfortunately, there is atpresent no commercially available instrument that operates onthis principle.

4.8 SPECIAL TECHNIQUES

Two old methods used to measure large coil impedancevariations (greater than 5%) are the ELLIPSE and SLITmethods. These methods analyse the AC signal directly on anoscilloscope (without converting it to DC). They were mainlyused for material sorting. They are obsolete methods and adetailed description is not warranted in this manual;a full description is contained in Reference 5.

Another technique, MODULATION ANALYSIS, is also described inReference 5. It works on the same principle as "frequencyspectrum analysis" where a discrete frequency component of awaveform can be analysed without interference from lower orhigher frequency noise. The inspection must be performed atconstant speed (in fact it only works if there is relativemotion between coil and sample). It is used in production-line testing at speeds up to 2 m/s or higher. It is a veryspecialized and complicated method and a detailed descriptionis not warranted in this manual.

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4.9 RECORDING EQUIPMENT

During inspection, eddy current instruments and recordingequipment are typically connected as in Figure 4.13. Theeddy current signal is monitored on a storage CRT (cathoderay tube) and recorded on X-Y and two-channel recorders.Recording on an FM tape recorder for subsequent playback isalso common.

The important characteristic of these recording instrumentsis FREQUENCY RESPONSE, or speed response, which limitsinspection speed. Section 4.4.1 indicated general eddycurrent instruments have a frequency response of 100 t;o 300Hz, limiting the inspection speed to 0.25 m/s. To becompatible, recording instruments must have the same orhigher frequency response.

ioX

X-Y RECORDER

X-YSTORAGEMONITOR

OEDDY CURRENTINSTRUMENT

X? ?Y

6 O

X , Y2-CHANNEL

CHART RECORDER

PROBE

FM TAPERECORDER

Fig. 4.13;Equipment

Block Diagram of Eddy Current Monitoring

-52-

X-Y Recorders

Signal analysis for signal discrimination and defect depthestimation is normally done on X-Y signal patterns. The CRTstorage monitors have a frequency response of at least 1 kHzand therefore do not restrict maximum inspection speed.However, to obtain a permanent visual record of the signal,it must be recorded on X-Y recorders. The fastest recordershave a speed of response of 8 Hz for small signals. Thisdrastically limits inspection speed if used on-line. It istherefore only used in the laboratory or to record playbackfrom tape recorders (this is done by recording at the highesttape speed and playing back at the lowest, a factor of 8:1for most tape recorder). One solution to on-line recordingof X-Y signals is to photograph the CRT display; however,this is not practical for recording many signals.

Another solution is to use storage monitors with hard copy(paper output) capability. These exist commercially butrequire custom-made control units. They have a frequencyresponse of 1 kHz or higher.

Strip Chart Recorders

Recording X and Y signal components against time is useful inlocating defects and determining their length.

Common two channel ink-pen strip chart recorders have a speedresponse of approximately 100 Hz. At maximum inspectionspeed (0.25 m/s) the recorded signal will decrease inamplitude and be slightly distorted.

Ink-ejection strip chart recorders have a speed response of1 kHz. These recorders are not readily available in NorthAmerica and use a lot of paper.

Ultraviolet light recorders have a speed response higher than p1 kHz, but require special paper. These recorders are rarely Jiused in eddy current testing.

FM Tape Recorders •

Tape recorders allow storage of eddy current signals (onmagnetic tape) for subsequent retrieval. They have a rffrequency response proportional to recording speed. The ;lowest recording speed is 24 mm/sec (15/16 ips) giving afrequency response of 300 Hz, and the fastest, 380 mm/s (15 •ips), will respond to 4.8 kHz. \

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4.9.1 Frequency Response

Eddy current instruments an^ recording instrumentation havelimited frequency response. This means they require finitetime to respond to an input signal. Frequency response,sometimes called speed of response, is defined as thefrequency at which the output signal falls to 0.707 (-3 dB)of the maximum input signal.

A test coil with an effective sensing width w passing overa localized defect at a speed s will sense the point defectfor a duration of w/s seconds. This signal is approximatelyequal to one wavelength with a frequency

f - s/w hertz (4.6)

where 3 is speed in mm/s and w is width in mm.

For example, at a probe speed of 0.5 m/s and probe sensingwidth of 2 mm, f » 250 hertz. If the instrumentation has afrequency response of 250 hertz, the output signal is reducedto 0.707 the input signal and the X-Y signal is distorted.If the instrumentation frequency response is 500 hertz, theoutput signal decreases only slightly. For this example, theeddy current instrument should have a frequency responseequal to or greater than 500 hertz to obtain undistortedsignals. Or inversely, if the instrument frequency responseis only 250 hertz, the maximum inspection speed should bereduced to 0.25 m/s

4.10 SUMMARY

Basic eddy current equipment consists of an alternatingcurrent source (oscillator), voltmeter and probe. When theprobe is brought close to a conductor or moved past adefect, the voltage across the coil changes and this is readoff the voltmeter. The oscillator sets the test frequencyand the probe governs coupling and sensitivity to defects.

For effective purchase or use of an eddy current instrument,the following information is needed:(a) type of instrument: impedance, send-receive, crack

detector, etc.(b) type of outputs: single (meter) or quadrature (X-Y)

component outputs(c) test frequency(d) type of lift-off compensation.

Most eddy current instruments use an AC bridge for balancingbut use various methods for lift-off compensation.Send-receive instrument should be used for accurate absolutemeasurements in the presence of temperature fluctuations.Multifrequency instruments can be used to simplify defectsignals in the presence of extraneous signals.

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4.11

Eddy current instruments and recording equipment have afinite frequency response limiting the Inspection speed tonormally 0.25 m/s.

Moat Instruments tolerate probe impedance between 10 and 200ohms.

Crack detectors operate close to coil-cable resonance. Theresonant test frequency Is given by

fr - l/2ir/LC (4.4a)

where L is coll Inductance in henries and C is cablecapacitance in farads. The lift-off signal is minimized byadjusting the frequency (slightly off resonance) until zero anda small probe lift-off gives zero output signal. High testfrequencies are normally used to inspect for shallow defects inhigh resistivity or ferromagnetic materials. Low testfrequencies are used for detecting deep defects or inspectinggood conductors. Crack detectors have a meter output, andcannot be used to reliably measure defect depth.

WORKED EXAMPLES

4.11.1 Impedance at Resonance

10-6PROBLEM: In a parallel L-C circuit, inductance is 80 ?henries, capacitance is 5 x 10~9 farads andresistance is negligible. Calculate (a) resonantfrequency, (b) inductive reactance and (c) capacitlvereactance.

SOLUTION:

(a) f (4.4a)

2TT ̂ (80 x 10~6) (5 x 10~9)

252 kHz

(b) Inductive Reactance, - 2irfL

2TT X 2 5 2 x 103 x 80 x 10"6 - 126.5 ohms

(3.4b)

(c) Capacitive Reactance,

1X C " 2ir x 252 x 1 0 3 x 5 x 10"9

l/2irfC

» 126 .5 ohms

( 3 .5 )

-55-

CHAPTER 5 - TESTING WITH SURFACE PROBES

5.1 INTRODUCTION

The goal of this chapter Is to present a practical approachto eddy current Inspections using surface probes. Theemphasis is on test variables such as test frequency, probesize and type; these are normally the only variables aninspector has at his control. These selections are usuallydetermined by skin depth considerations, defect size, andprobe size.

Impedance graphs and the Characteristic Parameter areincluded because the/ are tools that an inspector should notbe without. A thorough understanding of impedance graphs isessential to manipulate test conditions to minimize and/or to•cope with undesirable test variables. Erroneous conclusionsare often made by persons who do not have a working knowledgeof impedance graphs.

The scope of the approach to an eddy current inspection canbe very broad; a successful outcome usually depends on theproper approach.. When planning an inspection the firstquestions that must be answered before proceeding are; Forwhat type of defects is the inspection being conducted? Ifthe expected defects are cracks, how big are they? Do theyhave directional properties? What is the minimum acceptabledefect size? Does the material have ferromagneticproperties? Other variables will, of course, influence thetest but these questions must be answered in order to selectan appropriate probe size and test frequency.

5.2 SURFACE PROBES

The eddy current probe plays two important roles: it induceseddy currents, and senses the distortion of their flow causedby defects. Sensitivity to defects and other variables inthe test article can be affected by probe design. This isachieved by controlling direction of eddy current flow, bycontrolling the coil's magnetic field, and by selecting anappropriate coil size. The effects of undesirable materialvariations and/or variations in probe to test articlecoupling (lift-off) can often be decreased by using multiplecoils.

A surface probe, as the name implies, is used for inspectingsurfaces, flat or contoured, for defects or materialproperties. Defects can be either surface or subsurface.(Surface defects are those that bre-\k through, or originateat the surface - typically cracks, voids, or inclusions: asubsurface defect does not break the surface and is thereforenot visible).

Other names used for variations of surface probe designs arepancake probe, flat probe, spring probe or coil, spinningprobe, and pencil probe.

-56-

5.2.1 Probe Types

Simple Probes

Surface probe designs can vary from a simple, single coilattached to lead wires, to complex arrangements, as shown inFigure 5.1. Most eddy current instruments require two

ZIRCONIUM ALLOV

Fig. 5.1: Surface Probes

slmilai coils to satisfy their AC bridge network as discussedin Chapter 4. If only one coil senses the test material,it is an absolute probe; if both coils sense the testmaterial, it is a differential probe. The simple probe inFigure 5.1(a) is therefore undesirable because a second coilor electrical device with similar impedance will be necessaryfor bridge nulling. An exception would be in the use ofCrack Detectors; these instruments operate with an internalbalancing circuit (see Section 4.2.3).

A better arrangement is shown in the pencil probe of Figure5.1(b). This probe incorporates a second coil (reference)mounted far enough from the test article that it will not beinfluenced by it. The tvo colls have the same impedance whenthe probe is balanced In air, but will change relative toeach other when the test coil is coupled to a sample.However, the degree of coupling is usually small because ofthe inherent small size of pencil probes so the coils stillmatch well enough for most instruments over a reasonablefrequency range. The probe shown has ferrite cores; ferriteis used for three reasons:

-57-

1. higher inductance from a given coil size,2. small surface area in contact with the material,3. the coil can be further from the contact surface

providing greater wear protection.

A further improvement in reference coil arrangement is shownin Figure 5.1(c); it is attached to a disc whose propertiesare similar to the test material. With this arrangement therelative impedance of the two coils will not be affected bytest frequency.

The probe shown in Figure 5.1(d) is a spring loaded typedesigned to minimize lift-off. The shoe provides a broadarea for squarely positioning the probe on a flat surface,while the spring maintains probe contact at constant force.

Figure 5.1(e) shows a probe used for inspecting largediameter tubing. The probe can be rotated and/or movedaxially. The design shown incorporates a replaceable wearcap.

Other Probe Designs

A multi-coil array as shown in Figure 5.2(a) is useful forinspecting tubes. This type of probe could detect defects

TEST TUBESURFACE COILS

TORROIDAL REFERENCE COIL

PROBE CENTERING DISCS

TEST COILS

(a)

MUUI SURFACE -COIL PROBE

U)

DIFFERENTIAL SURFACE PROBE

. FERROMAGNETIC

CORE

•COILS

FIELD

(b)

GAP PROBE

SENSING CDII

(H)

LIFT-OFF COMPENSATING PROBE

Fig. 5.2: Special Surface Probes

-58-

that would not be detected by a conventional circumferentialcoil (discussed in Section 7.5).

A gap probe, Figure 5.2(b), uses ferromagnetic material toshape the magnetic field. The field is confined by the corecausing eddy currents to flow in circular loopsperpendicular to the flux lines.

A differential configuration is shown in Figure 5.2(c); thetwo coils are placed side-by-side. Both coils have highsensitivity to localized variations but tend to cancel outthe effect of lift-off, gradual material variations, orambient temperature changes.

A lift-off compensating probe is shown in Figure 5.2(d); thisprobe combines the signals from two coils to effectivelyrotate the defect signal relative to the lift-off signal.Therefore, even on "rough" surfaces, shallow defects can bedetected.

SEND .COIL \

(DRIVER COIL! \ I(a)

TEST ARTICLE

RECEIVER COIL

PICK-UP COILS(WOUND OPPOSINGEACH OTHER)

ELECTRICAL CONNECTIONS

(c)

Fig. 5.3: Send-Receive Probes

-59-

Send-Recelve Probes

Figure 5.3(a) shows a through-transmission probe arrangement.Current flowing in the SEND coil produces a magnetic field,part of which is transmitted through the test article. Thefield is detected by the RECEIVER coil, inducing a voltage.There will be no signal variation from the receiver coil whena defect-free test article is moved anywhere between the twocoils as long as the coil-to-coil spacing remains constant.

Figure 5.3(b) shows a reflection-type probe arrangement. Theprobe consists of a large send coil which generates a field,and two small receiver coils wound In opposite directions,as mirror images to one another, as shown in Figure 5.3(c).With the probe in air, net output is zero. However, If oneend is placed near a test article, the field differs at thetwo ends, and a net voltage appears across the two coils.

5.2.2 Directional Properties

Eddy currents are closed loops of induced current circulatingin a plane perpendicular to the direction of magnetic flux.Their normal direction of travel is parallel to the coilwinding and parallel to the surface. See Figure 5.4.Pancake type surface probes are therefore insensitive to poorbonding of coatings and flaws parallel to the surface of asample.

SURFACE CRACK EDDY CURRENTSLAMINAR CRACK

TEST PLATE

EDDY CURRENT FLOWS PARALLEL TO COIL WINDINGS- POOR SENSITIVITY TO LAMINATIONS

SURFACE CRACKIN PLATE

ZERO SENSITIVITY LOW SENSITIVITY MAXIMUM SENSITIVITYAT CENTRE OF COIL PARALLEL TO WINDINGS ACROSS WINDINGS

Fig. 5.4: Directional Properties of a Surface Probe

-60-

When testing for flaws such as cracks, It Is essential thatthe eddy current flow be at a large angle (preferablyperpendicular) to the crack to obtain maximum response. Ifeddy current flow is parallel to the defect there will belittle or no disruption of currents and hence no coilimpedance change*

When testing for flaws parallel to the surface, such aslaminations, a horseshoe shaped probe (a gap probe with avery large gap) has reasonable sensitivity.

5.2.2.1 Sensitivity at Centre of a Coil

Probe impedance changes with coil diameter, as will bediscussed further in Section 5.5. A simplified derivation ofthis diameter effect is derived below, for the case of noskin depth attenuation or phase lag and long coils. FromFaraday's Law,

*.-••»

The magnetic flux density, B, is approximately constantacross a coil's diameter, hence

(j> » BA

- (B)(irr2)

where r is radial distance from centre of probe;

therefore,

ora r

Ac

N(turns)

-61-

Reslstance to flow of current Is proportional to flow pathlength and resistivity and inversely proportional to cross-sectional area, Ac,

Rs

2urp

or

Since

and Z

and no

-V.skin

therefore,

or

s ince

Rs

s

CC Y

- V /ZS

1 + (U)L)2 - Rn ,

depth

s

Is

*s

effect,

R~ " ~

« r

= - Is

unit depth x unit width

by Ohm's Law

at low test frequency

from Lens's Law, it follows

that

Therefore, eddy current flow and its associated magnetic fluxare proportional to radial distance from the centre of acoil. Hence no current flows in the centre (r » 0) and thereis no sensitivity to defects at the centre of a coil.

5.2.3 Probe Inductance

The factor governing coupling and induced voltage in testmaterial is the magnetic flux surrounding the coil. Thetotal magnetic flux (<(>„) is proportional to probeinductance (L) and current (I), i.e., 4>poc LI. In mosteddy current instruments excitation current is kept Reasonablyconstant (in the milliampere range) but probe inductance couldvary by a factor of one thousand. The most important aspect ofinductance is that probe impedance, which is a function ofinductance, must be compatible with the instrument and signalcable,

and 6 Arctan -~K

where X 2 TfL when f is in hertz, L in henries and R iscoil wire resistance in ohms.

- 62 -

TABLE 5.1 SURFACE COIL IMPEDANCE

N -

N -

N -

N «

N »

21

50

98

136

200

Do

LR

40(0

L >R <

43(0

L >R >

46(0

L •

- 1.

• 0.- 0.

AWG.080

• 1.• 1

AWG.056

• 5.

• 4

AWG040

• 11R - 9

48(0.

L •R -

49(0.

AWG031

' 24• 1 7

AWG028

6 mm

27 yH2 ft

mm)

5

mm)

8

mm)

mm)

mm)

LR

34(0

LR

37(0

LR

40(0

LR

41(0

LR

43(0

» 3.2 mm

- 0.54 yH- 0.1ft

AWG.16 mm)

- 3,0- 0.5

AWG.11 mm)

- 12- 2

AWG.080 mm)

- 23- 3

AWG.071 mm)

- 49- 8

AWG.056 mm)

LR

28(0

LR

31(0

LR

34(0

LR

36(0

LR

37(0

- 6.3 mm

- l.i yH- 0.05 ft

AWG.32 mm)

= 6.1- 0.3

AWG.23 mm)

- 23- 1

AWG.16 mm)

« 45- 2

AWG.13 mm)

- 97- 4

AWG.11 mm)

LR

22(0

LR

25(0

LR

28(0

LR

29(0

LR

31

•12.7 mm

- 2,,lyH- 0.02ft

AWG.64 mm)

- 12* 0.1

AWG.45 mm)

- 47- 0.5

AWG.32 mm)

- 90- 0.9

AWG.29 mm)

- 195- 2

AWG.23 mm)

D >=0

LR

16(1

L >R >

19(0

L >R >

22(0

L »R >

23(0

L »R •

25(0.

25.4 .mm

- 4.3 yH- 0.01 ft

AWG. 3 mm)

- 24• 0.06

AWG.91 mm)

• 94« 0.3

AWG.6 4 mm)

• 180• 0.5

AWG57 mm)

• 3 9 0

• 1

AWG45 mm)

I

i PA .

•f = Dj =0.2 D o

-63-

The self-inductance of a long coil (solenoid) can becalculated from the equation

LQ -= 4ir x 1 0 " 1 0 y r N2A/Jt (5 .1a)

LQ is self-inductance in henrieswhere Hr is relative permeability of core (normally -1.0)

A is coil's planar surface area, millimetres2

& is coil length, millimetres.

This formula is a good approximation for coils oflength/diameter ratio greater than 10.

For a short coil, end effects will reduce inductance becauseof lower flux at coil ends. The N2 term remains since Nenters in N $_ (total number of flux linkages) and again since<(>_ itself is proportional to N. The following approximateequation can be used to calculate inductance of short coils:

L - 4 m r N 2Un ~ - 2) 10"10 (5.1b)Ox, JAI

D + Dwhere r is mean coil radius • ;—— , mm

and K • 0.112 (2£ + DQ + D ), mm

Most eddy current instruments will operate over a fairlybroad range of probe impedance (and probe inductance) withoutsubstantial reduction in signal-to-noise ratio and signalamplitude. An instrument input impedance of 100 ohms istypical, although any impedance between 20 and 200 ohms isgenerally acceptable, unless test frequency is too close toprobe-cable resonance; see Section 5.9. Exact probeinductance calculations are therefore not essential. Tofacilitate impedance calculations, Table S.I has beenprepared. This table lists coil inductance and resistance(with probe away from test material) for various outsidediameters and number of coil turns, keeping both the insidediameter and coil length equal to 0.2 times the outsidediameter. Wire diameter is chosen to fill available coilcross-sectional space. Using this table and the knowledgethat Inductance,

L « N2D2 (5.2)

where N is number of turns of wire and I) is average coildiameter, one can usually make a reasonable estimate of wiresize and number of turns required to achieve a particularinductance.

NORMALIZED DEFECT SIGNAL AMPLITUDE

ol-t(DHICO

n>

(D3CO

R)o

A)orter

NORMALIZED DEFECT SIGNAL AMPLITUDE

Vx./Vx=l

: iOS> OS?

II II

G9 CD O

CO—I

ro

CJI

CO

oCO

Ui

CJI

"o

CJI

en

oii

en

y

k

7(•i"

roCJI

LIF

/

^ ;

/ /

/

DEFE2

mm

crp

h

Ic2C

D —|NXn VCn v

; \ ' 7/A

T-OFF ^ — |

/

r

7

1 1\A

-65-

5.3 PARAMETERS AFFECTING SENSITIVITY TO DEFECTS

During eddy current inspection one must be aware of thelimitations of the technique and should take maximumadvantage of its potential. Although sensitivity to deepsurface defects is excellent, sensitivity to deep sub-surfacedefects is very poor. A subsurface defect only 5 mm from thesurface is considered very deep for eddy current testpurposes•

There are two factors that contribute to this limitation.The skin depth effect causes eddy currents to attenuate withdepth depending on the aaterial properties and testfrequency. This effect is normally minor and can becontrolled (within limits) by reducing test frequency. Thepredominant effect (rarely mentioned) is the decrease inmagnetic flux, and consequently eddy current density, withdepth because of the small diameter of most practical probes.One can increase penetration by increasing probe diameter,but as a consequence sensitivity to short defects decreases.One could optimize sensitivity if defect length is known;however,the maximum depth of detectability is still verysmall. Unlike ultrasonic inspection where a defect isdetected many transducer diameters away, eddy current testingis limited to detecting defects at a depth of less than oneprobe diameter. It is this effect of probe diameter thatlimits most volumetric eddy current inspection to materialsless than 5 mm thick. In following subsections, limitationsare discussed and empirical examples presented.

5.3.1 Sensitivity with Lift-Off and Defect Depth

There is a decrease in sensitivity to defects as a coil ismoved away from the surface. This is caused by the decreasein magnetic flux density with distance resulting from finiteprobe diameter. Figure 5.5(a) shows the extent of thisdecrease for three probes of different diameters. Note,forexample, the sensitivity of the smallest probe (5 mmdiameter) decreases a factor of four when moved about 1 mmfrom the surface.

This loss of sensitivity with distance will also apply todefects in a solid, in addition there will be a decrease dueto skin depth attenuation.

Figure 5.5(b) illustrates the decrease in signal amplitudewith subsurface defect depth without skin depth attenuation(solid lines) and with skin depth attenuation (dashedlines). With large skin depths (low test frequency) thedecrease in subsurface defect sensitivity with depth issimilar to the decrease in sensitivity with distance forsurface defects shown in Figure 5.5(a). This impliesmagnetic flux density decreases with distance from the coilin air as in a solid (without skin depth attenuation).

-66-

At a typical test frequency, where one skin depth equalsdefect depth (<5™ 2 mm for the dashed lines In Figure 5.5(b)),a further decrease, by about a factor of three, in signalamplitude at x » 2 mm is attributed to skin depthattenuation. This occurs since at one skin depth eddycurrent density Is 37% of surface eddy current density.

The decrease In defect sensitivity with depth In a finitethickness sample, without skin depth attenuation, isapproximately the same as In ar. infinitely thick sample.However, with skin depth attenuation, defect sensitivitydecreases less rapidly than the dashed lines in Figure5.5(b); the curve would fall somewhere in between the dashedand solid lines.

In general, the decrease in defect sensitivity with depth isdetermined by probe size rather than skin depth attenuation.Since most defects are not much longer than sample thickness,one cannot use probes with coil diameter much larger thansample thickness (because of loss In sensitivity with defectlength, Figure 5.6). Eddy current testing with surface probeis therefore normally limited to thicknesses less than 5 mm.

5.3.2 Effect of Defect Length

Eddy current flow is limited to the area of the inducingmagnetic field which is a function of coil geometry; defectsensitivity is proportional to coil diameter in a surfaceprobe, and to gap width in a horseshoe probe. As a generalrule, probe diameter should be equal to or less than theexpected defect length. The effect of probe diameter anddefect length is shown in Figure 5.6. For example, whendefect length equals probe diameter, the signal amplituderanges from one-third to two-thirds of the amplitude for aninfinitely long crack depending on probe diameter and testfrequency.

The sensing area of a probe is the area under the coil plusan extended area due to the magnetic field spread. Theeffective diameter-, Def% -, of a probe is approximately equal tothe coil diameter, Dc, plus four skin depths,

Deff - »e + " (5.3)

At high frequencies the 4 6 term will be small and thesensing diameter can be assumed to be about equal to coildiameter, but at low test frequencies the magnetic fieldspread can be significant. In this case it is common to useferrlte cups to contain the field. This results in aconcentrated field without affecting depth of penetration.

-67-

s

d

a

itn

UEF

EC1

.[Z

ED

1

100*

76*

50*

25*

1

///_/

/ / / .1

i

/ • '/

/ //

I

DEF

ECT

1 1

. » • • " " "

X " * ^ 100 KHJ

8 I MHz8100 KHz

1 1 1

7 mm PROBE DIMETER

1.3 mm PROBE 01 METER

= 0.36 mm

= 1.16 mm

1 1 1• 10 12 14

EDM NOTCH LENGTH, am

22

Fig. 5.6: Effect of Defect Length

5.4 COMPARISON BETWEEN SURFACE AND THROUGH-WALL INSPECTION

The major limitation of conventional eddy current methods islack of penetration. Figure 5.7(a) illustrates typicalresults obtained with the conventional eddy current method,where the probe is placed on one side only of a 4 mm thick,100 mm diameter tube. Test frequency is 30 kHz and skindepth, 6-1.7 mm. Note Che drastic decrease in signalamplitude and the significant phase rotation of the defectsignals with depth. A defect has to be long and very deepbefore it can be picked up from the opposite side of the tubewall. This decrease in sensitivity with depth is due to bothfinite probe size and skin depth attenuation.

Figure 5.7(b) illustrates typical results obtained withthrough-wall transmission equipment where excitation andreceive coils were located directly opposite each otheracross the wall. The probes were conventional absolutepancake type surface probes. The output signal appears as a'figure 8* because the signal was filtered (differentiated).

-68-

O.D. SURFACEGROOVE

25% 507 75?

AMFMTUOE OF DEFECT SIGNAL. V COUPONED

I.D. SURFACEGROOVE

Siso; 75?.

HOLES. 0.8 "™ blA. 13 ™" LONG

X-Y DISPLAY OF DEFECT SIGNALS

1 VOLT \

1 .D. GROOVE

0.8 *"• DEEP

13 <nm LONG

(a) Conventional Surface Probe Results

25% 50% 75%.O.D. " I.D.

GROOVE HOLES GROOVE

0.8 mm DEEP 0,8 nra DIA 0,8 mm DEEP

AMPLITUDE OF DEFECT SIGNALS, Y COMPONENT X-Y DISPLAY OF DEFECT SIGNALS(F ILTERED)

(b) Through-Wall Transmission Results

Fig. 5.7: Comparing Conventional and Through-WallTransmission Techniques

-69-

The Y-amplitude presentation in Figure 5.7(b) shows defectsignal amplitude does not change significantly with defectdepth. It is important to note the phase of the signals doesnot change with defect depth when using the send-receivemethod as shown in the X-Y display.

5.5 IMPEDANCE GRAPH DISPLAY

Impedance graphs are an indispensable aid in eddy currentinspections. An understanding of these graphs provides anoperator a clear picture of all variables and the ability forappropriate action to minimize effects of adverse conditions.

All information about the test article is transmitted to thetest coil via the magnetic field. The variation of themagnetic flux, <\>, with time induces a voltage, V, across thetest coil which, by Faraday's Law, depends on the magnitudeand rate of change of <j> and on the number of turns in thecoil, N

V = - N | (2.4)

= - Ldl/dt since <f> = LI/N.

The variation in amplitude and phase of this voltage vectorindicates the condition of the test article. The voltagevector can be resolved into the two quadratures, in-phascVQ» and out-of-phase VgQ • Since V = IZ and I is keptapproximately constant, the voltage graph can be replacedwith the impedance graph, as discussed in Section 3.3.

Impedance depends not only on test article variables but alsoon probe parameters. The probe parameters are coil diameter,number of turns, length, and core material. The instrumentparameter that affects impedance is test frequency (sincef « d<j>/dt ). To overcome the necessity of plotting impedancegraphs for each test coil, probe impedance is normalized. Thegraphs can then be used to study the effect of test articlevariations without dependence on probe details.

The inductive reactance component is normalized by dividingby the product of frequency and coil inductance (wL0) whenthe probe is away from test material (in air).

\ _ 0)LX Oiho o

where to is angular frequency, radians/secondL is inductance, henriesLo is inductance of coil in air, henriesX^ is reactance, ohmsXo is reactance of coil in air, ohms

-70-

pX I

1I X

rXI

AIR

IXI

INDUCTIVEREACTANCE

TEST ARTICLE

AIR

TEST ARTICLE

UL

TEST ARTICLE

RESISTANCE

( a ) BEFORE NORMALIZATION (b) AFTER NORMALIZATION

Fig. 5 .8; Coil Impedance Display

The r e s i s t i v e component i s normalized by subtracting c o i l wireand cable res i s tance , R__ , and then dividing by toL0 ,

X

R T " RDC

where R^ is coil resistive load due to eddy currents intest material.

The normalized components X^/XQ and R^/XQ are dimen-sionless and independent of both coil inductance and coilwire and cable resistance. Changes in the normalizedparameters indicate variations in eddy current flow into thetest article only. Figure 5.8 displays probe impedancebefore and after normalization. Changes in the test articleare reflected by a change in impedance point P. Figures 5.9to 5.11 are normalized coll Impedance graphs, produced bycomputer simulation, showing the change in the point P forthe following sample variables: electrical resistivity,permeability, and thickness. Figures 5.12 and 5.13 showeffects of test frequency and coil diameter.

-71-

% KESISTIVITJ (^n-eml

(TITANIUM ALLOY)

!•*'.

TEST FREQUENCY - 50 kHz

THICK PLATE

l.72 (COPPER)

.LIFT-OFF, ••

.L0.0 0.1 0.2 0.3 0.4

NORMALIZED RESISTANCE

FIG. 5,9: IMPEDANCE GRAPH-RESISTIVITY EFFECT

CONSTANT PERMEABILITY, /irCONSTANT RESISTIVITY, f

FREQUENCY - SO kHzLIFT-OFF " °THICK PLATE

0.2 0.4 0.6

NORMALIZED RESISTANCE

FIG. 5 . 1 0 : IMPEDANCE GRAPH-PERMEABILITY EFFECT

FQfff

'.05

FREQUENCY - 50 kHz

0.00.0 0.1 0.2 0.3 0.4

NORMALIZED RESISTANCE

FIG. 5.11: IMPEDANCE GRAPH-THICKNESS EFFECT

FREQUENCY, kHz

RESISTIVITY " S3jift.c«l

f i p • 1

LIFT-OFF " 0

THICK PLATE

0.0 0.1 0.2 0.3 0.4

NORMALIZED RESISTANCE

FIG. 5.12: IMPEDANCE GRAPH-FREQUENCY EFFECT

-72-

5.5.1 Effect of Resistivity

Figure 5.9 shows the effect of electrical resistivity for arange of conducting materials. The impedance point moves upthe curve with increasing resistivity. Impedance points forstep changes in coil to test article spacing between zero andinfinity are also included. Note that a small increase inspacing (lift-off) produces a large impedance change. Thisresults from decreased magnetic flux coupling to the sample.There would be a relatively larger effect on the impedance ofa small coil than on the impedance of a large coil for thesame change in spacing.

5.5.2 Effect of Permeability

Note in Figure 5.10 there is a large impedance increase for asmall increase in permeability. Small permeability changescan obscure other test variables.

5.5.3 Effect of Thickness

Figure 5.11 traces the impedance point path as samplethickness decreases from infinity to zero. As test materialbecomes thinner, causing increased resistance to eddycurrents, the impedance point moves up the curve. This wasalso the case in the resistivity graph, Figure 5.9. Thisimplies that any condition causing an increase in resistanceto flow of eddy currents, cracks, thinning, alloyingelements, temperature, etc., will basically move theimpedance point up the curve towards the probe impedance inair, XL/XO-1.

The impedance curve in Figure 5.11, from the knee down, makesa reversal swirl as the probe moves across a conductor withincreasing thickness. This is due to skin depth and phaselag effects which overshadow all basic movements of theimpedance point.

5.5.4 Effect of Frequency

Figure 5.12 shows the effect of test frequency (an instrumentparameter). As frequency is increased, eddy currents aresampling a thinner layer close to the surface (skin deptheffect, discussed in Chapter 2). When frequency is decreasededdy currents penetrate deeper into the material and theimpedance point moves up the curve.

Towards the upper end of the curve, impedance is mainlycomposed of resistance which has a great dependency ontemperature, both in the test article and in coil wireresistance (although the latter dees not appear on thisnormalized curve). It is therefore desirable, when possible,to operate near the knee of the curve say, 20 to 200 kHz inthis example.

II

-73-5.5.5 Effect of Probe Diameter

Figure 5.13 shows effect of coil diameter (a probeparameter). Note increasing coil diameter moves theimpedance point down the curve, similar to increasingfrequency. When test conditions dictate use of a lowfrequency, the operating point can often be brought down thecurve to the desired knee region by Increasing coil diameter(provided test conditions will permit a large probe).

1,0B,. t

NORMALIZED RESISTANCE

Fig. 5.13: Impedance Graph-Surface Coil Diameter Effect

5.5.6 Comparison of Experimental and Computer Impedance Diagrams

The impedance graphs shown in Figure 5.9 to 5.12, produced bycomputer simulation, can be verified using a standard eddycurrent instrument. Figure 5.14 shows probe response tovarious test variables: resistivity, perneability, lift-off,and test frequency. The solid lines are output voltagetraces generated by varying probe-to-test article spacing(lift-off) from infinity to contact with various conductingsamples, while keeping test frequency constant at 10 kHz, andagain at 100 kHz. The dashed lines, connecting the pointswhen the probe was in contact with the samples, were sketchedin to show the similarity between these graphs and thenormalized impedance graphs in the preceding section. Notethat the points move down the curve with increasingconductivity and also with increased frequency. For example,the operating point for 304 SS moved from the top of theimpedance diagram at 10 kHz to near the knee at 100 kHz.

- 7 4 -

INDUCTIVEREACTANCE

g.1

'" SAMPLE

MONEL 400

Cu

f - 1 0 kHz

LIFT-OFF

I , FERRITE

INDUCTIVEREACTANCE

- I ,

RESISTANCE

IRON

S

HONEL 400

304 SS

JLEAD

J! BRASS

f *1C0 kHz

R

"to

RESISTANCE

(b)

Fig 5.14; Probe Rtipome to Various Teat Parameters atTwo Frequencies

5.6 CHARACTERISTIC PARAMETER

In Section 5.5 impedance graphs were normalised to make testarticle parameters independent of probe properties such asinductance. Another method, proposed by W.E. Deeds, C.V.Dodd and f,o-worl:eri, combines frequency and probe diameterwith test material parameters, to form one characteristicparameter^?.).

f oi (5.4)

where r is mean coil radius(0 is angular frequencyUr is relative magnetic permeability (-1.0 for

nonmagnetic materials)and O is electrical conductivity.

-75-

Uslng this characteristic parameter, one impedance graph canbe plotted to describe four test parameters with Pc as theonly variable.

CJ

ELU

^ CONSTANT -— LIFT-OFF CONSTANT7 = COIL MEAN RADIUSt, = LI FT-OFF/?w = ANGULAR FREQUENCYM = MAGNETIC

PERMEABILITY<r = ELECTRICAL -

CONDUCTIVITY

0 0.05 0.10 0.1S 0.20 0.25 0.30 0.35

NORMALIZED RESISTANCE

Fig. 5.15; Impedance Diagram with CharacteristicParameter, PP

Consider Figure 5.15. The solid lines are generated bystarting with Pc equal to zero and increasing the value toinfinity (while holding coil to test article spacingconstant). The dashed lines are generated by starting withthe coil infinitely far away from the test article andbringing the coll closer until it contacts (while holdingP_ constant). Note the similarity between these curves andthe impedance graphs shown in precedingscale is twice the vertical scale)

sections (the horizontal

The usefulness of the characteristic parameter is that itprovides a modelling parameter. Conditions of similarity aremet when

Vrl°lor

rl r2

Test 1 Test 2

-76 -

PROBE

STORAGEOSCILLOSCOPE

D I S P U Y

NOMENCLATURE

VOLTAGE

CURRENT

u - ANGULAR FREQUENCY(«= 2»f)

PROBE INDUCTANCEIN AIR

RBC • PROBE WIRE I CABLEDC RESISTANCE

'SFECIMEN AC RESISTANCE

SUBSCRIPTS':T • TOTALi. - INDUCTANCER • RESISTANCEP -PRIMARYs - SECONDARY

IIIIIIIIIIIfI[f

V,

Fig. 5.16: Coil Impedance/Voltage Display

-77-

Test conditions with the same Pc value have the sameoperating point on the normalized impedance graph. If, forexample, test article resistivity measurements were required(for checking consistency of alloying elements for instance),the best accuracy would be achieved by operating near theknee of the curve where there is good discrimination againstlift-off. (Equation 5.4 does not include skin depth effects,which may be an overriding consideration).

To operate at the knee position in Figure 5.15 a probediameter and frequency combination are selected such thatP c£10. The value of Pc in equation 5.4 is given in SIunits; we can use the following version using more familiarunits.

Pc = 7.9 x 10~4 7 2 f/p (5.5)

where "r is the mean radius, mmf is frequency, Hzp is electrical resistivity, microhm-centimetre

( p r • 1 for nonferromagnetlc material)

It should be noted that the characteristic parameter, Pc,must be used in conjunction with Figure 5.15 (obtainedanalytically); it cannot be used to obtain Figure 5.15.

5.7 DEFINITION OF "PHASE" TERMINOLOGY

This section attempts to clarify the concept of phase. thevoltage/impedance graphs, presented in Section 5.5, are usedas a link between impedance diagrams and the display on aneddy current instrument monitor.

In eddy current work the most confusing and often incorrectlyused term is PHASE. Part of the problem arises because ofthe existence of two eddy current methods, coil impedance andsend-receive. In this section an attempt is made to clarifysome of the multiple uses of t;e word.

Figure 5.16 shows the impedance of a probe touching testmaterial. The two axes represent the quadrature components,V^ and Vg, of voltage across a coil. In the absence ofreal numbers, the axes can also be considered as thenormalized parameters wL/ioL and RL/ U)L0.

The following list summarizes uses of the term PHASE. One ormore of these are often used without adequate explanationbecause the term will have a colloquial meaning.

•-78- I

and resistive voltage vector. •

1. 0^, 0^ » Arctan ^— , angle between total voltage vector

NOTE: An impedance bridge measures amplitude of Kthe impedance vector Z and the angle 0 , where the £resistance includes RJJC . This vector could thereforenot be shown on Figure 5.16. (It is shown on the _-impedance diagram in Figure 5.8(a)). •

2. A©. f Change in phase of normalized resultant voltage vector ™as probe is moved over a defect.

3. 0,, Phase between secondary voltage (induced voltage) and •primary voltage (excitation voltage). Send-recelve ginstruments measure secondary voltage.

4. A@2> Change in phase of secondary voltage as probe is moved _over a r'efect. This is approximately the phase Imeasured by some send-receive eddy current instruments Hwithout X-Y outputs.

5. 0 , Phase between the voltage signals obtained from BLIFT-OFF and a crack or void. It is related to PHASE gjLAG '3 • explained below. (0^ 1 B about double the phaselag.) m

03 is used to estimate defect depth during ET. •6. g, PHASE LAG (not shown in Figure 5.16) of eddy currents II

below the surface relative to those at the surface. Itwas derived in the eddy current density equation Chapter m2, i.e. g • x/6for semi-infinite plates, where x is the Ifdistance below the surface and B is in radians.

7. 0,, Many eddy current instruments have a PHASE knob by whichthe entire impedance voltage plane display can be I;rotated. It is coataon practice to rotate the display I:to meke LIFT-OFF horizontal. (On an eddy currentinstrument display, absolute orientation of inductiveand resistive axes may be unknown). I!

8.0_, Phase between inductive voltage and current in acircuit; 0 - 90° •

I;5.8 SELECTION OF TEST FREQUENCY I!

5.8.1 Inspecting for Defects a?

The first question that must be answered before proceedingwith an inspection is: For what type of defects is theinspection being done? If the defects are cracks: What is fthe smallest defect that must be detected? Are the cracks Isurface or subsurface? Are they likely to be laminar cracksor normal to the test surface? A single general inspection r-procedure to verify the absence of any and all types of defects ||often has little merit. Inspections often require two or moreteit fraquencias and/or different probes to accurately identifydafacta. pTaat frequency can ba aalactad without knowledge of thecharacteristic parameter, P c, or tha oparatlnf point on tha r •impedance graph. It ahould ba chosen for good discrimination 'batwaan dafacta and othar varlablna. Tha moat troublasomavariabla la LIFT-OFF variations, ao aaparatlon of dafacts fromlift-off ia tha foraaoat consideration.

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Only the skin depth equation has to be used,

6 - nun (2.13a)

A test frequency where 6 is about equal to the expecteddefect depth provides good phase separation between lift-offand defect signals. Figure 5.17 illustrates the display on

COIL

\

LIFT-OFF SURFACE CRACK

1X1

4&W'\ SUBSURFACE

VOID (A)

SUBSURFACEVOID (A)

SUBSURFACEVOID (B)

INCREASINGLIFT-OFF

SURFACECRACK

SUBSURFACEVOID (B)

X -Y DEFECT SIGNALS

(a) (b)

Fig. 5.17; Typical Response Signals for Two Types of Defects

an eddy current instrument monitor as a probe passes oversurface and subsurface defects. Test frequency is such that$ equals depth of deepest defect, and instrument controlsare selected such that a signal from lift-off is horizontal.Note the difference in signal amplitude and angle relative tolift-off of subsurface voids A and B. This results from skindepth attenuation and phase lag.

If, during inspection, a signal indicating a defect isobserved, test frequency may be altered to verify whether thesignal represents a real defect or the effect of anothervariable. This discussion is expanded in the next chapterunder Signal Analysis.

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5.8.2 Measuring Resistivity

Resistivity can be measured at small localised areas or bysampling a larger volume of a test article to determine bulkresistivity. The volume of material interrogated depends onprobe size and test frequency. For bulk measurements a largeprobe would be used and a low frequency to maximizepenetration. The skin depth equation is again used toestimate depth of penetration at the test frequency.

Electrical resistivity measurement is a comparativetechnique; reference samples of known resistivity must beused for calibration. Variables that affect the accuracy ofresistivity measurement are lift-off, temperature, andchanges in the flow of eddy currents in test articles notrelated to electrical resistivity (such as cracks, thicknessand surface geometry).

For best discrimination between resistivity and othervariables the operating point on an impedance graph should beconsidered. Figure 5.12 illustrated the effect of testfrequency on normalized probe impedance. At the top of thegraph the angle, between lift-off variations and theresistivity curve, is small. Moving down the curve theangle, separating the two variables, increases towards theknee with no appreciable change beyond that. However,smalllift-off variations, at the bottom of the curve, produce alarge impedance change. The best operating point issomewhere between the two extremes, near the knee of theimpedance curve.

INCREASINGRESISTIVITY

MONITORDISPLAY

•

-_ —INC

' LI

REFERENCEPEDANCE

SAMFOIIi

H.E

p - 5S / « " em

|(EASING

FT-OFF

MPEDANCE POINTOF UHKNniM ^

s

REFERENCE "SAMPLE

EODV CURRENT INSTRUMENT MONITOR DISPLAY

RESISTANCE

<»> MKMKE MMM • KI ISTJWn fFFICT

Fig. 5.18; Resistivity Measurement and the Impedance Graph

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Figure 5.18 shows the method of manipulating test conditionsto best deal with lift-off. Figure 5.18(a) shows theresistivity impedance curve with a frequency and probeselected to operate near the knee. Figure 5.18(b) is anenlarged section of the curve rotated so lift-off signals areapproximately horizontal. This is the view on an eddycurrent instrument monitor.

Next consider temperature effects. First, test articleresistivity will be a function of temperature so test sampleand standards should be at uniform temperature. A greaterpotential error is in probe wire resistance, R-QQ . The coilwire resistance is a part of the probe impedance circuit, sovariations in temperature which affect coil resistance willappear as an impedance change. For greatest accuracy, theinductive reactance, Xj.» should be large compared to coilWire resistance; X^/R.. > 50 is desirable.

Obviously this condition is not easily satisfied at low testfrequencies where inductive reactance is low. One solutionis to use a large diameter probe cupped in ferrite. Thelarge diameter and ferrite cup will both increase X L/RQ^

Another solution is to use a Send-Receive instrument. Suchan instrument has a high input impedance, sensing onlyvoltage changes in the receive coil. Coil wire resistance isinsignificantly small in comparison to instrument impedanceand therefore has no effect.

Consider next the effect of changes in eddy current path notrelated to electrical resistivity. If the test is supposedto be a measurement of electrical resistivity, thicknessshould not influence the signal. The skin depth equation mustagain be used. Test article thickness should be equal to orgreater than three skin depths, t > 3 { ,

t > 3 x 50jf , mm

, r- 22500 Hzf >—-

where t is thickness, p is resistivity in microhm-centimetres, and f is frequency.

Other sources of signals are edge effects and surfacegeometry. When the test article's edge is within the probe'smagnetic field, an increase in resistance to eddy currentflow will be detected. Edge effect can be reduced by probedesign, such as a ferrite cupped probe, or by increasing testfrequency.

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If the surface of the test article Is contoured, the magneticflux coupling will differ from that of a flat surface and acorrection factor may be required.

Cracks or voids are usually less of a problem. The signalfrom a crack will be very localized whereas resistivityvariations are usually more gradual. The best procedure todetermine ii a localised signal is from a change inresistivity is to rescan with a smaller probe at higher andlower frequency (at least three times and one third the testfrequency). The angle between the signals from lift-off andresistivity should vary only slightly whereaB the anglebetween lift-oft and defect signals will Increase withfrequency.

An example of resistivity variations in a zirconium alloy,due to a change In oxygen concentration, is shown inFigure 5.19.

n

iiTEST ARTICLE WIDTH

X,VOLTS

(a) X-Y DISPLAY OF COIL IMPEOANCE FROMCHANGE IN ELECTRICAL RESISTIVITY

f!f!

fi

n

(b) MODIFIED C-SCAN DISPLAYING Y-COMPONENTOF COIL IMPEOANCE VECTOR FROM A CHANGEIN ELECTRICAL RESISTIVITY

Fig. 5.19: Eddy Current Signals from a Change In ElectricalResistivity on the Surtace of a Zr-Nb Test Article." TestFrequency • 300 kHz.

IIIIIIIIII1sIIIIIII

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5.8.3 Measuring Thickness

Test frequency should be chosen so 'lift-off and 'change inthickness1 signals are separated by a 50° phase angle, seeFigure 5.2O(a). This frequency can be calculated using theskin depth equacion. A 'reasonable approximation for thinsections is when obtained when

t/6 0.8

which converts to

f - 1.6

where

p/t5 kHz

(5.6)

(5.7a)

6 is skin depth, mmt is test article thickness, mmP is electrical resistivity, microhm-centimetresf is frequency, kHzVr is relative permeability (y • 1 for non-

ferromagnetic material).

In testing thick material, this equation can similarly baused to choose a test frequency to separate lift-off andsubsurface defect signals by 90°. Fquation 5.7(a) can beused by replacing t with x,

f - 1.6 p/x2 kHz (5.7)

where x is depth of subsurface defect.

INCREASINGRESISTIVITY

BAFOf

-TH

_ |

LIFT-OFF

ANCKOI

CKNI

MIM1SS-

NT

rmcKNESs

i

\

JHEREHICK

»SINIESS

G

(b) EDDY CURRENT INSTRUMENT MONITOR DISPLAY

RESISTANCE

(=> IUPEDSNCE GRAPH - RESISTIVITY AND THICKNESS EFFECT

Fig. 5.20: Thickness Measurement and the Impedance Graph

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Conventlonal thickness measurement Is to display the lift-offsignal horizontal (along the X-axis) and use the verticalsignal (along the Y axis) to measure thickness, see Figure5.20(b). It the signal on the instrument monitor is set tomove from right to left as the probe is moved away from thetest article, a vertical movement up or down denotesdecreasing and increasing thickness respectively.

5.8.4 Measuring Thickness of a Non-Conducting Layer on a Conductor

An insulating layer will not conduct eddy currents someasurement ox its thickness is essentially a lift-offmeasurement (provided it is non-ferromagnetic), i.e. thedistance between the coll and test article. At high testfrequency a small variation in lift-off produces a largechange in probe impedance as shown in the impedance graph ofFigure 5.9.

To minimize the signal from variations in the base material,the test should therefore be done at the highest practicalfrequency* The maximum frequency would be limited byprobe-to-instrument impedance matching, cable resonanceproblems and cable noise.

The measurement is a comparative technique so standardreference thicknesses must be used for calibration.

5.8.5 Measuring Thickness of a Conducting Layer on a Conductor

Measurement or the thickness of a conducting layer on aconducting test article can be done provided there is a

• ditterence in electrical resistivity (Ap) between the two.The measurement is essentially the same as the thicknessmeasurement described in Section 5.8.3. There is oneimportant difference; variables in the base plate, in additionto the variables in the layer, will affect the signal.

Figure 5.21(a) shows a computer simulation of a layerthickness measurement. The model shows the magnitude anddirection of variables when attempting to measure a layer(clad l), nominally 0.75 mm thick, with resistivity p •= 3 yfi.cmon a base (clad 2) with resistivity 5 yfl.cm. The plotis part of a normalized impedance graph. In addition tomaterial property variables, the parameter of space (gap)between the layers is shown as well as the effect of anincrease in test coil temperature. At 10 kHz, t/S Is 0.8and, as predicted, the angle separating signals from

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E00» CURRENT 1HPEDINCE PLANE

1 I F I I I I I I

t («)

7 l'"/1 I l l I I I I,170

.0S2O .0940 ,0H0 .OMO .MOO .0020 .0140 .OHD .0110 .0)00

IIOWJLIZED RESISTANCE. _! !L

!II RESISTKITr I c 3 i 2 0 t u f l - .« I

HIR CUf 0 TO .37 m

\ RESISTIVITY 3 -- 5 ! 101 nCl-

.1-ULJ»

RJNGE OF VARIABLES SHOIN IK COKPUTOR PLOTS

(D)

Fig. 5.21; Computer Simulation of a Multi-Layer Sample

lift-off and layer (clad 1) thickness is about 90°.Unfortunately, so are the signals from test coil temperature,gap, and resistivity of the base (clad 3 ) . Some of theseparameters can be discriminated against at higher and/orlower test frequencies.

5.9 PROBE-CABLE RESONANCE

Probe-cable resonance must be considered when operating athigh test frequencies and/or using long signal cables, e.g..frequencies greater than 100 kHz and cables longer than 30 m.Most general purpose eddy current instruments cannot operateat or close to resc-ince.

Probe-cable resonance can be modelled as shown in Figure 4.5.In simple terms, resonance occurs when inductive reactance ofthe coil equals capacitlve reactance of the cable, i.e. when

ti)L - 1/oiC

where to is angular frequency, in radians/secpnd, L is coilinductance in henries and C is total cablecapacitance In farads.

-86-

Transforming this equation and substituting U)=27rf showsresonance occurs when frequency is

fr - l/2ir i/LC (4.6a)

This approach is sufficiently accurate for most practicalapplications. A more rigorous approach to resonance ispresented in Section 4.3.

Resonance is apparent when a probe and cable combination,which balances at a low frequency, will not balance asfrequency is increased. At the approach of resonance, thebalance lines on the eddy current storage monitor will notconverge to a null. The two balancing (X and R) controlswill produce nearly parallel lines rather than the normalperpendicular traces, on the storage monitor. A number ofsteps can be taken to avoid resonance:

1. Operate at a test frequency below resonance, such thatf is less than 0.8fr.

2. Select a probe with lower inductance. (Since fr

is proportional to 1/ /U7 inductance must be decreased bya factor of four to double resonant frequency).

3. Reduce cable length or use a cable with lower capacitanceper unit length (such as multi-coax cables). This willraise the resonance frequency since capacitance isproportional to cable length and fr is proportionalto 1/ /cT

4. Operate at a test frequency above resonance, such thatf is greater than 1.2fr.However, above resonance the sensitivity of all eddycurrent instruments decreases rapidly with increasingfrequency because capacitive reactance (Xc»l/ 0)C)decreases, and current short circuits across the cable,rather than passing through the coil.

5.10 SUMMARY

Test probes induce eddy currents and also sense thedistortion of their flow caused by defects. Surface probescontain a coil mounted with its axis perpendicular to thetest specimen. Because it induces eddy currents to flow in acircular path it can be used to sense all defects independentof orientation, as long as they have a componentperpendicular to the surface. It cannot be used to detectlaminar defects.

For good sensitivity to short defects, a small probe shouldbe used; probe diameter should be approximately equal or lessthan the expected defect length. Sensitivity to shortsubsurface defects decreases drastically with depth; even a'thin' 5 mm sample is considered very thick for eddy curranttesting.

-87-

The analysis of eddy current signals is the most importantana unfortunately the most difficult task in a successfulinspection. A thorough understanding of impedance graphs isessential to manipulate test conditions to minimizeundesirable test variables. The characteristic parameter forsurface probes is used to locate the operating point on theimpedance diagram. It is given by

P - 7.9 x 10"4 i2 f/p (5.5)c

where r is mean radius, mm; f is test frequency, Hz; andp is electrical resistivity, microhm-centimeters.

The criterion for defect detection with impedance planeinstruments is phase discrimination between lift-off noise anddefect signals. Test frequency is chosen such that 'lift-offand 'change in wall thickness* signals are separated by a 90°phase angle. This can be derived from the followingequation:

f - 1.6 p/t2 , kHz (5.7)

where t is sample thickness, mm.

If inspection is performed at high test frequencies and/orwith long cables, it is desirable to operate belowprobe-cable resonance frequency. This is normally achieved byusing a probe of sufficiently low inductance.

To optimize test results, the inspector has control over probesize and test frequency. In choosing probe diameter thefollowing must be considered:

(a) operating point on impedance diagram(b) probe inductance and resistance(c) sensing area(d) sensitivity to defect length(e) sensitivity to defect depth(f) sensitivity to litt-off(g) sensitivity changes across coil diameter (zero at

centre)(h) sensitivity changes with ferrite core or cup.

Choice of test frequency depends on:

(a) depth of penetration(b) phase lag(c) operating point on impedance diagram(d) inductive reactance(e) probe-cable resonance.

-88-

5.11 WORKED EXAMPLES

5.11.1 Effective Probe Diameter

PROBLEM: Determine sensing diameter of a 5 mm probe when(a) testing 316 stainless steel (p - 72 microhm-

centimetres) at 2 MHz,

and(b) testing brass ( p -6.2 microhm-cm) at 10 kHz.

SOLUTION:(a)

6 - 5 (2.13a)

7250+h x 10

6- 0.30 mm

D -. « D + 46 - 5.Q + 1.2 - 6.2 mmeft c

(b) 1.25 mm

D ,- - D + 46 - 5.0 + 5.0 - 10 mmerr c

5.11.2 Characteristic Parameter

PROBLEM: If an available probe had coil dimensions of 10 mmouter diameter and 4 mm inner diameter, determinethe best frequency for resistivity measurements ofa zirconium alloy (P " 50 microhm-cm).

SOLUTION: The best frequency for resistivity measurements iswhen the operating point is at the knee locationon the impedance diagram. This occurs when thecharacteristic parameter Pc«10. Using equation5.5,

Pc - 7.9 x 10-4 /lO.O + 4.0 \ f/50 - 10

therefore, f 50 kHz.

(This calculation places no emphasis on skin deptheffect, which may be an overriding consideration).

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CHAPTER 6 - SURFACE PROBE SIGNAL ANALYSIS

6.1 INTRODUCTION

Manufacturing and preventive maintenance inspection of "flat"components with surface probes is one of the oldest and mostimportant applications of eddy current testing.Manufacturing inspection of small steel components fordefects and hardness is almost exclusively performed by eddycurrent methods. For safety reasons and preventivemaintenance (savings on replacement costs and downtime)inspection of aircraft components for cracks and heattreatment effects has been performed since commercialaircraft first went into service. Eddy current testing isone of the most effective NDT methods for the aboveapplications because it doesn't need couplants, it is fast,and 100% volumetric inspection is often possible. .

This chapter describes how to maximize signal-to-noise byproper choice of teat frequency and minimizing "lift-off"noise. Emphasis is given to signal analysis and how torecognize and discriminate between defect signals and falseindications. An attempt is made throughout this chapter toillustrate discussion with real or simulated eddy currentsignals.

6.2 EDDY CURRENT SIGNAL CHARACTERISTICS

6.2.1 Defect Signal Amplitude

A defect, which disrupts eddy current flow, changes test coilimpedance as f.he coil is scanned past a defect. Thiscondition is shown pictorially in Figure 6.1 which portrayseddy currents induced by a surface probe in a defectiveplate. Eddy currents flow in closed loops as illustrated inFigure 6.1(a). When a defect interferes with the normalpath, current is forced to flow around or under it or isinterrupted completely. The increased distance of thedistorted path increases the resistance to current just as along length of wire has more resistance than a short length.

Eddy currents always take the path of least resistance; if adefect is very deep but short, current will flow around theends; conversely, if a defect is very long (compared to thecoil diameter) but shallow, the current will flow underneath.In summary, defect length and depth (and width to somedegree) increase resistance to eddy current flow and this, inturn, changes coil impedance. (The effect of defect size onflow resistance in tube testing is derived in Section8.2.1) .

- 9 0 -

COIL BOUNDARIES

EBDY CURRENTS

TEST PLATE

CRACK

SURFACE COIL

WINDINGS

, TEST PLATE

EDDY CURRENT DISTORTIONAT CRACK

(b) EDDY CURRENTS TAKE THE PATH OF LEAST RESISTANCEUNDER OR AROUND A DEFECT

(9) EDDV CURRENTS FLOW IN CLOSED PATHS. A DEFECTINTERFERES WITH THE NORMAL PATH.

Fig. 6.1; Eddy Currents In a Defective Plate

In terms of the equivalent coll circuit of a resistor Inparallel with an Inductor and Its associated semi-circularImpedance diagram (Section 3.5), a defect moves the operatingpoint up the impedance diagram. Increasing resistance in atest article changes both probe inductance and resistance.

In the preceding discussion the defect was considered todisrupt the surface currents closest to the coil. Considerthe difference between surface and subsurface defects. Whena surface probe is placed over a deep crack of infinitelength, the surface currents must pass underneath the defectif they are.to form a closed loop, see Figure 6.2(a). Thisis not the case with subsurface defects as shown in Figure6.2 (b). Although the void in this picture is not as farfrom the surface as the bottom of the crack, the void maynot be detected. Eddy currents concentrate near the surfaceof a conductor,and therefore, tests are more sensitive tosurface defects than internal defects.

The skin depth equation helps in the understanding of thisphenomenon. In Chapter 2 it was shown that current densitydecreased with distance from the surface in the followingproportions:- 63% of the current flows in a layer equivalent in thickness

to one skin depth, 5 ,- 87Z flows in a layer equivalent to two skin depths, 2 5 ,- 95% flows in a layer equivalent to three skin depths, 36 .

-91-

IX'SURFACE COIL

TEST PLATE

CRACK

(a) EDDY CURRENT FLOW UNDER A CRACK (b) EDDY CURRENT FLOW AROUNO ASUBSURFACE VOID

Fig. 6.2: Eddy Current Flow in the Presence of (a)Surface and (b) Subsurface Defect

Since only 5% of the current flows at depths greater than the3 6 , there is no practical way to detect a subsurface defectat this distance from the surface. But in the case of a longsurface defect 3 8 or greater in equivalent depth, most ofthe current is flowing under the defect. Surface cracks willbe detected and depth can be estimated even if eddy currentpenetration is a small fraction of the defect depth. Onceeddy currents are generated in a metal surface, they willfollow the contour of a crack because a potential is set-upabout the crack.

6.2.2 Defect Signal Phase

From the above description one cannot predict a defect signalin detail, only its relative amplitude and direction on theimpedance diagram. A more complete explanation requiresinclusion of phase lag. Consider the cross section of asurface probe as shown in Figure 6.3(a). This pictorial viewshows the distribution of magnetic field magnitude and phasearound a coil as derived by Dodd(2). The solid lines arecontours of constant magnetic field strength; the dashedlines represent constant phase. Since the magnetic field andinduced eddy currents have approximately the same phase, thedashed lines will also represent the phase (g) of the eddycurrents. Amplitude drops off exponentially with distanceand eddy current flow increasingly lags in phase ([relative toeddy currents adjacent to the coil) both with depth and withaxial distance from the coil. Skin depth effect occurs inboth radial and axial directions.

Figure 6.3(a) permits an approximate derivation of eddycurrent signals for the shallow surface, subsurface and deepsurface defects illustrated. One needs to establish a

- 9 2 -

COHSUNT »«PUTUOE

OEEP DEFECT-

SHULLOI DEFECT -

SUBSURFUCE DEFECT-

DEFECT POSITION (a)

Fig. 6.3; Derivation of Eddy Current Signal Appearancefor Three Types of Defects"

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reference phase direction as starting point; the LIFT-OFFdirection Is convenient and can be defined as the signalresulting from Increasing the space between the coil and testarticle, starting from the point when the space is minimum.

The signal or effect of defects can be imagined as theabsence of eddy currents which were flowing in the areabefore the defect existed at this location. As the defectsapproach the coil from positions 0 to 5 in Figure 6.3(a), thesignal on the eddy current storage monitor moves from point 0to 5, tracing the curves illustrated in Figure 6.3(b). Thisprocedure is reasonably straight forward for shallow surfaceand subsurface defects since they are localized and onlyintersect one phase and amplitude contour at any givenposition. For the deep defect one has to divide the defectinto sections and determine weighted average values foramplitude and phase at each position.

The shallow surface defect in Figure 6.3(b) has a largecomponent in the lift-off direction; primarily its approachsignal makes it distinguishable from lift-off. As defectdepth increases, signals rotate clockwise due to increasingphase angle. The angle indicated in Figure 6.3(b) is not thevalue calculated from the phase lag equation,

3 = x/6 (2.14)

where (3 is phase lag (radians), x is distance of defect belowthe surface (mm) and 6 is skin depth (mm).

The angle between lift-off and defect signals is about 2 g .Although probably not strictly true, one can imagine defectphase angle as the sum of a lag from the coil to the defectand the same lag back to the coil.

The foregoing discussion assumes that the defect is a totalbarrier to the flow of current. Although this assumption isvalid for most cracks or discontinuities, some cracks arepartial conductors. Fatigue cracks, formed when the testarticle is under a tensile stress, can become tightly closedwhen stress is released. The result is that some fraction ofeddy currents could be conducted across the crack interfaceand the magnitude of the coil impedance change due to thedefect will be less. The phase lag argument is still valid;a deep crack will still be distinguishable from a shallowcrack by the shape of the eddy current signal, but thesensitivity to such a crack will be reduced because ofsmaller amplitude,

6.3 EFFECT OF MATERIAL VARIATIONS AND DEFECTS IN A FINITETHICKNESS

For each test, one must decide on the test frequency to useand on the phase setting. The conventional way of setting

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phase on an eddy current Instrument is to display the"lift-off" signal horizontally (on the X-axis) with theimpedance point moving from right-to-left as the probe israised. All material variables will then display an eddycurrent signal at an angle clockwise to the lift-off signal.

LIFT-OFF

7 iran

1.5 mm2.0 mm

- -At

LIFT-OFF

FREQUENCY = 10 kHz

LIFT-OFF

FREQUENCY = 50 kHz

LIFT-OFF

FREQUENCY - 200 kHz

Fig. 6.4: Probe Response to Various Test Parametersat Three Frequencies

Discrimination between defects and other variables isaccomplished through pattern recognition and varying testfrequency. Figure 6.4 displays the change in coil impedanceloci for various parameters at different test frequencies.The electrical resistivity (Ap) signal angle, relative tolift-off, increases only slightly as frequency is increased,whereas a change in plate thickness ( At) signal anglecontinually increases with frequency. The angle, between thesignal from lift-off and plate thickness change, equals abouttwice the phase lag across the plate thickness. The signalfrom a change in magnetic permeability (Ay) of the plate isapproximately 90° to the lift-off signal at low frequency anddecreases only slightly with Increasing frequency.

Figure 6.5(a) illustrates a computer simulation of coilresponse to various test parameters. The simulation is basedon the same probe and test sample used in the previousfigure. Comparison of these two figures reveals computersimulation gives very realistic results.

-95-

1.0

0.9

10 kHz

\ \ x

\ 0.25 - \ >

\\0.J5 m\

5 0 KHZ

(a)o.i

LlfT-OFF,2nn

A/. = t25*

At - -25»

A M = .25*

p. - 1.0

0.2

1.5 mi. M

' \

O.I

0.7

0.6

0.5

0.4

v -..V •-

\

\ \ •

0.25 m

\I

0.25 MH' * » /

0.1 0.2 0.3

(b)

IIFT-OFF

I 2 Ml

0.25 mm *>3( to kHz *J—2 r»

SC kHz

J 1 J L.

I1«.f- 1 . 5 mm

= 72 f>a • cm

= •25%

= 1 . 0

0.4 0.5

Flg» 6.5: Computer Simulation of Probe Response toVarious Test Parameters

-96-

Note at 50 kHz the increase in magnetic permeability signal(Ay) is to the right of the electrical resistivity signal forthe 7 mm probe. For the 25 mm probe at 50 kHz it is to theleft of the Ap signal. As the operating point moves down theimpedance curve with increasing probe diameter, aresistivity signal rotates CW relative to a permeabilitysignal. Note also that the permeability signal is notperfectly parallel to the inductive reactance axis. This isdue to the skin depth and phase lag changing withpermeability, rotating the signal CW.

During general inspection for all parameters in a thin platetest frequency is normally chosen such that 'lift-off and'change in plate thickness1 signals are separated by 90° onthe impedance plane. This frequency is empirically derivedby setting ratio between plate thickness and skin depthequal to approximately 0.8,

t/5 = 0.8 < 5. 6 )

Substituting in equation 2.13 yields

f - 1.6 p/t2 , kHz (5.7)

where p is electrical resistivity (microhm-centimetres), andI; is plate thickness (mm).

This frequency has been proven in practice on variousconductivity samples and various probe diameters. The 90°phase angle increases only slightly with increasing probediameter, see Figure 6.5(b). All defect signals (fromsurface or subsurface defects) will fall inside this 90°band. Shallow defects, cracks or pits, on the opposite sideof the plate will produce a signal whose angle approachesthat of wall thickness, i.e. 90°. Shallow defects on thesurface nearest the probe will produce a signal whose angleis close to that of lift-off.

The two methods of discriminating between defects and othervariables, pattern recognition and varying test frequency,complament each other. Consider signal pattern behaviour dueto nominal wall thickness and resistivity variations. Thesevariables normally change gradually along a sample. Whereascracks, pits, and subsurface voids or inclusions exhibit astep change. Discrimination between these variables isenhanced by analyzing their behaviour at different testfrequencies, as shown in Figures 6.4 and 6.5. An extremelyimportant point to remember is that all defects will fallbetween the 'lift-off signal angle and the'decrease-in-wall-thickness' signal angle regardless of frequency. (Forpractical applications this statement is valid; however,the signal from a shallow defect with length greater than aprobe diameter may dip slightly below the lift-off signal).

-97-

mm

SAMPLE : p = 50 ̂ ii • cm

fir = 1.00

CALIBRATIONCRACKS

CRACKCRACK

LIFT-OFF

2 mm DEEP NOTCH2 mm DEEP NOTCH

LIFT-OFF0.5 mm DEEP NOTCH 0.5 mm DEEP NOTCH

FREQUENCY = 50 kHz FREQUENCY =300 kHz

6.4

Figo 6.6: X-Y Display of Coil Impedance Vector fromCalibration Grooves and a Real Crack. Estimated Depth*!.3 mm,

COIL IMPEDANCE CHANGES WITH DEFECTS

6.4.1 Surface Defect Measurement

Figure 6.6 illustrates the method used to predict depth ofsurface defects. Pattern recognition is used where coilimpedance response from the defect is compared withcalibration defects. To estimate defect depth by patternrecognition, the real and calibration defect signals must becomparable in amplitude. This can be achieved by changingthe gain of the display (normally by decreasing thecalibration defect signals). Defect depth is estimated byinterpolation.

Amplitude of defect signals is not a reliable parameter forestimating defect depth. Amplitude is affected by length andthe degree of contact across the two interfaces (e.g., crackclosure). Whereas the coil impedance locus (the X-Y displayof coil impedance) depends mainly on the integrated responsewith depth of the eddy current phase lag.

6.4.2 Subsurface Defect Measurement

Signals from subsurface defects, Figure 6.10(b), have anaverage phase angle relative to lift-off of approximately 23where 3 is the phase lag of the eddy currents at depth x.This signal is similar tc a change in wall thickness signaland its phase was denoted by G3 in Figure 5.16.

-98-

6.5 COIL IMPEDANCE CHANGES WITH OTHER VARIABLES

6.5.1 Ferromagnetic Indications

In eddy current testing the test coil is sensitive to manytest parameters. One variable that often causes problems ismagnetic permeability. At common test frequencies one caneasily mistake a signal due to increased permeability(ferromagnetic indication) for a serious defect. Thefollowing discussion briefly outlines the problem and showshow one can differentiate between defects and ferromagneticindications.

It is generally recognized that magnetic saturation isrequired for eddy current testing of ferromagnetic alloys.Conversely, saturation is not usually employed when testing"non-magnetic" alloys such as austenitic stainless steels andnickel base alloys. Unfortunately, these alleys and anyalloys containing iron, -nickel or cobalt can displayvariations in magnetic permeability. This is caused by thestrong dependence of magnetic properties on metallurgicalvariables such as composition, grain size, thermalprocessing, cold work, contamination and segregation.

The following are examples of ferromagnetic indications innominally nonmagnetic alloys which have been encountered:

- Ferromagnetism associated with manufacturing defects inInconel 600 extrusions (possibly from chromium depletionat the surface).

- Ferromagnetism associated with EDM calibration grooves inType 304 stainless steel.

- Permeability variations occurring in austenitic stainlesssteel castings probably due to segregation (or possiblycontamination).

- Ferromagnetic inclusions in zirconium alloys resulting frompick-up during forming.

- Magnetite (Fe30^) deposits on heat exchanger tubes dueto steel corrosion somewhere else in the cooling system.

The first two types of defects would have made defect depthpredictions seriously inaccurate, and the last three types offerromagnetic indications could have been mistaken fordefects such as cracks or pitting.

Some of the anomalous ferromagnetic indications listed abovecould be suppressed by saturating the test area with apermanent magnet possessing a flux density of a fewkilogauss. If saturation is not possible (or Incomplete)there is another way to determine if an indication is due toa defect or a magnetic effect. The method involves retestingat a much lower frequency. It is illustrated in Figure 6.7for the case of a surface probe passing over defects and aferromagnetic inclusion.

At typical test frequencies (100-500 kHz) there is littlephase separation between the signals from defects and magneticinclusions* As test frequency is reduced, the operating

I

IIIIIIIIIIIIIIIIII

- 9 9 -

t.ooFERROMAGNETIC

/ INCLUSION FERROMAGNETICINCLUSION

CALIBRATIONCRACKS

PROBE Dl« = 7 mmSAMPLE p = 50/ifl-cm

2 mm DEEP/ NOTCH

L.O

100 kHz

0 0.05 0.10 0.15 0.20 0.25

NORMALIZED RESISTANCE _K

«,L0

10 kHz

2— • —

LJFT-OFF

100 kHz

2

L 0 .

500 kHz

mm

mm

0

DEEP

- ^ .

0.5

DEEP

10.5

.5 mm DEEPNOTCH

/ FERRI

/

mm DEEP

, FERRO

mm DEEP

Fig. 6.7: Coil Impedance/Voltage Display at Three Frequencies

point moves up the impedance curve and defect signals rotate asshown. The important point to note is that relative tolift-off, defect signals rotate CCW whereas the magneticinclusion signal rotates CW and approaches 90° at low frequency(approximately 10 kHz or lower for the above probe and sample).On the impedance diagram of Figure 6.7 the direction of theferromagnetic signal would not vary appreciably with frequency;increased permeability primarily increases coil inductance.

When a magnetic inclusion is not on the surface - if it issubsurface or on the opposite side of a thin test plate -there is the added complication that the angle of the signalwill be rotated relative to the angle of a ferromagneticindication on the surface adjacent to the coil. This arisesfrom phase lag across the plate thickness. The previousapproach of retesting at reduced frequency will also serve todistinguish between defects and magnetic inclusions. If thephase of the signal from the indication increases to 90°relative to 'lift-off1, it is a ferromagnetic anomaly; if itdecreases to nearly 0°, it is a defect.

-100-

To summarize:(a) Many nominally "non-magnetic" alloys can exhibit

ferromagnetic properties and almost any alloy can pick upmagnetic inclusions or contamination during manufacture orservice.

(b) At normal eddy current test frequencies magneticindications will often appear similar to defects.

(c) Magnetic indications can be distinguished from defects byretesting at a reduced test frequency.

6.5.2 Electrical Resistivity

Electrical resistivity is a material parameter which, unlike adefect, usually varies over a significant area. However, if itis localized, and the eddy current signal is small, it could bemistaken for a small defect. The best means of distinguishingthe two is to rescan with a smaller probe at the same testfrequency, at three times the test frequency, and at one thirdthe test frequency. Unlike a defect signal, the angle betweenresistivity and lift-off changes little with frequency. Seeimpedance graph in Figure 5.9.

As with the detection of any signal source, resistivity isaffected by skin depth. At high frequency, when skin depth issmall, there will be greater sensitivity to surface resistivityvariations. At lower test frequency, eddy currents penetratedeeper into the material so the measurement will represent alaiger volume.

6.5.3 Signals from Changes in Sample Surface Geometry

Abrupt changes in surface curvature result in eddy currentsignals as probes traverse them. It causes changes in couplingcreating a large lift-off signal and the curvature also changeseddy current flow distribution creating an effective resistancechange, yielding a signal at an angle to the lift-off direction.The combined effect may be a complicated signal, as shown inFigure 6.8. The appearance of this type of signal will notchange significantly when rescanned at higher and lower testfrequency.

Such signals can be difficult to analyze because they depend onhow well the probe follows complicated surface curvatures.Basically the direction of the impedance change obeys thefollowing rules when using surface probes:

- decreasing radius of curvature on an external surface, e.g.,ridge, produces a change in the direction of increasingresistivity,

- decreasing radius of curvature of an internal surface, e.g.,groove, produces a change in the direction of decreasingresistivity.

-101-

Figure 6.8(a) illustrates the signal as a probe traverses ashallow groove (decrease in surface radius) on the internalsurface of a 100 mm tube. Figure 6.8(b) shows the signal as aprobe traverses a flat (increase in surface radius). The testwas done with a 9 mm diameter probe at a test frequency of 300kHz.

1 VOLT

1 VOLT

(a) WIDE SHALLOW GROOVE (b) LOCAL FLAT SPOT

Fig. 6.8; X-Y Display of Surface Coil Impedance for InternalSurface Variations in a 100 mm Diameter Tube

6'. 6 CALIBRATION DEFECTS

Analysis of eddy current signals is, for the most part, acomparative technique. Calibration standards are necessary forcomparing signal amplitude and phase (shape) of unknown defectsto known calibration defects. Calibration signals are also usedfor standardizing instrument settings, i.e., sensitivity andphase rotation.

Existing national specifications and standards only supply broadguidelines in choice of test parameters. They cannot be used toestablish reliable ET procedures for most inspections. Figure6.9 shows a calibration plate proposed by the authors forgeneral application. The effect of the following can beestablished using this plate:

1. Varying Electrical Resistivity2. Varying Thickness3. Surface Geometry (Curvature)4. Defect Length for Constant Depth5. Defect Depth for Constant Length6. Increasing Subsurface Defect Size for Constant Defect

Depth7. Increasing Distance of Subsurface Defects from the

Surface with Constant Defect Size8. Varying Thickness of a Son-conducting Layer (lift-off)9. Varying Thickness of a Conducting Layer10. Ferromagnetic Inclusions

- 1 0 2 -

I I I I I I

m-CONDUCTING

LAYER

0.2 mm

0.1 mm

0.05 n

COPPER

LAYER

1.0 mm

0.5 mm

0,1 m

CHROMIUM

PLATE

0.1 Him

.05 mm

.01 mm

( b ) BACK SIDE

2 mm -

1 . 5 mm - .

0 . 7 mm -

T E l f - 1 2 0 I f - 7 0 f f = 5 0 I P - Z 5 \ P = 7 f P = 4 | P - 1

• c m

r I I I I I II— 0.12 0.25 0.5 1.0 2.0 4.0

d = 1 mm DEPTH, mm

— 0.5 mmI I

I . 2. 4. 10

LENGTH, mm

d=2nm CONSTANT DEPTH = 0.5 mi

> • COPPER • 00.S mm t=0. I m 1'ilN VOID

R, - 2 0 Rj=5 Rc = 5 R j = I O R , = 5 5

25

( a ) FRONV SIDE

Fig. 6.9: Calibration Standard

-103-

More than one calibration plate would be required to cover acomplete range of materials. A group of three would normallysuffice, comprising base materials: aluminum alloy, p=4 lift.cm;bronze, p = 25 \iQ . cm; and Type 316 stainless steel,P =74 yfi.cm.

Figure 6.10(a) i l lustrates eddy current signals obtained withan absolute surface probe from some of the calibration blockdefects. Figure 6.10(b) i l lustrates signals from the samedefects using a differential surface probe, similar to thatin Figure 5.2(c) .

0 .5 mm CEEP4 mm DEEP ; 4 mm DEEP

1 mm DEEP

0 .5 mm DEEP

LIFT-OFF

SURFACE DEFECTS

LIFT-OFF

0 .7 mm DEEP

LIFT-OFF

0 . 7 mm DEEP

LIFT-OFF

SUBSURFACE DEFECTS

(a) (b)

Fig. 6.10; Eddy Current Signals With (a) Absolute and (b)Differential Surface Probes

-104-

6.7 SUMMARY

Defect signal amplitude is a function of defect length, depthand closure (if a crack)* Signal phase is primarily afunction of defect depth. For volumetric inspection of thinmaterial the following test frequency should be used:

f - 1.6 p/t2 , kHz (5.7)

where p is electrical resistivity, microhm-centimetre, andt is wall thickness, mm.

At this frequency there is good discrimination betweendefects and lift-off signals but not between defects andferromagnetic signals. Magnetic indications can bedistinguished from defects by retesting at reduced frequency.Defect signals rotate CCW (approaching 0") whereasferromagnetic signals rotate CW (approaching 90°) relative tolift-off signals.

There are few national standards governing eddy currentinspections with surface probes. For effective inspection, acalibration block should simulate the test piece and containappropriate surface and subsurface defects along withferromagnetic inclusions. Basic knowledge of phase lag andimpedance diagrams is also required for reliable analysis ofeddy current indications.

-105-

CHAPTER 7 TESTING OF TUBES AND CYLINDRICAL COMPONENTS

7.1 INTRODUCTION

Tubes or rods up to about 50 mm diameter can be inspected fordefects with encircling coils* Defect sensitivity in largerdiameter components decreases because the inspected volumeincreases while defect "volume" remains the same for a givendefect. For larger diameters, surface probes should be usedto obtain higher defect sensitivity, see Chapter 5.

The components can be in the form of wire, bars or tubes andround, square, rectangular or hexagonal in shape, as long asappropriate coil shapes are used. Inspection is fast andefficient since an encircling coil samples the completecircumference of the component, allowing 100% inspection inone pass.

Defect detectability depends on disruption of eddy currentflow. Therefore, the best probe is the one which induceshighest possible eddy current density in the region ofmaterial to be inspected, and perpendicular to the defect.

When planning an inspection, the following questions mustfirst be answered:- For what type of defects is the inspection to be performed?- If cracks are expected, do they have directional

properties?- Does the material or components in close proximity haveferromagnetic properties?

Once these questions have been answered one can decide onsuitable probe design, test frequency and calibrationstandards. With the proper procedures one can discriminatebetween defect signals and false indications as well asdetermine depth once a defect Is located. These proceduresare based on a knowledge of impedance diagrams and phase lag.

7.2 PROBES FOR TUBES AND CYLINDRICAL COMPONENTS

7.2.1 Probe Types

Four common probe types for testing round materials areillustrated in Figure 7.1: (b) and (d) are differentialprobes, (a) and (c) show absolute probes. Each type containstwo separate coils to satisfy AC bridge circuit requirements,which is the typical mode of operation of most eddy currentinstruments, see Chapter 4. These bridges require matchingcoils on two separate legs of the bridge to balance, thuspermitting amplification of the small impedance differencesbetween the two coils. If the two coils are placedside-by-side, both equally sensing the test material, theprobe is "differential". If one coil senses the test article,the other acting only as a reference, the probe is absolute.

-106-

Figure 7.1(a) and (c) show effective designs for absoluteprobes; the piggy-back reference coil is separated from thetest article by the test coil and therefore couples onlyslightly to the test article ( f i l l factor«l ) .

CENTERING DISCS

TEST COIIREFERENCE COIL

- GUIDES

TEST COIL

' REFERENCE COIL

(A) ENCIRCLING PROBE. ABSOLUTE(PIGGY-BACK REFERENCE)

(C) INTERNAL PROBE. ABSOLUTE(PIGGY-BACK REFERENCE)

(B) ENCIRCLING PROBE, DIFFERENTIAL(D> INTERNAL PROBE, DIFFERENTIAL

Fig. 7.1: Tube Probe Types

Coil Size

The best compromise between resolution and signal amplitudeis obtained when coil length and thickness equal defectdepth. See Figure 7.2 for a labelled diagram of a probecross section.

As a general guideline for tube inspection, coll length anddepth should approximately equal wall thickness. However, toimprove coupling a rectangular cross section with thicknessreduced to one-half the length can be used. For greatersensitivity to small near surface defects, coil length andthickness can both be reduced further. Unfortunatelythis will result in a decrease in sensitivity to external (farsurface) defects.

Coil spacing, in differential probes, should approximatelyequal defect depth or wall thickness for general inspections.

-107-

///AY///

COIL THICKNESS

TUBE-COILCLEARANCE

COIL SPACING

*- COIL WIDTHy////////

D (AVERAGE COIL DIAMETER)

///X/7/. //////////s

Fig. 7.2 Probe Coll Nomenclature

For increased sensitivity to near surface defects, spacingcan be reduced at the expense of a reduction in sensitivitywith distance from the coil.

Probe-to-tube clearance or gap should be as small aspossible. In most internal tube inspections, a gap equal tohalf the wall thickness is common. A larger gap (smallerfill-factor or coupling) results in a small decrease In nearsurface defect resolution and a large decrease in signalamplitude for all types of defects.

7.2.2 Comparing Differential and Absolute Probes

Absolute probes with a fixed reference coil are essential tobasic understanding. They enable study of all physicalproperties of a test article by plotting characteristicimpedance loci.

When an absolute coil signal is plotted as a function ofdistance (as the probe travels along a tube axis) dimensionalvariations and discontinuities can be separated. See theexample of Figure 7.3(b). The signal is a function ofeffective cross-sectional area of eddy current flow, i.e.,wall thickness in the case of tubes, and can be analyzed likea surface roughness trace with the extra advantage thatsubsurface flaws can be sensed.

-108-

In tube testing with an internal coil, absolute probe signalsfrom defects and supports are simple and undistorted; signalsfrom multiple defects and defects under support plates areoften vectorially additive.

Differential probes have two active coils usually wound inopposition (although they could be wound in addition withsimilar results). When the two coils are over a flaw-freearea of test sample, there is no differential signaldeveloped between the coils since they are both inspectingidentical material. However, when first one and then theother of the two coils passes over a flaw, a differentialsignal is produced. They have the advantage of beinginsensitive to slowly varying properties such as gradualdimensional variations and temperature: the signals from twoadjacent sections of a test article continuously cancel.Probe wobble signals are also reduced with this probe type.However, there are disadvantages; the signals may bedifficult to interpret, even to the extent of beingmisleading. Defect signals under support plates can beextremely complicated. The signal from a defect is displayedtwice: once as the first coil approaches the defect and againfor the second coil. The two signals form a mirror image andthe signal direction from the first coil must be noted. If aflaw is longer than the spacing between the two coils onlythe leading and trailing edges will be detected due to signalcancellation when both coils sense the flaw equally.

u SUPPORT PLATE POSITION

SECTION THROUGH TUBESNOWING CORRODED ARE*

DIFFERENTIAL COILS ABSOLUTE COIL

fALL LOSS y COMFONENT

TRACE WITH ABSOLUTE PROBE

WALL LOSS T r COMPONENT

TRACE WITH DIFFERENTIALPDOBE

(c)

F i g . 7 . 3 ; Eddy Current Y-Channnel Recordings from a BrassHeat Exchanger Tube

OP "26.9 mm, t-1.1mm, fqp -21 kHz

-109-

An even more serious situation occurs with differentialprobes when the ends of a flaw vary gradually; the defect maynot be observed at all. An example of this is shown inFigure 7.3; this brass heat exchanger tube suffered generalcorrosion as well as localized corrosion on either side of asupport plate. The gradual upward trend of the Y-DISTANCErecording in Figure 7.3(b) shows the pronounced grooves at Aand B are superimposed on an area of general wall shinning inthe vicinity of the support plate. Note the response of adifferential probe to the same defect in Figure 7.3(c). Thedifferential probe senses the localized grooves but theY-DISTANCE recording shows no indication of the gradual wallthinning which was apparent in Figure 7.3(b).

Table 7.1 compares advantages and disadvantages of the twoprobe types.

TABLE 7.1COMPARISON OF ABSOLUTE AND DIFFERENTIAL PROBES

ADVANTAGES: DISADVANTAGES:

ABSOLUTE PROBES

respond to both sudden and gradualcharges in properties and dimensionscombined signals are usually easy toseparate (simple interpretation)show total length of defects

- prone to drift fromtemperature instability

- more sensitive to probewobble than a differentiaprobe

DIFFERENTIAL PROBES

- not sensitive to gradual changes...in properties or dimensions

- immu le to drift from temperaturechanges

- less sensitive to probe wobblethan an absolute probe

- not sensitive to gradualchanges (may miss longgradual defects entirely)

- will only detect ends oflong defects

- may yield signals diffi-cult to interpret

7.2.3 Directional Properties

When inspecting for defects, it is essential that flow ofeddy currents be as perpendicular as possible to defects toobtain maximum response. If eddy currents flow parallel to adefect there will be little distortion of the eddy currentsand hence little change in probe impedance.

The eddy current flow characteristics of circumferentialinternal or external probes are listed and illustrated inFigure 7.4.

110-

C O I l con EDDY CURRENTS COIL EDDV CURRENTS

EDDY CURRENTS FLOW IN CLOSED PATHSLIMITED TO CONDUCTING MATERIAL

EDDY CURRENT FLOWS PARALLEL TOCOIL WINDINGS - NOT SENSITIVETO PURELY CIRCUMFERENTIAL CRACKS

EDDY CURRENT FLOW DIMINISHES TOZERO AT THE CENTRE OF A SOLID RODNO SENSITIVITY AT CENTRE

COIL COIL

EDDY CURRENT FLOWS PARALLELTO TUBE SURFACE - NOT SENSITIVETO LAMINAR SEPARATIONS.

EnDY CURRENTS CONCENTRATE NEAR THESURFACE CLOSE TO THE COIL - DEPTHOF PENETRATION IS CONTROLLED BY TEST FREOUENCY.

Fig. 7.4: Directional Properties of Eddy Currents InCylindrical Test Articles

In addition to considerations of eddy current flow direction,the following are important:

- Magnetic flux is not bounded by the tube wall but willinduce eddy currents in adjacent conducting material, e.g.tube support plates in heat exchangers.

- Eddy current coils are sensitive to ferromagnetic materialintroduced into a coil's magnetic field. Theferromagnetic material need not be an electrical conductornor need it form a closed path for eddy currents.

- Eddy current coils are sensitive to all materialvariations that affect conductivity or permeability.

7.2.4 Probe Inductance

The equations quoted in Section 5.2.3 to calculate inductancefor surface probes are also used to calculate Inductance ofprobes for testing tubes and cylinders. The important aspectof inductance is that probe impedance, which is a function ofinductance, must be compatible with the eddy currentinstrument and signal cables,

Jprobe -V? 2

where XL - 2 ir f L when f is in hertz and L in henriesand R is coil wire resistance in ohms.

-111-

TABLE 7.2 ENCIRCLING OR INTERNAL COIL IMPEDANCE

K

N

N

N

N

» 25

* 49

- 81

- 144

- 225

D -0

L •

R -

L -

R •

L »

R *

L -

R »

L -

R -

8 . 9

6 . 1

0 . 3

23

1

64

3

200

9

4 9 0

24

mm

liH

ft

D -0

L -

R -

L -

R •

L »

R «

L »

R >•

L -

R -

1 2 . 7 mm

1 1 uH

0 . 4 fl

42

1 . 5

110

5

360

14

880

35

D " 1 5 . 9 mm0

L

R

L

R

L

R

L

R

L -

R

- 15 liH

» 0 . 5 ft

- 59

- 2

- 160

- 6

» 5 1 0

- 18

1 . 2 4 mH

- 45

D0

L

R

L

R

L

R

L

R

L

R

» 1 9 . 1 mm

• 20 UH

- o .6 a

- 77

- 2

- 210

- 8

- 660

- 22

-1.62mH

- 55

D0

L

R

L

R

L

R

L

R

L

R

« 2 2 . 2mm

- 25 uH

- 0 .7 a

- 96

- 3

•= 260

= 9

- 830

- 26

-2.02mH

- 64

Wire Size

31 AWG( 0 . 2 3 mm)

34 AWG( 0 . 1 6 mm)

37 AWG( 0 . 1 1 mm)

39 AWG(0 . 0 8 9 mm]

41 AWG( 0 . 0 7 1 mm)

Most eddy current instruments will operate over a fairlybroad range of probe impedance without a substantialreduction in signal-to-noise ratio or signal amplitude. Aninstrument input impedance of 100 ohms is typical, although aprobe impedance between 20 and 200 ohms is normally acceptable,unless the test frequency is too close to probe-cable resonancefrequency, see Section 7.2.5. Exact probe inductancecalculations are therefore not essential.

To facilitate impedance calculations Table 7.2 has beenprepared. This table lists coil inductance and resistance(with probe in air) for various diameters and wire sizeswhile keeping coil cross section constant at 1.2 mm x 1.2 mm.(These dimensions are fairly typical of tube wall thicknessin heat exchangers). With the aid of this table, andknowledge that inductance is proportional to the square ofnumber of turns aad the square of mean coil diameter(L a N2D2),one can usually make a reasonable estimate ofwire size and number of turns for a particular probe.

I

-112- r

i,

7.2.5 Probe-Cable Resonance j"i

Probe-cable resonance must be considered when operating athigh test frequencies and/or using long signal cables, e.g. >;frequencies over 100 kHz or cables longer than 30 m. Most [general purpose eddy current instruments cannot operate at orclose to resonance. r

Probe-cable resonance can be modelled as shown in Figure 4.5. 'In simple terms, resonance occurs when inductive reactance ofthe coil equals capacitive reactance of the cable, i.e. when j~

U)L • 1/uC

where u> is angular frequency, radians/second ]L Is coil inductance,henriesC is total cable capacitance, farads

Transposing this equation and substituting to - 2irf 'shows resonance occurs when frequency is

This approach is sufficiently accurate for most practicalapplications. A more rigorous approach to resonance ispresented in Section 4.3.

IResonance is apparent when a probe and cable combination, 'which balances at a low frequency, will not balance asfrequency is increased. At the approach of resonance, the Hbalance lines on the eddy current storage monitor will not i iconverge to a null. The two balancing (X and R) controlswill produce nearly parallel lines, rather than the normal <~rperpendicular traces, on the storage monitor. A number of !steps can be taken to avoid resonance:

1. Operate at a test frequency below resonance, such that jftest is less than 0.8 fr . *

2. Select a probe with_lower inductance. (Since fr isproportional to 1//L , inductance must be decreased a "'factor of four to double the resonant frequency). I

3. Reduce cable length or use a cable with lower capacitanceper unit length (such as multi-coax cables). This will ~-raise the resonance frequency since capacitance is |proportional to cable length and f is proportional to1/^C , r

4. Operate at a test frequency above resonance, such that [ftest l s greater than 1.2 fr.However, above resonance the sensitivity of all eddycurrent instruments decreases rapidly with increasing ";frequency because capacj.tive reactance (Xc. • 1/WC) idecreases, and current short circuits across the cablerather than passing through the coil. -•

-113-

7.3 IMPEDANCE PLANE DIAGRAMS

Eddy current probes for testing cylindrical components differmechanically from those for plate testing, but coil impedancecan be treated similarly for both test coil configurations.The impedance display treatment introduced in Chapter 5applies for internal and external circumferential coils withthe following changes:

1) Lift-off, becomes "fill-factor". Fill factor is ameasure of coupling between the coil and test object.In general, it is the fraction of magnetic field thatcrosses the test object; for a long coil, this is thefraction of the test coil area filled with teetmaterial. Fill-factor, n (eta), is the ratio

* " D o / ¥ 2 (7.1a)

for an encircling coil,

and n = D2/D* (7.1b)

for a bobbin type internal coil,

where D o is cylinder diameterD is average coil diameter

and T)± is tube internal diameter

Fill-factor is always a quantity less than or equal toone (n < 1.0). For a coil inside a tube the impedancechange due to decreasing r) Is the same as an increasein D± (with constant wall thickness). For a coilaround a tube or cylinder, decreasing r| is the same asdecreasing D Q.

ii) Probe diameter in plate testing is replaced by tube orcylinder diameter in ET of cylindrical components.They have a similar effect on the operating point onthe impedance diagram.

Figure 7.5 summarizes the effect of test and materialvariables on a simple semicircular impedance diagram. Notethe similarity of changes in resistivity, test frequency,diameter and fill-factor with the surface probe results ofFigures 5.9 to 5.13.

-114-

i.o

0.8

0.6

0.4

0.2

INCREASINGRESISTIVITY (/>) COIL

THIN • MIL TUBE

INCREASINGFREQUENCY (f) andDIAMETER (D)

0.2 0.4 0.6

NORMALIZED RESISTANCE

Fig. 7.5; Simplified Impedance Diagram of a Long Coil Arounda Non-magnetic Thin-wall Tube Showing Effect of Test andMaterial Variables

Impedance diagrams presented in the literature are often onlystrictly valid for long colls (much longer than materialthickness), coil lengths for inspection are normally only afraction of material diameter. Decreasing coil length has aneffect similar to decreasing fill-factor, it causes theimpedance diagram to be smaller than expected (but similar inshape) from coil and test material geometry. Followingsections will present impedance diagrams for tubes and solidcylinders. For simplicity a fill-factor of unity will beused.

-115-

7.3.1 Solid Cylinders

The Impedance diagram for a solid cylinder (diameter, Do)inside a long coil is shown in Figure 7.6. As in Figure 7.5an increase in test frequency or diameter moves the operatingpoint (the point on the impedance diagram that specifies thenormalized inductive reactance and resistance of the testcoil) down the curve while an increase in resistivity movesit up the curve. This diagram applies to both wires andround bars.

UJ

I

ACT

2

IDU

CT

IVE

_

RH

ALI

ZEC

i

\ \

Ai

11|

A1

i

' \

\

/

/

\

i \ \ -

//INCREASI

f

\iCOIL

INCREASING RESISTIVITY

DECREASING FIU-MCT0R

NORMALIZED RESISTANCE

Fig. 7.6: Impedance Diagram for a Solid Cylinder

The shape of impedance diagrams for cylinders differ markedlyfrom a semicircle, particularly at higher test frequencies.The shape difference is due to skin effect and phase lag,factors which were not included in arriving at thesemicircular shape in Chapter 3. At high test frequenciesthe curve approaches the X and Y axes at 45°.

In testing cylinders with an encircling coil it should berecognized that sensitivity to defects at the centre of baror wire is zero, regardless of test frequency. The reasonfor this is illustrated schematically in Figure 7.7 which

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LOW FREQUENCY » > J!°4

INTERMEDIATE FREQUENCY 8 ---£-

HIGH FREQUENCY 8 <-

Fig. 7.7; Schematic of Eddy Current Distribution in aCylinder Surrounded by an Encircling Coil

shows plots of eddy current density across a cylinder.Defects have to disrupt eddy current flow in order to affectprobe impedance. It is apparent from Figure 7.7 that eddycurrent density is always zero at the centre of a cylinderresulting in no sensitivity to defects.

7.3.1.1 Sensitivity in Centre of a Cylinder

It was stated in the previous section that eddy currentdensity in the centre of a cylinder is zero and hence thereis no sensitivity to defects. The relationship of currentflow with depth into a cylinder is derived (veryapproximately) below, for the case of no skin depthattenuation and long coils. From Faraday's Law,

The magnetic flux density, B, is approximatelyconstant inside a long coil, hence

<j> - BA

- <B)(irr2)

where r is radial distance from centre of cylinder;

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therefore,

\ • -»«2 If

orV oc r '

s

Resistance to flow of current is proportional to path lengthand resistivity and inversely proportional to cross-sectionalarea, Ac,

_ 2trrp m 2irrps Ac unit length x unit depth

or Rs « r

Since by Ohm's Law

and Z - /B.23 + (iuL)2 " Rs

skin depth effect,

therefore,V

i - =* -

at low test frequency and no

or

Therefore, eddy current flow is proportional to radialdi-stance from centre of a cylinder. Hence no current flows atthe centre (at r-0) and there is no sensitivity to defects.

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7.3.2 Tubes

The impedance diagram for an extremely thin-wall tube witheither an internal or external circumferential coil is asemicircle. This shape is only obtained when wall thickness,t, is much less than skin depth (t <<<5 ), i.e. skin effect andphase lag are negligible. This situation will rarely beencountered In practice, especially at intermediate and hightest frequencies, but the concept is useful since it definesone of the coil Impedance limits.

With an external coil the other limit is defined by theimpedance curve for a solid cylinder (maximum possible wallthickness). The impedance diagram for any tube tested withan external coil,hence,has to lie between the two brokencurves in Figure 7.8, for example the solid line applies to

s

ENCIRCLING COIL

°0 D|

CYLINDER <D| * 0)

TUBE (Oj /D, = 0.8)

THIN WALL (D|KD0)

DECREASING «ALL THICKNESS

N0RUALI2ED RESISTANCE

Fig. 7.8; Impedance Diagram for a Tube with Encircling CoilShowing Effect of Decreasing Wall Thickness

a tube with internal diameter 802 of the outside diameteri.e., I>i/D0 - 0.8. Tubes with Dj/D,, greater than 0.8would lie to the right of the solid line. The dotted linesin Figure 7.8 trace the shift in operating point as wallthickness decreases (Do constant, Dj increasing). Notethe spiral shape of the wall thickness locus. The thick wallend of the curve deviates from a semicircle locus.

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This is attributed to phase lag across the tube wall andforms the basis for eddy current signal analysis which willbe treated in detail in Chapter 8.

Figure 7.8 also illustrates the dependence of the terms"thick-wall" and "thin-wall" on test frequency. Near the topof the diagram (low frequency) a tube with D^/DQ - 0.8qualifies as thin wall, there is no phase lag across the tubewall, t <<6. Near the bottom (high frequency) the same tubebecomes thick-wall because thickness becomes much greaterthan skin depth, for eddy current purposes the tube nowappears as a solid cylinder.

When a tube is tested with an internal circumferential coilthe impedance diagram for a thin-wall tube remainssemicircular but that for a thick-wall tube differs markedlyfrom a solid cylinder; compare Figures 7.8 and 7.9. The

as

THICK WALL TUBE (D|«D0)

TUBE (D|/0o = 0.8)

TUBE (D|/Do = 0.9)

THIN MLl

DECREASING MLL THICKNESS

NOHMLIZE0 RESISTANCE

Fig. 7.9: Impedance Diagram for a Tube With Internal CoilShowing Effect of Decreasing Wall Thickness

impedance locus for any given tube will again fall betweenthe dashed curves at intermediate frequencies and approachthe thin-wall curve at low frequency and the thick-wall curveat high frequency as shown for tubes with Dj/D0 ~ 0.8 and0.9. As in the previous figure, a change in wall thicknessproduces a coil impedance change along the dotted linestracing a spiral shaped curve. Again, this departure from asemicircle is attributed to phase lag across the tube wall.

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7.3.3 Characteristic Frequency for Tubes

Section 5.6 described how the Characteristic ParameterPc" "r2cjycf, introduced by Deeds and Dodd, enabledpresentation of the effects of changes in T, w , y anc* a on asingle impedance diagram. This allowed test coil impedanceto be specified in terms of a single quantity rather thanfour independent variables. One could use this parameter intesting cylinders and tubes. However, most eddy currentliterature refers to a similar variable, the characteristicor limit frequency, fg usually attributed £o Forster.It differs from 1> because probe radius, r, is replaced withtube or cylinder dimensions.

By definition, fg is the frequency for which the Besselfunction solution, to Maxwell's magnetic field equations fora finite test object, equals one. (Bessel functions aresimilar to, but more complex than trigonometric sine andcosine functions). For a solid cylinder or thick-wall tubetested with an encircling coil,

* - 5.07cg ,,2 , kHz

^r Do <7.2a)

with p in microhm-centimetres and D Q in millimetres.

For a thick-wall tube with an internal coll,

» kHz(7.2b)

For a thin-wall tube with internal or externalcircumferential coils,

•»• ( 7 . 2 c )

The ratio f/fg defines the operating point on impedancediagrams. For non-magnetic materials (yr*l), frequencyratio for cylinders and thick-wall tubes tested with externalcoils is given by

( 7. 3 a )

where f is test frequency in kilohertz.

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For a thick-wall tube tested with an internal coil,

f/fg - fD*/5.O7p ( 7. 3 b )

For thin-wall tubes tested with internal or external coils,

(7.3c)f/f - fD.t/5.07p

© 3.

THICK-(ALL TUBE (INTERNAL COIL)t/lg = f 0 | » / 5 .07 />

SOLID CYLINDER (EXTERNAL COIL)M , = tB0'/i.mP

THIN-WALL TUBE(INTERNAL ( EXTERNAL COILS)f/r, = to, t /s.07/3

0.2 0.4 0.6

NORMALIZED RESISTANCE

Fig. 7.10; Impedance Diagrams for Tubes and Rods with LongColls and Unity Fill-factor Showing Variation of f/f_ AlongImpedance Loci -S-

Figure 7.10 shows impedance diagrams for thin-wall tubes,solid cylinders and thick-wall tubes with values of f/fg(from 0 to infinity) on the curves. The impedance plots areboth different in shape and have drastically differentf/fg ratios. For example, at the "knee" in the curves athin-wall tube has f/fg "1, for a cylinder f/fg»6 and athick-wall tube has f/Fg» 4. These differences originatein the defining equations which contain Do , D/ andD^t. To find the operating point on an impedance diagramusing frequency ratio one has to know the geometry (tube orcylinder). For tubes which do not satisfy the conditions for

-122-

elther thin or thick wall, calculation of f/fg i8 notpossible except near the top and bottom of impedance diagramswhere curves for intermediate wall tubes converge with thethin-and thick-wall curves, respectively.

In addition to defining operating point, frequency ratio canalso be used for extrapolation or scale modelling using thesimilarity condition* this condition states if two objectshave the same f/fg then eddy current distribution isidentical in each. Hence if test frequency f} meets testrequirements for article No. 1, one can calculate f£ forarticle No. 2 from the following:

For cylinders,flDolP2

for thin-wall tubes,

" f2Di2t2Pl

and for thick-wall tubes (internal inspection),

f l D i l P 2 * f2Di2<>l

7.3.4 Computer Generated Impedance Diagrams

As indicated in the previous section, exact analyticalsolutions (BesBel function solutions) for impedance loci oftest coils around or Inside tubes are only possible forlimiting cases. These solutions have the additional drawbackthat they are only strictly true for long coils. Analternative was made available by C.V. Dodd and hisco-workers(£) at Oak Ridge National Laboratories. Theydeveloped computer programs to calculate coil impedance.These are valid for all coll lengths, internal and externalcoils and all tube wall thicknesses. Such computer programspermit paper experiments to define operating point as well atthe effect of variations in coil size and shape, resistivity,wall thickness and test frequency.

Figure 7.11 is an example of computer generated impedancedisplay for a short Internal coil in an Inconel 600 tube atvarious test frequencies. Fill-factor and the effects ofsmall changes in resistivity (Ap), wall thickness (At) andmagnetic permeability (Ay) were examined at each frequency.Note the similarity with the impedance plots of Figure 6.5obtained for a surface probe. The angular (phsse) separationbetween fill-factor, Ap , At and Ay provides the basis foreddy current signal analysis which will be treated inChapter 8.

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25 kHz

TUBE': Do = 12.7 m

D| = 10.2 mm

t = 1.J5 Hi.p - 100

1.00k-

0.18 .

0.92 -

O.H _

O.H .

0.10

o.n o.oaNORMALIZED RESISTANCE

0.12

Fig. 7.11; Computer Simulation of Probe Response to VariousTest Parameters

7.4 CHOICE OF TEST FREQUENCY

Test frequency is often the only variable over which theinspector has appreciable control. Material properties andgeometry are normally fixed and probe choice is oftendictated by test material geometry and probe availability.Choice of a suitable test frequency depends on the type ofinspection. Testing for diameter variations normallyrequires maximum response to fill-factor which occurs at highfrequencies. Testing for defects requires penetration topossible defect locations; surface defects can be detected athigher frequencies than subsurface defects. Maximumpenetration requires a low frequency which still permitsclear discrimination between signals from harmless variationsin material properties and serious defects. The abovefactors show choice of test frequency is usually acompromise.

7.4.1 Test Frequency for Solid Cylinders

As discussed in Section 7.3.1, the sensitivity at the centreof a cylinder, with an encircling coll, is zero at all testfrequencies. Therefore, there is no advantage in using avery low test frequency to increase penetration.

Maximum test sensitivity is obtained when the impedancediagram operating point is near the knee of the curva. This

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7.4.2 Test Frequency for Tubes

IIcondition occurs when f/f„ = 6. At this point balanced

sensitivity to defects, resistivity and dimensions isobtained. At this test frequency, DQ/6 ^3.5. Increasing _•the frequency ratio f/fg to 15 or 20 improves discrimination Vbetween surface defects and fill-factor variations (probewobble), at the expense of reduced sensitivity to subsurfacedefects. Maximum sensitivity to diameter variations is Mobtained at higher test frequencies, f/fg * 100 or more. §;

A frequency ratio lower than 6 will result in a decrease in _.phase lag and therefore less phase discrimination between •defects and fill factor. To distinguish between ™ferromagnetic variations (or inclusions) and defects, theoperating point should be on the top quadrant of theimpedance diagram. A frequency ratio of approximately two(f/fg - 2) would achieve this. I

IWhen inspecting tubes for defects, the criterion to satisfyis (a) phase discrimination between defect signals and other Bindications and (b) good phase separation between internal J|and external defect signals. A test frequency, proven inpractice on many types and sizes of tubes, is the frequencyf nn which yields 90° phase separation between fill-factor Bvariations (and internal defect signals) and external defect Isignals. The frequency f 90 is empirically derived from theratio between thickness and skin depth, slightly larger than Wone, £

t/6 = 1.1

and converts to2

fg0 = 3p/t . kilohertz (7.4)

where p is resistivity in microhm-centimetres and t is tube I!wall thickness in millimetres. This equation is valid forboth internal and external coil inspection and is roughly •-independent of tube diameter. At f 90 , there is good ff|sensitivity to internal and external defects and littlesensitivity to magnetite deposit and ferromagnetic supportplates. If

I?The characteristic frequency ratio f/fg cannot be used tosatisfy the criterion of phase discrimination, because the ••fg equation is not a function of phase lag. It would also Ifbe wrong to use it for defect detection because it is a '-function of tube diameter. The latter would requiredifferent test frequencies for different diameter tubes to Ifkeep f/fg constant. ft

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7.5

If one desires to distinguish ferromagnetic signals fromother indications, the operating point should be on the topquadrant of the impedance diagram for thin-wall tubing,Figure /.10. This point is located by calculating the testfrequency to make the characteristic frequency ratio equal toor less than 0.5 (f/fg <0.5).

Inspection Standards and Specifications

A number of industrial codes cover eddy current tubeinspection. The various ASTM specifications are E-215(aluminum alloys), E-243 (copper and copper alloys), E-426(stainless steels) and E-571 (nickel alloys). None of theASTM standards specify test frequencies, they sometimespresent normal ranges such as 1 to 125 kHz for aluminumalloys. Such numbers are of little use in deciding on asuitable test frequency for a particular test. The ASMEBoiler and Pressure Vessel Code, Section V, Article 8 (1980)specifies test frequency in terms of the angle betweenthrough-wall and external defect indications from acalibration tube. The procedure specified will normallyyield a frequency higher than fgo, perhaps as high as 2fgg .

Most calibration tubes consist of drilled holes of variousdiameters and/or various depths from the external surface.Some calibration tubes have EDM (electric dischargemachining) notches in the circumferential and axialdirections and on both internal and external surfaces.

PROBES FOR DETECTING CIRCUMFERENTIAL CRACKS

A conventional internal circumferential (bobbin) probeinduces a flow of eddy currents parallel to the coil windingsand therefore circumferential in direction (Figure 7.4). Tosense a defect, coil impedance must change; this will occuronly if the eddy current flow path is disturbed.Circumferential defects parallel to this current, whichpresent no area perpendicular to this path, will thereforenot be sensed.

(a) (b)

Fig. 7.12: (a) Probe No. 1- Multi-pancake Coil Probe(b) Probe No. 2 - Zig-zag Coil Probe

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To detect circumferential defects the coil must inducecurrents at an angle to the cracks. Two possible types ofprobes are (a) surface probes and (b) zig-zag probes. Figure7.12 shows examples of such probes. The surface probeinduces currents in a circular pattern whereas the zig-zagprobe induces currents to follow the 30° coil angle. Theprobes shown in Figure 7.12 are differential. In the surfaceprobe configuration a multi-coil array is used; the foursurface coils in each row are connected in series and the tworows are connected differentially. A single absolute surfacecoil can also be used, provided the probe maintains contactwith the tube surface by spring force or other means(otherwise lift-off noise would be intolerable). See Figure7.13 for the cross section of a typical spring-loadedinternal probe for tube testing.

C « L E TUBE IHI P u m i cCONNECTOR **LL B 0 D y

/REFERENCE SPRING

COIL

Fig. 7.13: Spring Loaded Internal Surface Probe forTube Inspections

A single surface probe is unquestionably the easiest to use;signal analysis is discussed in Chapter 6. The maindisadvantage is the partial circumferential coverage;multiple passes or helical scanning are necessary for 100 51coverage. Another disadvantage of the surface probeconfiguration (single or multiple) is the loss of sensitivitywith distance from the coil. If surface coils are small, aswill be the case for most tube inspections, the reduction insensitivity with distance from the surface will be greaterthan with circumferential coils, see Section 5.3.1. Thesensitivity to small localized defects originating from theoutside surface could be as much as 10 times lower than thesensitivity to internal defects. A zig-zag coil has lessattenuation to outside defects, it falls into thecircumferential class in this respect. Neither zig-zag norsurface coil probes will give uniform sensitivity aroundtheir circumference. There will be peaks of maximum andminimum sensitivity depending en the angle between eddy

-127-

current path and defect orientation. This can best bevisualized by considering a short circumferential crackpassing over the coils: there will be areas, such as at thepeaks of the zig-zag, where eddy current flow is almostparallel to the crack, resulting in poor sensitivity.

Figure 7.14 shows examples of signal response to realcircumferential fatigue cracks with the probes discussedabove.

(a) MULT I-PANCAKECOIL PROBE —41U

( b ) ZIG-ZAGCOIL PROBE

WVlr

( e ) BOBBINCOIL PROBE

yv*

Fig. 7.14; Eddy Current Scans of Circumferential Cracks inInconel Tubing (Signal Amplitude Normalized to a 1.6 mmDiameter Through Hole), f - 400 kHz.

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7.6 SUMMARY

Test coils induce eddy currents and also sense the distortionof their flow caused by defects. Encircling or bobbin probeshave test coil(s) mounted with their axes parallel to the tubeor rod axis. Since the coils are wound circumferentially theinduced eddy currents also flow circumferentially. They cannotbe used to detect circumferential cracks, laminar defects, nordefects in the center of a rod.

As a general guideline for tube inspection, probe coillength, depth, and spacing (if differential) shouldapproximately equal wall thickness.

An absolute bobbin probe (single test coil) should be usedfor general in-service heat exchanger inspection. However,for short localized defects, differential probes (two testcoils side-by-side) are normally preferred.

Analysis of eddy current signals is the most important andunfortunately the most difficult task in a successfulinspection. A thorough understanding of impedance diagramsand effect of phase lag is needed to manipulate testconditions to minimize undesirable test variables. TheCharacteristic Frequency for tube inspection is used tolocate the operating point on the impedance diagram. It isgiven by

f - 5.07p/Dt kHz (7.2c)g

where p is electrical resistivity and D is tube internaldiameter (for bobbin probe) and external diameter(for encircling probe); t is tube wall thickness.

One needs to know the operating point on the impedancediagram to determine effects of fill-factor, electricalresistivity, and magnetic permeability. The optimumsensitivity to fill-factor is near the bottom of theimpedance diagram, in the middle for electrical resistivityand at the top for magnetic permeability•

When inspecting tubes for defects, criteria to satisfy are(a) phase discrimination between defect signals and other

-129-

indications and (b) good phase separation between Internaland external defect signals* For general purpose testing thefrequency given by

- 3p/t' kHz (7.4)

7.7

Is used where t Is wall thickness In mm. This frequencyyields 90° phase separation between Internal and externaldefect signals and little sensitivity to magnetic deposit!and ferromagnetic support plates.

Special probes are needed to inspect for circumferentialcracks or defects close to tubesheets. Single, springloaded, surface probes are effective.

WORKED EXAMPLES

7.7.1 PROBLEM:

SOLUTION:

Calculate frequency to operate at the kneelocation of the impedance diagram for a cylinder5 mm in diameter and electrical resistivityp m io microhm-centimetres.

(7.3a)

therefore6 x 5>O7 x 10

S2

12 kHz

7.7.2 (a) Calculate the test frequency to inspect InconelPROBLEM: 600 tubing with D± = 10.2 mm, t - 1.1 mm and

p * 98 microhm-centimetres.

SOLUTION: Best test results are obtained when there issufficient phase separation between internal andexternal defect signals. A phase separation of90° allows good discrimination between the two andreasonable defect depth estimates. To achieve 90°phase separation, the test frequency is determinedby

90 t2

(derived from t/<5

_ 3 x 98

(l.l)2

(7.4)

1-D

245 kHz

Therefore 245 kHz is the required frequency.

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7.7.2 (b)PROBLEM

SOLUTION:

Determine the approximate operating point on theimpedance diagram, for problem (a).

Since t/6 =1.1 this tube cannot be consideredthick or thin walled. Therefore, neither equation7.2(b) nor 7.2(c) is strictly valid. However,for t/6 > 1, equation 7.2(c) for thick-wall tubingwill yield an approximate solution.

7.7.2 (c)PROBLEM

SOLUTION:

f/fg - fD^/5.07 p

= 245 x 103(10.2)2/5.07 x 98

= 51.3

(7.3c)

This would place the operating point on the lowerquadrant (much lower than the knee location) ofthe thick-wall curve of Figure 7.10.

Calculate a test frequency for the above tubesuitable for discriminating between ferromagneticInclusions and defects, when testing with aninternal probe.

The operating point should be on the top quadrantof the impedance diagram for thin-wall tubing,Figure 7.10. This point is located by calculatingthe test frequency to make the ratio of Forster'scharacteristic frequency equal to or less than0.5.

f/fg - fD1t/5.07p

- 0.5

(7.3b)

therefore

f - (0.5)(5.07p)/D±t

- 0.5 x 5.07 x 98/10.2 x 1.1 = 22 kHz

Therefore, at 22 kHz (9% of fgQ ), it should bepossible to discriminate between defects andferromagnetic indications.

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CHAPTER 8 - TUBE TESTING - SIGNAL ANALYSIS

8.1 INTRODUCTION

Manufacturing and in-service inspection of tubes is one ofthe most important applications of eddy current testing. Forin-service inspection of small-bore tubing in particular,eddy current is by far the most frequently used method.Access is usually limited to tube ends which makes other NOTtechniques difficult or impossible to apply.

This chapter emphasizes in-service testing of tubes usinginternal probes. This approach is taken because testing ofsolid cylinders and tubes with external coils (manufacturinginspection) is generally less complicated. If the readerunderstands in-service inspection he should encounter noproblems applying similar principles to other tests ituations.

Reasons for the appearance of impedance plane eddy currentsignals are presented first. Repetition from previouschapters is intentional, it was desired to keep this chapteras independent as possible without excessive cross-referencing. Discussion of simple defect indications isfollowed by superimposed signals which are frequentlyencountered during in-service inspection such as defects atbaffle plates and tubesheets. A section dealing with surfaceprobe internal tube inspection is included, difficult testsituations have been resolved with this technique. Signalswhich could be mistaken for real defects (anomalousindications) are the subject of another section. The chapterconcludes with a discussion of multifrequency testing,including its advantages and limitations.

An attempt is made throughout this chapter to illustratediscussion with real or simulated eddy current defect

• signals.

8.2 EDDY CURRENT SIGNALS

8.2.1 Defect Signal Characteristics

A defect, which disrupts eddy current flow, changes test coilimpedance as the coil is scanned past the defect. A non-rigorous derivation of this effect can be obtained usingFigure 8.1 which portrays eddy currents induced in a tube witheither an internal or external coil. Consider & unit lengthof tube as being the secondary winding of a transformer(similar to treatment in Chapter 3). The resistance of aconductor of length H, cross-sectional area A and resistivity Pis

R = £p/A, ohms

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Wlthout a defect, resistance around this tube is

2ir7p/t (8.1a)

Introduction of a long defect, of depth h, which constrictseddy current flow over the distance A© (In radians),increases total resistance to

R • 2irrp/t + A9hrp/t(t-h)

or R - Ro (defect free resistance) +

(8.1b)

(due to defect).

Fig. 8.1; Schematic Illustration of Eddy CurrentDistribution Around a Defect in a Tube

A short defect will also increase resistance but by a smallerAR since current can flow both under and around it. Notethat it is width of affected zone, A6 , rather than actualdefect width which determines effect of the defect onresistance. In summary, the above argument illustrates thatdefect length, depth and width (to some extent) all increaseresistance to current flow and hence defect signalamplitude.

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In terms of the equivalent coil circuit of a resistor inparallel with an inductor and its associated semicircularimpedance diagram (Chapter 3), a defect moves the operatingpoint up the impedance diagram. Increasing resistance in aspecimen changes both probe inductance and resistance.

The above discussion does not predict a defect signal indetail, only its approximate amplitude and direction on theimpedance diagram. A more complete explanation requiresinclusion of phase lag. Consider an absolute coil around acylindrical sample as in Figure 8.2(a). (The treatment for adifferential coil would be similar but more complicatedbecause the twin coil configuration generates two mirrorimage signals and cross-coupling between the two coils causesfurther complications). Figure 8.2(a) shows the distributionof magnetic field amplitude and phase around a coil asderived by Dodd(£). The solid lines are contours of constantmagnetic field strength; the dashed lines are constant phase.Since magnetic field and induced eddy currents have about thesame phase, the dashed lines also represent the phase of theeddy currents. Similar diagrams could be derived for coilsinside or around tubes. Amplitude drops off exponentiallywith distance and eddy current flow increasingly lags inphase (relative to eddy currents adjacent to the coil) bothwith depth and with axial distance from the coil. Skin deptheffect occurs in both radial and axial directions.

Figure 8.2(a) permits derivation of eddy current signals forthe surface, subsurface and deep defects illustrated. Oneneeds to establish a reference phase direction as startingpoint, the fill-factor direction is convenient and can bedefined as the signal resulting from a very shallow surfacedefect which only decreases coupling without changing phaselag distribution. Hence choosing the phase contour whichjust touches the surface under the coil as the 0° contourfixes fill-factor direction as in Figure 8.2(b). The signalor effect of defects can be imagined as the absence of eddycurrents which were flowing in the area before thedefect existed at this location. On moving the coil (ordefects past the coil) from positions 0 to 5 in Figure8.2(a), one observes the change in amplitude and phasesketched in Figure 8.2(b). This procedure is reasonablystraight forward for the surface and subsurface defects sincethey are localized and only intersect one phase and amplitudecontour at any given position. For the deep defect, one hasto divide the defect into sections and determine weightedaverage values for amplitude and phase at each position.

The surface defect in Figure 8.2(b) has a large fill-factorcomponent, primarily its approach signal makes itdistinguishable from fill-factor. As defect depth increases,signals rotate clockwise due to increasing phase angle.

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CUOTWT WFLiniDE

DEEP OEFtCf

S M L l O i DEFECT —

SUI1UHMCE DEFECT

(a)

1 2 3 4DEFECT rOSITIDH

(b)

S U B S U R F A C E

D E F E C T < X 2 J

FILL-MCTOR

Fig. 8.2: Derivation of Eddy Current SignalAppearance for Three Types of Defects

-135-

The angle between fill-factor and defect signals in Figure8.2(b) is about 2 3 , where S = x/6. Although probably notstrictly true, one can imagine defect signal phase angle asthe sum of a lag of & from the coil to the defect and thesame lag back to the coil.

8.2.2 Effect of Tast Frequency

We can now combine Figure 8.2 results with impedance diagramsfrom Chapter 7 to illustrate the effect of test frequency ondefect signal appearance. Figure 8.3(a) shows part of Figure7.9, the impedance diagram for a tube with D i/D o =» 0.8tested with a short internal coil. The dotted lines tracethe impedance change with decreasing D Q. An externaldefect (0D defect) in a tube is essentially a decrease inDQ with D^ held constant, therefore the dotted linestrace the change in impedance as a coil is scanned past an ODdefect. Note the similarity between the subsurface defect inFigure 8.2(b) and the OD defect at 2 f90 in Figure 8.3(a).The display is normally rotated counter-clockwise to make asignal from fill-factor approximately horizontal. This isachieved by rotating the phase control knob on the eddycurrent instrument.

NORMALIZED RESISTANCE

Fig. 8.3(a): Relation Between Impedance Diagram and DefectSignal Appearance

-136-

With this phase setting and at fgg an OD defect shows wallloss (+Y) In a tube without a change In fill-factor as InFigure 8.3(b). An ID defect consists of wall loss (+Ycomponent) as well as a large fill-factor (-X component)because of decreased coil/tube coupling. The through-walldefect (hole) signal contains elements of both ID and ODdefects and hence yields a signal which falls between thetwo. Note that all defect signals must fall betweendecreasing fill-factor and OD defect signals.

OD DEFECTTHROUGH-WALL

DEFECT

ID DEFECT

-X -*- -•- +X

DECREASING FILL FACTOR

]

-Y

Fig. 8.3(b): Defect Signal Appearance at fg.Q

Figures 8.3(a) and 8.4 show what happens to defect signalswith changing test frequency. Reduced frequency results inrotation of defect signals towards the fill-factor direction.At very low frequencies (less chan f 90 M ) signals fromdifferent types of defects become difficult to distinguishdue to small phase angle separation.

-137-

Increasing test frequency increases phase separation betweenID and OD defect signals as predicted by phase lag. At fgQ

the ID and OD defect signals are separated by about 90° withlow sensitivity to tube supports and external deposits. Athigher test frequencies, 2 fgQ and above, higher sensitivityto probe wobble and dents is obtained and the increasedangular separation of defect signals makes it difficult todiscriminate between OD defects and probe wobble or fill-factor variations, see Figure 8.4(c).

ID DEFECT

(0) \ f, (b) f 9 0

(c) 2 f 9 0

Fig. 8.4: Appearance of Calibration Defect Signals atDifferent Test Frequencies

-138-

8.2.3 Calibration Tubes and Simple Defects

Both manufacturing and in-service inspection require calibra-tion tubes with artificial defects for initial instrumentset-up and subsequent signal analysis and Interpretation.These tubes should be identical in material and size tc tubesto be tested. Minimum calibration requirements include ID,OD and through-wall defects (see also the ASTM and ASME codescited In Section 7.4.2). Vox in-service inspection, expectedsignal sources such as baffle plates, magnetite deposits anddents are useful and often essential for reliable signalanalysis. Figure 8.5 shows typical signals, at ft)o . from acalibration tube suitable for in-service heat exchangerInspection. Both absolute and differential probe signals areshown. The 90° phase separation between ID and OD defectsalso exists for differential probes. Note the similaritywith the signals derived in the previous section.

STEELSUPPORT PLATE

OUTSIDEGROOVE

INSIDEGROOVE

THROUGHHOLE DENT

12.7 mm

PROBE9 = 98 uSl- cm

OUTSIDE

OUTSIDE

DECREASINGFILL FACTOR

HOLE « _ ^

INSIDE ^ \

cSUPPORTPLATE

ABSOLUTE

1 PROBE1 WOBBLE

/ / DENT

MAGNETITE

s*^—-*T/SUPPORT VPLATE

DIFFERENTIAL

Fig. 8.5; Eddy Current Signals from a Typical CalibrationTube. Test Frequency fQn • 250 kHz.

Qualitative reasons for the appearance of ID, OD and through-wall defects were presented in Section 8.2.2. The othersignals in Figure 8.5 can be explained in a similar fashion.Magnetite is a ferromagnetic non-conductor, its signal is due

-139-

to its high permeability. As indicated in Figure 7.11increasing permeability of tube material yields a signal whichfalls between OD and through-wall defects. The magnetitesignal in Figure 8.5(b) is essentially such a signal rotatedabout 90° clockwise because of phase lag across the tube wall.A dent places tube material in closer proximity to the coilresulting in improved coupling (increased fill-factor) andhence yields a signal opposite to decreasing fill-factor.Probe wobble yields a signal very close to the fill-factordirection because radial displacement of the coil reduces thecoupling to the tube. The reason for baffle plate signalappearance is due to a combination of factors. For carbonsteel baffles, the effects of high magnetic permeability andintermediate resistivity partially cancel resulting in smallsignal amplitude. Phase lag across the tube wall rotates thissignal clockwise.

5* ID GROOVE 10* OD GROOVE 1.6 m2.5 mm WIDE 2,5 mm WIDE HOLE

0.25 mi

DENT

V CHANNEL

JCARBON STEEL

SUPPORT

<r

XTDISTANCE

X CHANNEL

Fig. 8.6: Appearance of Quadrature Components on a ChartRecording for a. Calibration Tube

In eddy current tube testing one normally records the quadra-ture components (vertical, Y; horizontal, X) of coil impedanceon a two—channel strip chart recorder as shown in Figure 8.6.With phase adjusted as shown, any real defect will exhibit aY component. The X-channel information is required for detail-ed signal analysis to decide type and depth of defects which

-140-

can only be performed reliably through phase analysis.Accurate phase analysis can be done on-line by monitoring thesignals on an eddy current instrument storage monitor.Alternatively an X-Y recorder or similar device permits hard-copy storage of quadrature signals.

A flaw indication on an X-Y monitor is normally a curvedlocus; it does not have a simple and unique phase angle. Ifan absolute probe is used the significant angle to measure isthe tangent angle at the defect signal tip, see Figure8.7(b). If a differential probe is used, the phase angle isthe slope of the straight line joining the end points of the"figure-8" signal, see Figure 8.7(c). Figure 8.7(a)illustrates the change in phase angle with defect depth.This curve should be used only as a guide since defect signalphase angle can change with defect and probe geometry.

UJ

o

onw

90

80

70

60

5040

30

20

10

00

I .D .DEFECTS .

/

/

1- 1- /

/" /- //

1 1

10 20

O.D.DEFECTS

\

\ -\

\

\

I I I i

30 40 50 60

\

\

\\

70 80 9C

0.0 . DEFECT

THROUGH 1

^ V 1

1.0. DEFECT r T̂

( b ) ABSOLUTE

THD0U6H

g f ^ v ^ V ^ \ V , O.D. 0EFEC1

1.0. DEFECT l \ > ^\ \

, (c) DIFFERENTIAL

SIGNAL PATTERN PHASE ANGLE ( 0 ) , DEGREES

(a)

Fig. 8.7: Eddy Current Phase Angle/Defect Dept.t CalibrationCurve at fqn

-141-

When an eddy current signal source is located it is oftenuseful to retest at other frequencies to confirm a defectexists and/or to improve depth estimate. Defect depth isestimated from signal pattern recognition and verified bycomparison with calibration defect signals at various testfrequencies. Normally, frequencies of one-half and twiceare sufficient. However, to check for magnetic deposits orinclusions a frequency of one-tenth fgg or less may berequired (see Sections 7.4.2 and 8.3.1). Figure 8 . 'i showseffect of changes in frequency on calibration signals.Increasing test frequency increases phase separation betweenID and OD defects as predicted by phase lag. It alsoincreases sensitivity to probe wobble and dents but lowerssensitivity to tube supports and external deposits. Onemight question the validity of comparing machined holes andgrooves in calibration tubes with real defects to estimatetype and depth. The following examples justify thisapproach.

Figure 8.8 shows external corrosion in a copper tube. Attackis general but non-uniform with localized severe pitting.An absolute internal probe was used to obtain signals fromartificial defects and three of the localized pits. Thephase angle of the first two corrosion indications shows theyare OD defects, comparison with the calibration defect led toa depth estimate of 25 to 50%. Independent mechanicalmeasurement found deepest penetration to be 50% for bothdefects. The third defect has a noticeably different phaseangle from the first two. It approaches the angle for athrough-wall hole, hence its depth was estimated to be 50 to75% (actual measurement yielded 75%).

1.6 mmCALIBRATION HOLE

DEFECTS ,„. ,„ECCENTRIC

GROOVE

CORROSION

DEFECTS

F i g . 8 . 8 : E x t e r n a l C o r r o s i o n i n a C o p p e r Tube ( D o = 1 5 . 9 mm,f l . O mm, fqp = 5 . 3 kHz)

-142-

An example of stress corrosion cracking (SCC) in Type 316stainless steel, from a heavy water plant heat exchanger, isshown in Figure 8.9. The crack extends nearly half wayaround the tube. Phase angle of the crack signal shows itextends through the tube wall. Since the eddy currents flowparallel to coil .windings, circumferentially, tht large cracksignal is due entirely to the component of the crtck alongthe tube axis. The intergranular, branching nature of SCCgenerally permits their detection. Since a defect must dis-rupt eddy current flow to be detectable, if circumferentialcracks are suspected, fatigue cracks for example, specialprobes are required, see Section 7.5 and 8.2.5.

30 40 50 60 70 SO 90liliI.lililJihLl.iiiililiiilifililil.liLlJililiLiiLLluilil.lililil.iilililililililihl.Mililii.l.lil.linl 1.1,1.1,1.;,,

CRACK

SIGNAL

, 50% 0D•i.Z mm I CONCENTRICHOLE ̂ -* - ^ | GROOVE1 .6 mmHOLE

CALIBRATION

DEFECTS

Fig. 8.9: Stress Corrosion Cracking in Type 316 StainlessSteel Tubing (Dn =19.1 mm, t»1.8 mm, fqp =68 kHz)

8.2.4 Vectorial Addition and Defects at Baffle Plates

During in-service inspection of tubes in heat exchangers,tube supports (baffle plates) are frequently defect proneregions. Inspection for defects at baffles is possiblebecause eddy current signals are often vectorlally additive.This permits analysis of superimposed signals; the signalscan be (mentally or graphically) subtracted from the totalindication with resultant separated signals appearing similarto calibration defects. Vectorial addition provides thebasis for multifrequency eddy current testing (Section 8.4).

=143-

Figure 8.10 illustrates how signals from a steel baffle plateand an external groove are added to obtain a superimposedindication. The difference between the end points of thebaffle plate and baffle and groove signals equals theindication obtained from the groove by itself.

BAFFLE

RROOVE

OD fiROOVE

CARBONSTEELBAFFLE

Fig. 8.10: Vectorial Addition of Eddy Current Signals

Figure 8.11(a) shows a section of stainless steel tuberemoved from a power plant heat exchanger with part of thecarbon steel support plate still in place. The support showsconsiderable corrosion; originally there was about 0.25 mmclearance between the tube and the hole in the plate.Corrosion products have completely filled the gap leading tocrevice corrosion evident in Figure 8.11(b) which is asimilar tube with the plate removed. Calibration signals arepresented in Figure 8.11(c). The eddy current signal fromthe baffle plate region of Figure 8.11(a) is shown in Figure8.11(d). This seemingly simple signal is actually quitecomplex. The upward component is due to external pittingsimilar to that in Figure 8.11(b). The presence of a supportplate should result in -X, -Y signal components; in fact a+ X deflection is observed. This is the result of denting ofthe tube. Denting is circumferential constriction of tubesdue to compressive stresses exerted by baffle plate corrosion

-144-

products such as magnetite. The presence of magnetite canalso contribute to signal distortion particularly at low testfrequencies. Tube denting is of concern because, in additionto complicating eddy current signal analysis, it can lead tofurther tube damage such as stress corrosion cracking orthermal fatigue because tubes are no longer free to expandand contract during thermal cycling.

(a)

ID GROOVE

(c)

DEFECT

(d)

Fig. 8.11: Corrosion and Denting Under a Steel Baffle Plate(Dp -15.9 mm, t-1.25 mm, fqp = 80 kHz)

Another example of defects near a carbon steel tube supportis shown in Figure 8.12. These were obtained from a brass,thermal power plant condenser tube which sufferederosion/corrosion on either side of supports. This is thesame tube as in Figure 7.3. Defect signals from the baffleplate vicinity are so large the support signal is obscured.The main point of this example is the advantage of usingphase angle, rather than amplitude, to judge defect severity.Defect B with both differential and absolute probes has aphase angle approaching that of a through-wall hole, i.e., itprobably extends at least 75% through the wall. Defect A onthe other hand is vertical and hence is probably no deeperthan 50% even though it exhibits greater amplitude than B.

-145-

DEFECTSIGNALS

STEELBAFFLE

5R00V! 00GROOVE 1.6 mm

HOLE

(a) CALIBRATION DEFECT SIGNALS

(b)

ABSOLUTE DIFFERENTIAL

Fig. » 12: Quadrature Eddy Current Signals from the BrassTube In Figure 7.3

T.

To this point we have only considered ferromagnetic tubesupports, carbon steel is the material used in most heatexchangers. With magnetic baffle plates vectorial additionappears to apply for all types of defects. Unfortunatelydeteriorating water quality, denting problems and longer ser-vice life requirements have made it necessary to constructsome heat exchangers with non-ferromagnetic support plates.Vectorial addition of eddy current signals involving nonmag-netic supports is generally not valid. Several factors con-tribute to this situation, nonmagnetic supports yield muchlarger signals than magnetic supports. The large signal fromnonmagnetic baffle plates effectively reduces signal-to-noisemaking small defects more difficult to detect.

Possibly the most difficult defects to detect under non-magnetic supports are those of the same width as the plate,e.g., fretting wear from tube vibration.

Figure 8.13(a) illustrates such a situation, a brass baffleplate with a copper-nickel tube containing simulated 50%. deepfretting wear. The same defect with a magnetic baffle plateIs shown in Figure 8.13(b) for comparison.

Problems in detecting defects at non-magnetic supports cannot be overcome by employing a multifrequency eddy currenttechnique. The multifrequency approach relies on vectorial

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MAX1MUM GAP

50% ODECCENTRIC GROOVE

BAFFLE WITHMAXIMUM RAP

BRASS BAFFLEIN CONTACT

OD GROOVE

BRASSBAFFLE

(a)

MAGNETICBAFFLE

(b)

Fig. 8.13; Wear Under (a) Non-Ferromagnetic and (b)Ferromagnetic Baffle Plates

addition being valid (Section 8.4). Sensitivity can beimproved by employing special probes as will be shown inSection 8.2.6.

8.2.5 Tube Inspection at Tubesheets

Heat exchanger tubesheets are usually made of carbon steel,eddy current response should therefore appear similar to abaffle signal. In addition, a large fill-factor (tubeexpansion) signal is also obtained as a result of tubes beingrolled into tubesheets. Rolling eliminates corrosion pronecrevices and also helps hold tubes in the tubesheet. Withcarbon steel tubesheets, expansion usually yields the largestsignal component, the tubesheet only contributes appreciablyat test frequencies below f90 Figure 8.14 shows tubeconfiguration at a tubesheet and typical eddy currentsignals.

Occasionally one may encounter a tubesheet clad with acorrosion resistant alloy such as stainless steel orInconel. If the cladding is non-magnetic the samecomplications arise as with non-magnetic baffle plates(Section 8.2.4). Fortunately, most tubesheets are only cladon the primary side (near tube ends) where service relateddefects rarely occur.

-147-

END OF' ROLLED JOINT

121

EXPANSION SIGNAL

Fig. 8.14; Schematic of Tube Geometry at Rolled Joint InTubesheet and Associated Eddy Current Signals

The end of the rolled joint at the Inboard edge of a tube-sheet is a defect prone area because of high residual andservice stresses and also because deposits tend to accumulateat this location which can lead to corrosion. Eddy currentIndications with bobbin-type probes froa defects in thisregion can be difficult to Interpret because of excessivesignal distortion from tube expansion. Sensitivity may beImproved by employing a spring loaded surface probe asdiscussed in next section.

8.2.6 Testing Tubes with Internal Surface Probes

During in-service inspection of tubes, situations arise whereconventional circumferential probes (both differential andabsolute) prove inadequate. The case of circumferentialcracks was treated in Section 7.5. Surface probe designshave also been found to yield improved test results In thecase of defects at non-magnetic baffle plates and at heatexchanger tubesheets.

Surface probes have several advantages over bobbin-typeprobes. They can be made much smaller than tube diameter andhence sample a smaller volume of tube periphery, thisprovides inherently greater sensitivity to small defects .Spring loading of a surface probe against the tube walleliminates much of the fill-factor (lift-off) distortioncaused by tube expansion in tubesheets. The main drawback to

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surface probe tube testing is that a number of scans have tobe made for complete circumferential coverage. Conventionalprobes sample the entire tube in a single scan.

TUBESHEET END 0* 1NCONEL 600ROLLED JOINT TUBE WALL

TUBESHEET

JOINT

CONVENTIONALPROBE SURFACE

PROBE

Fig. 8.15; Comparison of Eddy Current Test Results in HeatExchanger Tubesheet Region with Conventional and SurfaceProbes (Cn -12.5 mm, t - 1.2 mm, fop -200 kHz)

Figure 8.15 illustrates surface probe testing at the tube-sheet region of a power plant steam generator. It comparessignals, from what is believed to be OD corrosion damage atthe end of the rolled joint, obtained with conventional andsurface probes. The reason for the characteristic A'B'C'surface probe signal is as follows. As the probe is with-drawn from the tube (direction of arrow) it encounters thestart of the expanded area. Failure of the probe to followthis contour exactly results in an increasing lift-offsignal, A'B', superimposed on the impedance change, A'C dueto the presence of the tubesheet. Both defect signals wereobtained from the same tube, note the considerable improve-ment in sensitivity obtained with the surface probe. Thistube was In fact leaking.

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IBRASS

BAFFLE

TUBE WALL507, OD

ECCENTRIC

GROOVE

CALIBRATION

BRASS BAFFLE

BAFFLE

(MAXIMUM GAP)

Fig. 8.16; Internal Surface Probe Testing for Fretting Wearunder a Non-Magnetic Baffle Plate. (Compare with Fig. 8.13Results)

A second example of improved sensitivity with an internalsurface probe involves fretting wear under non-magneticbaffle plates. Figure 8.16 shows results. Compare withFigure 8.13(a) which shows test results for the same defectobtained with an internal circumferential probe. With no gap,the 50% groove was barely detectable with a conventionalprobe,while Figure 8.16 shows this defect is easily detectedwith a surface probe.

8.3 ANOMALOUS EDDY CURRENT SIGNALS

Some eddy current signals can be mistaken for defect indica-tions; these are called false or anomalous signals. Theyarise because of the high sensitivity of eddy currents tomany variables and demonstrate tha need for thorough analysisbefore concluding that every eddy current signal represents adefect. The following examples illustrate more common oneswhich have been encountered in practice.

8.3.1 Ferromagnetic Inclusions and Deposits

Materials with relative magnetic permeability greater than1.0 affect eddy current response drastically. Skin depth andprobe inductance are both affected by permeability; permea-bility values of 50 to several hundred are typical.

-150-

Before citing specific examples consider the general approachto identifying signals from magnetic materials. Such signalscan be distinguished from real defects by reducing test fre-quency to move the operating point near the top of the impe-dance diagram. Figure 8.17 illustrates the procedure where1, 2 and 3 represent ferromagnetic material on the inside, inthe tube wall and on the outside respectively. It may bedifficult to achieve a sufficiently high operating point withsome instruments and probes when testing low resistivity,large diameter tubes. However, if a low enough frequency isachieved, real defect Indications will fall nearly parallelto fill-factor whereas high permeability indications arenearly perpendicular to fill-factor. At 240 kHz (fgrj) l n

Figure 8.17, 1 and 2 could easily have been mistaken for IDdefects. There is no confusion at 10 kHz since it is knownthat all defect indications must fall between fill-factor andan 0D defect signal. The following two examples demonstratethe procedure to discriminate false defect (ferromagnetic)indications.

0 DGROOVE

I DGROOVE

FERROMAGNETICANOMALIES

© © ©1.1 mmt

12.7

ABSOLUTE PROBE INCONEL 600 TUBE

0.05 0.10 0.19RL

I.D.

©I®

DECREASING FILL FACTOR

O.D.

DECREASING FILL FACTOR

NORMALIZED RESISTANCE,uL.

Fig. 8.17; Coil Impedance Display at Two Test Frequencies

-151-

Ferromagnet1c Inclusions are occasionally encountered duringeddy current testing of non-magnetic materials. These arisefrom chips or filings from steel tooling and handling equip-ment which are embedded during manufacture. The surface ofnominally non-magnetic stainless steels and nickel-basealloys can also become magnetic as a result of cold workingor through alloy depletion from oxidation or corrosion.

O.D. DEFECT I.D. DEFECT —B.

250 kHz FERROMflGNETIC T M~INCLUSION \

0.0.

FERROMAGNETICINCLUSION

50 kHz

\ I INCLUSION

10 kHz

Fig. 8.18; Defect and Magnetic Inclusion Signals Obtainedfrom a New Inconel 600 Tube (D n - 13 mm, t =* 1.1 mm) with anAbsolute External Coil. fgp »250 kHz

Though one might consider a magnetic inclusion a defect,there are several reasons why it is important to identify theorigin of an indication. Even very small, perhaps insignifi-cant, magnetic inclusions can yield sizeable eddy currentsignals because of the extreme sensitivity to magnetic perme-ability. A second reason to determine defect origin is someasures can be taken to,minimize further damage; magneticinclusions are nearly always manufacturing defects. Figure8.18 shows the signal from a magnetic inclusion in new Incon-el 600 tubing at various test frequencies. These resultswere obtained with an external encircling probe; this ex-plains the reversal in appearance of ID and OD defects fromprevious examples. The magnetic inclusion yields a signalwhose angular separation from the fill-factor directionincreases as test frequency is reduced. The response of realdefects is just opposite.

••152-

i O.D. DEFECT

I.D. DEFECT

INTERNALMAGNETITE

250 kHz

MAGNETITE

MAGNETITE

SO kHz

m MAGNETITE

10 kHz

Fig. 8.19; Defect and Magnetite Signals from an Inconel 600Tube (Dn • 13 mm, t - 1.1 mm) Obtained with an Absolute"Internal Probe. fQn • 250 kHz)

Figure 8.19 shows eddy current response to magnetite(Fe30^) deposits inside an Inconel 600 tube at varioustest frequencies. As in the previous example, ti <> existenceof ferromagnetic material is verified by lowering testfrequency; magnetite signals rotate clockwise whereas defectsignals rotate counter-clockwise. One could easily mistakethe magnetite signals for real defects at 250 kHz and 50 kHz.Reducing test frequency can also be used to verify thepresence of magnetite on the outside of a tube. Thisapproach has been used to measure the height of sludgedeposits (containing magnetite) above tubesheets duringin-service inspection of vertical heat exchangers.

Figure 8.20 shows the eddy current signals from a Monel 400steam generator tube with external wall thinning near a tubesupport. The tube was inspected with an absolute saturationprobe and the signals recorded with wall thinning giving avertically upward signal. At 50 kHz the vertical componentof the complex signal is from wall thinning and thehorizontal signal is primarily from magnetic deposit. At 200kHz (2 fgo) the vertical component is again from wallthinning but the horizontal signal is primarily from anincrease in tube magnetic permeability because of incompletemagnetic saturation under the carbon steel tube support.At 400 kHz eddy currents just barely penetrate through thewall. In this case the signal is primarily from tubemagnetic permeability variations.

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O.D. GROOVE

+6/U.

O.D. / D E N T

' BAFFLEBAFFLE p m E

PLATE

f,*=5O kHzf 2 = 2 0 0 kHz

CALIBRATION TUBE

SIGNALS

MAGNETITE

+&/L

DENT

f j = 400 kHz

00

f , = 5 0 kHz f 2 = 200 kHz

BAFFLE

f 3 = 400 kHz

ACTUAL DEFECT SIGNAL

Fig. 8.20: Eddy Current Signals from Monel 400 Tube at BafflePlate Location.. (fqn - 100 kHz)

I

8.3.2 Conducting Deposits

The most probable conducting deposit which may be encounteredduring in-service tube testing is copper. Copper taken intosolution in one part of a cooling circuit, from brass tubes forexample, can re-deposit at another location at the expense of aless noble metal such as iron. An example is shown in Figure8.21 which is a copper-alloy tube from an air conditioner heatexchanger. Copper deposits occur near tube supports, maximumthickness was 0.05 mm. Even such a thin deposit yields a largeeddy current signal since copper is a good conductor. Figure8.21 shows response from both absolute and differentialinternal probes. The absolute probe gave eddy current signalswith no +Y component, clearly indicating the non-defect natureof the anomaly.

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Th e differential probe signal is not nearly as clear andillustrates another limitation of differential probes. Com-parison of the deposit indication with calibration defectscould easily lead one to conclude the presence of an ODdefect; particularly if the eddy current results were com-pressed on X and Y channel recordings as is often the caseduring in-service inspection. With a differential probe, onehas to observe defect sense (arrows) to distinguish betweendeposit signals and those from real defects.

Copper Deposits

.30 -10 50 »0

CALIBRATIONOEFEC7 SIGNALS

ABSOLUTE DIFFERENTIAL

DEPOSIT SIGNALS

g. 8.21; Eddy Current Indications from Copper Deposits on"per Alloy Tube (Dp » 19 mm, t = 1.1 mm, fo,Q * 57 kHz)

8.22 shows simulated copper deposit signals at differ-.est frequencies. There is a noticeable change in phasewith increasing deposit thickness as well as test fre-

quency. At frequencies above fgg there exists a possibilitydeposits could be mistaken for ID defects, even with anabsolute probe. The procedure for in-service inspection ofnuclear power plant boilers specified by ASME(^l) leads totest frequencies between and 2fgg. This appears to be a

l d i i ifgg ggweakness in the code which may lead to revision if copperdeposits prove more common as boilers age. Inspection ofFigure 8.22 reveals clearer discrimination between copper anddefects is achieved at fgQ /2 than at fgg • Optimum testfrequency for copper coated tubes appears to be the frequency

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which just leaves copper signals below the horizontalfill-factor direction.

A - 701 OD ECCENTRIC GROOVEfi - 10J |D COKCENTSIC GROOVEC - 0.13 mm THICK COPPER WOUND TUBEB - 0.06 mm THICK COPPER AROUND TUBE

Fig. 8.22; Eddy Current Signals Obtained with an InternalCircumferential Probe from Simulated Copper Deposits on Tubes

S.4 MULTIFREQUENCY EDDY CURRENT TESTING

8.4.1 Background

Successful in-service Eddy Current inspection relies on eddycurrent probes that can sense defects and on analysis of eddycurrent signal. Both aspects are equally important. Whilescanning each tube, eddy current signals pee obtained frombaffle plates, magnetite deposits, dents, tubesheets, tubeexpansion, etc. and maybe defects. One must, therefore,discriminate between defects and insignificant signals andeven more important, estimate defect severity when it occurstogether with other signal sources. It would be much easierif the data could be processed to contain only defectsignals; Multiffrequency ET can do this.

In multifrequency testing, two or more sinusoidal signals ofdifferent frequencies are fed simultaneously to a single eddycurrent probe. Gain and phase of the output signal from eachfrequency can be separately controlled.

-156-

ID GROOVE O.D. GROOVE

THROUGHWALL HOLE

DENT

BAFFLEPLATE

MAGNETITE

CALIBRATION TUBE

1.3 mmJ

IS.5 mm

100* O.D.

100*

DENT

(C)

Fig. 8.23; Internal Probe Response to Various TestParameters. f g Q • 130 kHz.

A f, = 100 kHz

(h)

Fig. 8.24; Eddy Current Signal at Baffle Plate Positionin Tube of Eigure 8.11. fg0 - 130 kHz.

-157-

These signals can then be combined to eliminate unwantedsignals and leave only the defect signal. This method isonly effective if a defect signal differs characteristicallyfrom unwanted signals and if signals are vectoriallyadditive. The first condition makes detection of internaldefects, in the presence of internal variations, impossible.The second requirement makes the method ineffective fordetection of fretting wear under non-ferromagnetic baffleplates (Section 8.2.4). As a consequence of combiningsignals from three different frequencies, defect signalamplitude decreases and instrument noise increases.

Eddy current penetration and phase lag are a function offrequency; increasing test frequency reduces penetration andincreases phase lag. Since an eddy current signal is afunction of current density and phase lag, it is possible tochange the response to various signal sources by changingtest frequency.

If one simulates a heat exchanger tube with defects,deposits , dents and support plates, one obtains the followingresults:(a) at high frequencies only internal defects and dents are

detectable, Figure 8.23(c).(b) at intermediate frequencies, all features are

detectable and there is phase discrimination betweeninternal and external defect signals (because of phaselag across the wall) and other signals, Figure 8.23(b).

(c) at low frequencies, baffle plates and magnetite depositsyield predominant signals with little phase separationbetween internal and external defect signals, Figure8.23(a).

With this background in mind, one can decide which combina-tion of frequencies should be used to eliminate extraneous(unwanted) signals. The following two examples illustratethese effects .

For the dented tube example described in Section 8.2.3(Figure 8.11), the extraneous signals making up the compositesignal at f = 100 kHz can be determined by re-inspecting thetube at higher and lower test frequencies. If the signalsfrom the actual defect in Figure 8.24 are compared with thecorresponding calibration signals in Figure 8.23, one can seeat 500 kHz the signal is primarily from a dent while that at20 kHz contains a large baffle plate signal component.

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8.4.2 Multifrequency Testing of Dented Tubea

With single frequency eddy current inspection, tube supportsand dents tend to mask signals from tube damage under tubesupports. This makes detection and estimation of severitydifficult and time-consuming. In the remaining section weshow how multifrequency simplifies the inspection of thedented tube described previously.

Figure 8.25 illustrates the tube stripping sequence; one ormore signals are removed by each mixing of two frequencies.By proper manipulation of the signals from the two lowerfrequencies, baffle plate and magnetite deposit signals canbe eliminated. However, the resultant eddy current signal isstill distorted by the 'denting' signal. Again, by combiningthis resultant signal with the signal from a higher testfrequency, the dent signal can also be eliminated. The tubenow looks bare. If a defect existed under the baffle plate,it would be very easy to detect, the resultant signalcontains only information from the OD corrosion. Thisprocess of unwanted signal elimination is like solving threesimultaneous equations with three unknowns and solving forthe parameter Xx - defect.

20 kHz

T2

100 kHz 500 kHz

c, = f, - f,

Cj = C, - f,

Fig. 8.25: Tube Stripping Sequence by Multifrequency

1-159-

As shown in Figure 8.23, the signal at each baffle plate is acomposite signal comprising a baffle plate, magnetite deposit(or baffle plate corrosion products), dent and defect signal.Figure 8.26 illustrates elimination of baffle plate andmagnetite signals. The probe is moved back-and-forth underthe baffle plate and the signal is monitored on the storagescope in the chopping mode, where both frequency signals aredisplayed simultaneously.

BAFFLEPLATE

BAFFLEPLATE

MAGNETITE

MAGNETITE

COMPRESSED BY 0.7Hfj WITH PHASE

ROTATION OF 19°

RESIDUAL BAFFLE PLATE SIGNAL

RESIDUAL MAGNETITE SIGNAL

Fig. 8.26: Suppression of Baffle Plate and Magnetite Signals

The f£ signal is first rotated to match the Z± signalorientation. Then fy amplitude is changed to match, asnearly as possible, the f^ signal size. In this case, thismethod by itself doesn't work. However, by decreasing thevertical component of the f^ baffle plate signal, oneobtains a good match. On subtracting the signal, through anelectronic mixer (C^), the signals from the baffle plateand the magnetite deposit both nearly disappear. A smallresidual signal remains due to different approach signalsat the two test frequencies, indicated in Figure 8.26 bythe two open circles. Although the baffle plate signals

-160-

are identical, the two points do not coincide; the baffleplate is sensed earlier at the lower test frequency. Thisresidual signal is insignificant for this application thoughit can become quite serious when testing for small cracksunder non-ferromagnetic baffle plates.

C2-Ci-f3 /

* RESIDUAL DENT SIGNAL

Fig. 8.27; Suppression of Dent Signal

Figure 8.27 illustrates how one can eliminate the 'denting'signal from the resultant (C^ - £"i~£\) signal. Thisis achieved by first matching the phase and amplitude of theC± and fj 'dent' signals and then using a second mixingmodule (C£) for subtraction.

Figure 8.28 traces the above sequence for two defective tubes,and shows the eddy current signal becoming simpler to analyzewith each step. On comparing defective tube signals with thosefrom a calibration tube, one observes the f2 defect signal isdistorted by the baffle plate, dent and/or magnetite deposit.The C^ signal is only distorted from the dent signal,and C2is a clear signal indicating OD pits approximately 50% deep.Even an inexperienced inspector could analyze these results.

IIIIIIIIIIIIIIIIIT

- 1 6 1 -

CHUBMTION TUBE

c, = c , - ( ,

OEFECTIVE

TUBE NO I

OEFECTIVE

Fig. 8.28: Multifrequency Eddy Current Signals fromDefective Tube

When using multi-frequency to eliminate "ID noise", such assignals from cyclic internal diameter variations ("pilgernoise or die chatter"), dents and probe wobble, the signalamplitude from internal defects is drastically reduced.However, signal amplitude from external defects is notaltered significantly. Multifrequency is more effective forexternal defect detection than for detection of internal defectsin tubes.

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8.5 SUMMARY

Defect signal amplitude Is a function of Its axial andcircumferential extent as well as depth. Defect signal phaseIs primarily a function of depth. For general purposevolumetric inspection of heat exchanger tubes, a suitabletest frequency is

3 p/t' kHz (7.4)

where p is electrical resistivity and t is wall thickness.

Inspection at fgQ allows defect depth to be estimated on thebasis of signal phase. To discriminate between defects andferromagnetic deposits a lower test frequency should be used;normally 10 or 20% of f 90*

Signal response from most significant service induced defectsis usually comparable in amplitude to that from a 1.6 mmdiameter through hole. Stress corrosion cracking, generalcorrosion and fretting wear give large signals whereaspitting corrosion and fatigue cracks give small signals.

Testing for fretting wear under non-ferromagnetic supportplates is difficult and unreliable with bobbin type probes,because defect and support plate signals are not vector.tallyadditive. A surface type probe should be used.

Multifrequency equipment can be used to eliminate unwantedcomponents from complex signals such as support plates andprobe wobble, this greatly simplifies signal analysis.

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CHAPTER 9 - METALLURGICAL PROPERTIES AND TESTINGFERROMAGNETIC MATERIALS

9.1 INTRODOCTION

One can find numerous references in NDT publications dealingwith eddy current measurement of material properties,such aschemical composition, hardness, strength, corrosion damage,degree of cold work and extent of both carburization anddecarburization. In fact, none of these properties andmaterial conditions are measured directly. Eddy currenttesting is sensitive to material properties through theireffect on resistivity and magnetic permeability. As such,eddy currents only provide indirect measurement of materialproperties and care must be taken to insure that someunforeseen material variation does not lead to falseconclusions. Two precautions will help avoid false testresults:

(a) a sound basic understanding of ET as outlined inprevious chapters

(b) use of suitable standards for any particular test;the condition of such standards should be verifiedby independent methods, e.g., hardness tests, tensiletests.

A complete treatment of materials property evaluation by eddycurrent testing is beyond the scope of this manual. Thebasics are covered and a few examples presented.

9.2 ELECTRICAL CONDUCTIVITY

9.2.1 Factors Affecting Resistivity

All materials possess intrinsic resistance to electron flow(current) which is termed resistivity ( P, microhm-centimetres).The resistance of a conductor is given by

R = pfc/A ohms

where H is length (cm) and A is cross-sectional area (cm^).Resistivity values for various materials are listed in Table 9.1.

Conductivity (CT, siemens/metre) * is the ease with whichelectrons can move through a material. It is the reciprocal

•Conversion:a = 108/p, S/m or mho/m

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TABLE 9.1 ELECTRICAL RESISTIVITY OF COMMON CONDUCTORS AT 2O°C

MATERIAL

SilverCopperGoldAluminum7075-T6 <A1 Alloy)ZincMagnesiumAdmiralty BrassIronPhosphor BronzeLead70 Cu-30 NiMonelZirconiumTitanium304 SSZircaloy-2Inconel 600Hastelloy XWas paloyT1-6A1-4V

RESISTIVITY(yfl.cm)

1.61.72.42.85.35.94.67.09.71620.637.448.25054.8707298115123172

CONDUCTIVITY(siemens/m)

6.14xlO7

5.814.103.551.891.702.171.431.030.630.490.270.210.200.180.140.140.100.0870.0810.058

CONDUCTIVITY(Z IACS)

10510070613229372418118.44.53.63.43.12.52.41.71.51.41.0

of resistivity. In eddy current testing, conductivity isfrequently given as a percentage of the InternationalAnnealed Copper Standard (Z IACS). In this systemconductivity of pure, annealed copper at 20°C is set to 100%and conductivity of other materials is given as a percentageof copper. Conductivity of a material can be calculated fromits resistivity,

X IACS - 172/p

Increasing temperature normally increases resistivity(decreases conductivity) as shown in Figure 9.1. Over alimited temperature range the variation is usually linearaccording to the relation

po(l +

where P is resistivity at temperature T <°C), Po isresistivity at a reference temperature To, a ("C "

1 )is thermal coefficient of resistivity and AT is thetemperature difference (T-To). For common metals andalloys values of a range from less than 0.001 to over 0.01,0.004 is fairly typical.

-165-

Alloying normally increases resistivity. Figure 9.2 showseven small alloy additions to aluminum can increaseresistivity appreciably. The conductivity of binary Cu-Ni

ime

tre

s)

ohm

-cen

t(m

ici

'ITY

RE

SIS

TS

60

00

4 0

30

2 0

10

/TITANIUM/ :»0.04

- / /

l^ 1-—•—T i

v^PLATINUM/ a « 0.004

COPPERa «* 0.005 -

1 1 1 1ZOO 400 600 800 1000 1200

TEMPERATURE ( ° K )1400

Fig. 9.1; Effect of Temperature on the Resistivity ofCopper, Platinum and Titanium

2 7.0 MANGANESE

MAGNESIUM

1-0 2.0 3.0 4.0 5.0 6.0ALLOY CONTENT (wt. %)

Fig. 9.2; Effect of Alloying Elements on the ElectricalResistivity of Aluminum.

-166-

alloys Is shown In Figure 9.3. The dependence ofconductivity on composition provides one basis for eddycurrent sorting of mixed alloys. Oxygen impurity inzirconium and titanium alloys changes resistivityconsiderably. Figure 5.19 showed a non-uniform oxygendistribution in a zirconium-niobium alloy detected by eddycurrent testing.

100 -

80

COPPER/NJCKEL ALLOYS

CO60

40

20 40 60

WEIGHT % COPPER80 100

Fig. 9.3: Variation in Electrical Conductivity of Nickel-Copper Alloys with Composition

Cold work increases resistivity through introduction oflattice defects in metals. At normal temperatures, cold workhas a relatively small effect on conductivity (<1Q%) and canusually be ignored. The degree of cold work in someaustenitic stainless steels can be determined by ET, this ispossible because cold work makes them, ferromagnetic, notbecause of a resistivity change.

9.2.2 Material Sorting by Resistivity

TwoThis is normally an eddy current surface probe method,instrument types are commonly used. Impedance displayinstruments offer a comparative method as treated in Section5.8.2; the lift-off curves for unknown materials are comparedwith those of known standards and the resistivity of theunknown is estimated by interpolation. Meter readoutinstruments are also available with built-in "lift-off"

-167-

compensation which are calibrated directly in % IACS. Bothtypes of instruments require care on the part of the operatorto insure meaningful results. Effects which can contributeto erroneous results follow (for more detail see Section5.8.2):

(a) too low a test frequency can make material thicknessappear similar to resistivity changes.

(b) sample curvature affects ceil coupling and hence itsresponse (edge and other geometry effects have a similarresponse).

(c) too high a test frequency could sense alloy changes atthe surface of oxidized or corroded materials.

(d) conducting and nonconducting coatings affect test coilimpedance.

(e) ambient temperature variations result in changes insample resistivity and test coil resistance.

The above potential error sources can largely be overcomethrough use of suitable standards which duplicate materials tobe tested.

IIT

S

z=>

TR

AR

YkN

BI1

^4

>vTENSILE STRENGTH

. .^HARDNESS ^ ^

^CONDUCTIVITY

1 1 1I 10 100

TIME AT TEMPERATURE (h)1000

Fig. 9.4: Variation of Mechanical Properties and Conductivityin 7075-T6 Aluminum Exposed at 205°C

-168-

An example of eddy current testing to determine heattreatment state of an aluminum alloy is shown in Figure 9.4These results are from PellegriniC^LO) who indicates thetechnique can be used to judge the fitness of overheatedmaterial for further service. A similar approach has beenused to assess heat treat condition of titanium alloys.

9.3 MAGNETIC PROPERTIES

For eddy current purposes one can classify materials asferromagnetic (magnetic) or non-ferromagnetic (nonmagnetic).Diamagnetic and paramagnetic materials can be considerednonmagnetic. Ferromagnetism has its origin in a quantummechanics effect, the "exchange interaction". It occurs inthe elements iron, cobalt, nickel and some of the rare earthmetals. These elements have partially filled d and felectron shells. Alloying with elements which have a higherelectron to atom ratio fills these d and f shells and makesthe resulting alloys less! magnetic, e.g., copper added tonickel (Monel) and chromium added to iron (stainless steel).

The main feature separating magnetic from nonmagneticmaterials is magnetic permeability, U , which is a measure ofa material's intrinsic ability to conduct magnetic flux. Itis defined as the induced magnetic flux density, B, dividedby external magnetic field intensity (magnetizing force), H,

li = B/H

For air and nonmagnetic materials y is a constant,

liQ = 4ir x 10 webers/ampere-metre

when B is in teslas* (T) or webers/metre^ and H is inampere/metre (A/m).

Simplification results if one uses relative permeability,which is defined as

Ur ~ l-'/̂o (dimensionless)

Relative permeability has the same value in all magneticsystems of units. For magnetic materials Pr can be verylarge, whereas for nonmagnetic materials Ur ~ 1.0.

*Conversion: 1 tesla = 10^ gauss; 1 A/m=0.012566 oersted.

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9.3.1 Magnetic Hysteresis

When a material is magnetized in a coil, the magnetic fieldintensity, H, is proportional to coil current. Ifalternating current is applied to a magnetizing coil a B-Hloop results as shown in Figure 9.5. As H increases fromzero for the first time, B increases along the DC curve, pathNo. 1. When H decreases, B also decreases but along path No.2. The difference between paths 1 and 2 is termedhysteresis. When H has fallen to zero a residual fluxdensity remains in the material, Br, called retentivity orresidual flux density. On decreasing H further (reverse ornegative current) flux density decreases to zero at Hc

which is the coercive magnetic intensity or coercive force.Decreasing H still more drives the curve to point S ^Additional AC cycles will retrace the loop. At point S2the material is saturated, from S2 to S3 the B-H curve islinear with slope Mo . Flux density at saturation depends onmaterial; carbon steel saturates at about B = 2 tesla (20kilogauss) whereas Monel 400 saturates at about 0.3 tesla (3kilogauss).

Fig. 9.5; Hysteresis (or B-H) Loop

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9.3.2 Magnetic Permeability

For eddy current inspection of ferromagnetic materialsseveral kinds of permeability play an important role. Normalpermeability, Ur , is a measure of a material's ability toconduct magnetic flux; it is an important factor whendetermining the ease with which a magnetic material can besaturated.

Another permeability of concern in ET is relative incrementalor recoil permeability, U^ • It Is defined as

AB/AH

where AB is the change in flux density which accompanies achange in magnetizing force, AH, created for example by aneddy current coil's alternating current. An incrementalAH can be superimposed at any point on a DC magnetizationcurve as illustrated in Figure 9.6.

0.8 -

0. 6 -

O.H

0. 2

100 200 300 100

MAGNETIZING FORCE (A/mi

500

Fig. 9.6;Iron

DC Magnetization Curve and Recoil Permfability for

-171-

At H-0 we have the relative Initial permeability, Pi . In amagnetic material without a biasing DC magnetic field, thenormal permeability is equal to the incrementalpermeability,

In eddy current testing, test coll inductance and depth ofpenetration are influenced by incremental permeability notnormal permeability. However, throughout this report it isassumed that the eddy current test is performed without DCbias and with a low magnetizing force (low alternating coilcurrent). In this case, V and for simplification

x &purposes pr is used in the skin depth and inductanceequations and impedance diagrams; Pr is used throughout themanual to denote incremental permeability (p.) unlessotherwise stated.

When an increasing DC magnetizing field is applied, anonlinear B-H relationship results as shown in Figure 9.7.The incremental permeability continuously decreases untilsaturation is achieved. At saturation U^ "1.0. The normalpermeability,instead, first increases to a maximum value andthen decreases gradually, see Figure 9.7; at saturation itcan still be very large.

0.3 "

0. 2 -

0. 1 -

20 -

H x 10 ! A /m )

Fig. 9.7: Magnetization Curve, Incremental Permeabilityand Normal Permeability for a 3R&6O Tube Sample

-172-

9.3.3 Factors Affecting Magnetic Permeability

Ferromagnetic materials do not have unique magnetizationcurves but depend strongly on factors such as

thermal processing history,mechanical processing history,

- chemical composition,- internal stresses,

temperature (If close to Curie temperature).

The following examples illustrate the effect of some of theaevariables.

Figure 9.8 shows B-H curves, at room temperature, for threesupposedly identical Monel 400 tube samples. The differencesare attributed to variations in nickel/copper content withinthe normal alloy specification range.

Figure 9.9 shows variation of magnetic permeability with coldwork in Type 300 series stainless steels(^). In these"nonmagnetic" austenitic steels a ferromagnetic martensitephase forms during cold working increasing the magneticpermeability. In contrast, most normally ferromagneticmaterials exhibit a decrease In permeability as a result ofcold work. The 300 series stainless steels can also becomeferromagnetic as a result of welding, a magnetic deltaferrite phase forms during solidification.

o.g

D.E

0.2-

HOKEL 100 TUBESFROH NUNIICQKE C.S.

J I— 25

/ TUBE *-250

_ /_15 ML.- —

TUBE «"251

'TUBE »-252

10 12 11

H OERSTEOS

Fig. 9.8: Magnetization Curves for Various Monel 400Samples

IIIIIIIIIIIIIIIIII

- 1 7 3 -

l O O j r

xatHia.

- AUSTENITIC STAINLESS STEEL

40 6 0

% COLD WORK

BO 100

Fig. 9.9: Variation of Relative Permeability with ColdReduction for Various Austenitlc Stainless Steels (.2.)

1. 5

0. 5

6 HPa NO STRESS

21 HPa

ANNEALLED IRON

I25 50 75

MAGNETIZING FORCE ( » / • !

100

Fig. 9.10: Effect of Elastic Strain on the Magnetization ofIron 19_)

-17 4-

Figure 9.10 shows changes in B-H curves for iron withinternal stress. Note that these stress levels are purelyelastic, well below the yield strength. The changes in B-H(and permeability) are due to magnetostriction.

The above examples illustrate the inherent variability of B-Hand hence permeability of ferromagnetic materials.Incremental permeability affects an eddy current coil'sinductance as well as depth of eddy current penetration intoa material< The large variations in permeability shown abovemake conventional eddy current testing for defects inmagnetic materials very difficult if not impossible.

The best solution to eddy current testing of a magneticmaterial for defects is to bring it to a condition where1>A "1.0. A few slightly magnetic materials can be heatedabove their Curie temperature to make them nonmagnetic.Monel. 400 heated to between 50° and 70°C has been tested inthis manner. Most materials have too high a Curietemperature to be tested by this approach. The only otherway to decrease U^ to unity is by magnetic saturation. Thistopic is treated in a subsequent section.

9.4 TESTING MAGNETIC MATERIALS

9.4.1 Simplified Impedance Diagrams

A qualitative understanding of the effect of permeability oncoil impedance can also be obtained by the equivalent circuitand its associated semicircular impedance diagram treatmentof Section 3.5. Coil inductance is a function of magneticflux through it; flux increases in the presence of a magneticmaterial. For a cylinder surrounded by an encircling coil,coil inductance is proportional to both the cylinder'spermeability and its cross-sectional area,

2Lp " yr o

where L_ is primary coil (probe) inductance, U r * V ^ is thecylinder's incremental permeability and D Q its diameter.An increase in permeability or diameter will increase coilinductance. By a similar treatment to that presented inChapter 3, one can generate the impedance diagrams of Figure9.11. Figure 9.11(a) is obtained by plotting the encirclingcoil Impedance normalized to the inductive reactance in sir.It illustrates the effect of permeability and cylinderdiameter. As permeability or cylinder diameter increases(with constant coil diameter) coil impedance increasesdrastically. (This explains the good response toferromagnetic inclusions and deposits discussed in Sections6.5.1 and 8.3.1). There is no phase separation and hence nodiscrimination between variations in permeability andcylinder diameter. However, there is about 90° phaseseparation and hence excellent discrimination betweenvariations in permeability and resistivity.

-175-

uiL

1.0 1.0

0.5RL/<uLp

(b) CYLINDER

0.5RL/wL0

(c) PLATE

Fig. 9.11: Simplified Impedance Diagrams for FerromagneticCylinders and Plates

Figure 9.11(b) is obtained by plotting the encircling coilimpedance normalized to its inductive reactance with theferromagnetic cylinder inside the coil. This figureindicates the effect of permeability and cylinder diameter onoperation point location. An. increase In both permeabilityand cylinder diameter moves the operating point DOWN theimpedance curve (for constant fill factor).

Surface probe inductance also depends on test samplepermeability (L „ is proportional to Vr ). An increase inpermeability moves the operating point UP the impedance locusas shown in Figure 9.11(c). However, unlike curves for acylinder where the semicircle increases drastically in size,the curve for a surface probe increases only a small amountas previously shown in Figure 5.10. This results from muchless efficient coupling with surface probes as compared toencircling coils. A surface probe with a ferrite core (orcup) coll permits better magnetic coupling (decreasedmagnetic reluctance) and hence yields a larger impedancediagram than a similar air core coil. An additionalobservation can be made from Figure 9.11(c); magneticpermeability has the same effect as electrical resistivityand hence these two parameters cannot be separated when usinga surface probe.

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70329 STAINLESS STEEL

I kHz

10 20 30 10

NORMALIZED RESISTANCE

Fig. 9.12; Experimental Normalized Impedance Diagrams forThree Type 329 Stainless Steel Samples Tested with a LongEncircling Coil

9.4.2 Impedance Diagrams

Figure 9.12 shows experimental impedance curves for threedifferent Type 329 stainless steel samples tested with longencircling coils. These curves differ markedly from asemicircle at the lower section of the impedance diagram,where the curve approaches the Y-axis at 45° rather than 90°.These curves are nearly identical in shape to that presentedin Figure 7.6 for a nonmagnetic cylinder. But, while thenonmagnetic curve intersects the reactance axis (Y-axis) at1.0, the Figure 9.12 curves intersect this axis at theirrespective PT values. Magnetic saturation of thesesamples would reduce them to a common curve intersecting theaxis at 1.0. This figure is another example of typicalpermeability variations which may be encountered insupposedly "identical" samples.

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1.0

INCREASING PROBEDIAMETER

INCREASINGPERMEABILITY

INCREASINGRESISTIVITY

NORMALIZED RESISTANCE

Fig. 9.13: Impedance Diagram for Ferromagnetic MaterialShowing Effect of Material and Test Parameters

Figure 9.13 shows an actual surface probe impedance diagramfor magnetic material. The shape differs appreciably from asemicircle. Most test variables have a similar effect on theimpedance diagram as for surface probes on nonmagneticmaterial (Section 5.5). To measure magnetic permeability inthe presence of lift-off noise, probe diameter and testfrequency should be chosen to operate in region A.

Eddy current inspection of magnetic materials for defects isdifficult or impossible because of random permeabilityvariation as discussed in Section 9.3.3. In addition thereare skin depth limitations. Without saturation, the initialpermeability can range from 50 to over 500. Since depth ofpenetration is inversely proportional to the square root ofpermeability and test frequency,

to obtain equal penetration requires a reduction in frequencyby the same factor of 50 to over 500. Unfortunately,lowering frequency moves the operating point to Region B inFigure 9.13 where there is poor signal separation betv;-nlift-off, permeability and resistivity as well as redc="dsensitivity to defects.

-178-

Before leaving Figure 9.13 consider the characteristicparameter, F20Jjira (Section 5.6;. Figure 9.13 shows Cheparameter is not generally valid for ferromagnetic materials.It indicates an increase in Pr should move the operatingpoint down the impedance curve like increasing frequency orprobe diameter. In practice exactly the opposite occurs.The characteristic parameter should only be used for findingoperating point of surface probes on nonmagnetic materials.

9.4.3 Material Sorting by Magnetic Permeability

Detailed treatment of this topic is beyond thf scope of thismanual. This section is essentially a warning,

Many properties of magnetic materials affect permeability asdiscussed in Section 9.3.3. Eddy current testing has beenused to sort mixed alloys as well as measurement of hardness,decarburization, carburization, degree of cold work,strength, ductility,etc. A standard, ASTM E566-76, offersbroad guidelines on this eddy current application.

Meaningful results with such testing requires at least thefollowing:

understanding of the variables affecting a material'selectrical and magnetic propertiesa sound knowledge of eddy current testing

- adequate standard samples verified by destructiveexamination or other independent methods.

9.4.4 Testing for Defects in Magnetic Materials

Previous sections explained why saturation is required tosuppress effects of usually harmless permeability variationswhich could be mistaken for, or obscure, defect signals. Weonly consider testing of cylindrical materials; similartechniques can, at least in theory, be applied to surfaceprobe •esting.

Manufacturing inspection of rods, wires and tubes isaccomplished fairly simply by external, water cooledmagnetizing coils through which the material is passed. ASTMstandard E309 covers such testing. In-service inspectionagain presents the most difficult situation due to access andspace limitations.

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II

O.D.DEFECT

SUPPORTPLATE HOLE

FLAT PITS

SLIGHT BENDIN TUBE

/L

CALIBRATIONTUBE

EDDY CURRENTTEST WITHOUTSATURATION

EDDY CURRENTTEST WITHMAGNETICSATURATION(10 X ABOVE GAIN)

Fig. 9.14: Eddy Current Signals from a High MagneticPermeability Monel 400 Tube. TestFrequency - 50 kHz

-180-

Figure 9.14 compares Y-channel eddy current signals from aMonel 400 tube at fgg without and with magnetic saturation.Saturation results In good defect detection. Permeabilityvariation due to cold work and internal stresses at a slightbend in the tube are completely suppressed by saturation.This tube was saturated by superimposing the AC eddy currentsignal on DC magnetization power. Saturation of Monel 400 Isalso achieved by incorporating permanent magnets in theprobe(6).

Saturation with DC magnetization is limited by coil heating.Heat dissipation is proportional to current squared and coilwire resistance (Power=I2R). To increase magnetization (His proportional to I) pulse saturation is used. Thesaturation current (DC) is switched on-and-off at regularintervals thereby reducing the heating effect. The testcurrent (AC) is superimposed on the saturation current andthe eddy current signal is sampled only at maximumsaturation. One commercial Instrument, operating on thisprinciple, is currently available. Testing speed id afunction of pulse rate, in general it is much slower thanconventional testing.

If magnetic saturation at defects is not complete, an eddycurrent test becomes a test for permeability, not eddycurrent testing as described in previous chapters. This canbe understood from Figure 9.15 which illustrates the changein eddy current signals from calibration defects in amagnetic stainless steel tube as degree of saturation isincreased. The eddy current signals were obtained with anabsolute bobbin type probe. Since defect signal amplitudedecreases as saturation is approached, instrument gain wasdoubled for the 20 and 40 ampere saturation results.Magnetization was achieved with an external, water cooledcoll; 10 amperes produced about 2.8 x 10* A/m or 350oersteds. Figure 9.15 shows one has to be saturated wellpast the knee in the magnetization curve (over 20 amperes)before eddy current defect signals appear normal, like thosefrom nonmagnetic materials.

The reason for the characteristic eddy current signals frompartially saturated tubing is more clearly apparent In theeddy current impedance display of Figure 9.16 which includesimpedance response as magnetization level increases. Thisfigure shows, at partial saturation (less than 10 amperes),defect signals consist nearly entirely of increasing anddecreasing permeability. The initial increasing permeabilitysignal component is attributed to less saturation on eitherside of machined calibration defects while the decreasingpermeability component is due to more intense saturation inthe reduced tube-wall region at defects.

Similar results are obtained with internal saturation usingDC magnetization or permanent magnets. A single rare-earthpermanent magnet was found to be equivalent to about 5amperes (175 oersteds) of an external magnetizing current forthis tube size and material.

fi

III

IIIIIIII1IIIII

- 1 8 1 -

THROUGH HOLE

\

EXTERNAL MAGNETIZING COIL

—INTERNAL ABSOLUTE PROBE

BC

A PROBE WOBBLEB THROUGH HOLEC O.Q GROOVED I D GROOVE

10 IS 20 25

MAGNETIZING CURRENT ( A )

Fig. 9.15: Eddy Current Signals from E-Brite 26-1 Tube WithIncreasing Saturation, (fo.Q • 100 kHz at Complete Saturation)

Eddy current testing at partial saturation may seemattractive since defect sensitivity is very high, it may infact develop into a useful NDT technique. However, there aredrawbacks; U^ is greater than one and is variable. Thismeans eddy current penetration is not defined andconventional phase analysis is impossible. Testing tubes fordefects at magnetic supports could be a very questionableprocedure since large permeability signals would beencountered which could be mistaken for or obscure defects.Even the best available saturation methods still encounterproblems in detecting defects at steel baffle plates In someMonel 400 tubes which are only slightly magnetic.

Eddy current testing at partial saturation only measurespermeability in a thin surface layer adjacent to the testcoil. This classifies the technique with NDT methods such asmagnetic particle inspection and leakage flux testing.Leakage flux testing responds to the distortion of magneticflux at defects in a magnetized material using pickup coils orHall effect sensors. Partial saturation ET with surfaceprobes has an advantage over encircling (or internal) probesin the ability to separate permeability from lift-offvariations (Figure 9.13).

-182-

A BC P

BALANCE POINTIN * AIR

\ \FACTOR

\ \

INCREASINGFLUX DENSITY -(DECREASINGPERMEABILITY)

\\\ .

MAGNETIZING /CURRENT (AMPS)\

THROUGH HOLE

\

EXTERNAL MAGNETIZING COIL

-INTERNAL ABSOLUTE PROBE

10 GROOVE^ /OD GROOVE

B C D

A - PROBE WOBBLEB - THROUGH HOLEc - a a GROOVED • I. D GROOVE

1

i1

Fig. 9.16: Eddy Current Signals from E-Brlte 26-1 Tube withIncreasing Saturation, f90 - 100 kHz

IIIIIIIIIIIIIIII11T

-183-

An example of the dangers of ET ferromagnetic materials atpartial saturation is Illustrated in Figure 9.17. It showseddy current signals from calibration defects in a 3Re60 heatexchanger tube tested with a differential probe. <3Re60requires a flux density of about 0.6T for completesaturation). Calibration defects yield signals which changein phase with increasing depth leading to the conclusion onemay have a viable test technique. However, elasticdeflection of the tube at a support plate gives change ofpermeability signals nearly identical to serious (50% and75%) defects. This is due to magnetostriction: changes inmagnetic properties due to elastic stress such as shown inFigure 9.10.

PARTIAL SATURATION PROBE

CALIBRATION ASIGNALS I)

0

BAFFLE PLATE

SIGNALS

&' = 7 mm 6 mm 5 mm 4 mm 3 mm 2 •• 0

Fig. 9.17: Eddy Current Signals from 3Re6O Tube WithPartial Saturation for Various Levels ofElastic Stress. Test Frequency fqn - 230 kHz.

-184-

The problem of Figure 9.17 was overcome with a multimagnetprobe similar to that developed for Monel 400 tubing (8).This eliminated the false defect signals at tube supports andmade these heat exchangers inspectable by conventional ETtechniques. It was fortunate these particular heatexchangers had nonmagnetic, Type 304 stainless steel, supportplates. This permits tube saturation in the vicinity ofsupports. If the supports had been magnetic they would haveprovided a low reluctance alternative path to the saturationfield leaving the tube only partially saturated. Nonmagneticsupport materials improve inspectability of ferromagnetictubes even though fretting wear may be difficult to detectwith a conventional bobbin-type probe as discussed in Section8.2.4.

9.5 SUMMARY

Eddy current testing can be used to measure electricalresistivity and magnetic permeability. This parameter, insome cases, can be correlated to a material's chemicalcomposition, hardness, heat treatment, etc. and thereforeprovide an indirect measurement of material properties.Material sorting by electrical resistivity can be done withgeneral purpose eddy current instruments or with specialinstruments with meter output calibrated in % IACS. Caremust be taken to obtain reliable results. Material sortingby magnetic permeability is not simple. It requires a soundknowledge of magnetic properties and eddy current testing.Most of the commercial equipment make use of hysteresisdistortion and the method is empirical. It is more reliableto use general purpose eddy current equipment to roughlymeasure magnetic permeability and then correlate to materialproperty.

Testing ferromagnetic materials for surface defects ispossible but often unreliable. If material can bemagnetically saturated, it appears as non-ferromagneticmaterial to the eddy currents. Testing at partial saturationresults in good sensitivity to defects and to ferromagneticanomalies but can result in false indications. It ispossible to magnetically saturate some ferromagnetic tubealloys in unsupported tube sections, but nearly impossibleunder ferromagnetic baffle plates.

Magnetic permeability affects the following:- depth of penetration- probe inductance- operating point on impedance diagram- characteristic defect signal is no longer dependent onphase lag

- drastically decreases signal-to-noise ratio.

III

-185-

9.6 WORKED EXAMPLES

9.6.1 PROBLEM: Convert resistivity of 5.5 microhm-centimetres to

SOLUTION:

% IACS - 172/p

- 172/5.5 - 31.3%

I a.o.i FKUBLEH: convert resistivity oconductivity in % IACS

I9.6.2 PROBLEM: Pure annealed iron under a magnetizing force, H, of

140 A/m results in a magnetic flux density, B, of0.028T. Determine magnetic permeability andrelative permeability in (a) the tesla,ampere/metre system of units and (b) the gauss,

I oersted system.

SOLUTION:

| (a) vi = B/H = 0.028/40 = 7.0 x 10"4 henry/m

• Ur W/Uo = 7.0 x 10 /4ir x 10~7 = 557 (dimensionless)

I (b) B 0.028 x 104 = 280 gauss

* H = 40 x 0.012566 = 0.503 oersted

_ U B/H = 280/0.503 = 557 gauss/oersted

u = p/u = 557/1.0 = 557 (dimensionless)

IIIIr

-186-

I

9.6.3 PROBLEM:Calculate standard eddy current depth ofpenetration in carbon steel at a test frequency of10 kHz (a) without saturation and (b) with completesaturation. P - 15 microhm-centimetres, y. - 300

SOLUTION:

(a) From Equation 2.13(a)

(b)

- 50 15

0"x 300

- 0.11 mm (0.004")

6 -

- 1.0 at saturation

50,'V; 15

10 x 1.0

- 1.94 mm (0.077")

I1111111

II

I

IIIIIIIIIII

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CHAPTER 10 - DEFINITIONS, REFERENCES AND INDEX

10.1 DEFINITIONS

This section lists the most common terms covered in themanual. For each term, the symbol, the SI units and thesection where the topic is covered is given, followed by thedefinition.

ABSOLUTE PROBE - See Sections 5.2 and 7.2.- A probe having a single sensing coil.

ALTERNATING CURRENT - I A C , amperes; see Chapters 2 and 3.~ A current flow changing in amplitude anddirection with time.

ANOMALY - See Sections 6.5 and 8.3.

I- An unexpected, unclassified eddy currentsignal.

- A false defect indication.

BRIDGE - See Section 4.2.1.~ Electrical circuit incorporating fourimpedance arms.

CALIBRATION STANDARD - A test standard used to estimatedefect size and set-up instrument.

CAPACITIVE REACTANCE - Xc, ohms; see Section 3.2.- The opposition to changes in alternatingvoltage.

CHARACTERISTIC PARAMETER - r2wau , dimensionless, seeSection 5.6.

- It allows test coil operating point to bespecified in terms of a single quantityrather than four independent variables.

CHARACTERISTIC OR LIMIT FREQUENCY - f~, hertz, see Section7.3.3. g

CHARACTERISTIC FREQUENCY RATIO -f/fg - dimensionless, seeSection 7.3.3.

- It allows the test coil operating point to bespecified in terms of a single quantity ratherthan four independent variables.

CIRCUMFERENTIAL COIL - see encircling and internal probes.

CONDUCTIVITY - a(sigma), siemens/m; see Sections 2.4 and9.2.

- Measure of the ability of a material toconduct current (alternating or directcurrent).

CONDUCTOR - Material capable of carrying electrical current.

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COUPLING - The coil's magnetic field couples to the test |sample.

- The change in probe impedance is directly pro- mportional to probe-sample coupling. I

CURRENT - I, amperes, sea Section 3.3 - Flow of electrons.

DEPTH OF PENETRATION (STANDARD) - 6 (delta), millimetres; Isee Section 2.4.

- The depth at which the eddy current density has |decreased to 1/e or 36.8% of the surface |density.

- Also referred to as skin depth. -

DEFECT - A flaw or discontinuity that reduces a material's •integrity or load carrying capacity - mayinvolve a loss of material. I

DIFFERENTIAL PROBE - see Sections 5.2 and 7.2.- A probe having two sensing coils located side-by- *side. I

DIRECT CURRENT - I DC » amperes; see Section 3.3. _- A current flow that is constant in 1

amplitude and direction with time. '

DISCONTINUITY - A defect. I

EDDY CURRENTS - see Chapter 2 and Sections 5.2.2 and 7.2.3.- A closed loop alternating current flow

induced in a conductor by a varying magnetic ifield.

EDDY CURRENT METHOD - An electromagnetic NDT method based onthe process of inducing electrical currentsinto a conductive material and observing theinteraction between the currents and the 'material. In France it is known as the'Foucault currents' method.

EDGE EFFECT - see Section 5.8.2.Signal obtained when a surface probe approachesthe sample'8 edge.

EFFECTIVE DEPTH OF PENETRATION - see Section 2.4.- Depth at which eddy current density drops offto 5% of the surface density. "•

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END EFFECT - see Section 5.8.2.- Signal obtained when an internal or encircling

probe approaches the end of a tube or rod(similar to edge effect).

ENCIRCLING PROBE (Coil)-see Section 7.2.R - Also referred to as a feed-through coil.

I- A probe which completely surrounds test

material; can be absolute or differential.

FEED-THROUGH COIL - see encircling probe.

IFERRITE - Ferromagnetic oxide material.

- Used for cores in high frequency transformers

FLAW - A defect .

FERROMAGNETIC - see Section «>.3.- A material with a relative magnetic

permeability greater than 1.0

FILL-FACTOR - n (eta), dimensi on.less; see Section 7.3.- It is a measure of coupling between the coil

and test object.- Fraction of the test coil area filled by the

test specimen.

FOUCAULT CURRENTS METHOD - In France the Eddy Current Methodis known as the 'Foucault currents' method.

FREQUENCY - f, hertz, see Section 2.4.- Number of cycles of alternating current per

second.

FREQUENCY (ANGULAR) -u) (omega), radians/second; see Section 3.2.- Angular velocity, where u) « 2 irf.

HYSTERESIS - See S e c t i o n 9 . 3 .- Magnetization curve.

IACS - OIACS , %, see Section 9.2.- International Annealed Copper Standard.- Conductivity as a percentage of pure copper.

INDUCTANCE - L, henries, see Section 3.2.- Ratio of the total magnetic flux-linkage in a

coil to the current flowing through the coil.

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IMPEDANCE - 7., ohms, see Section 3.2.- The total opposition in an electrical circuit

to flow of alternating current.- Represents the combination of those electrical

properties that affect the flow of currentthrough the circuit.

IMPEDANCE METHOD - Eddy current method which monitors thechange in probe impedance; both phase andamplitude.

INDUCTIVE REACTANCE - XL, ohms, see Section 3.2.- The opposition to a change In alternating

current flow.

INDUCTOR - A coil.

INTERNAL PROBE (COIL) - see Chapters 7 and 8.- A probe for testing tubes (or holes) from the

inside. The coil(s) is circumferentially woundon a bobbin.

LIFT-OFF - L.O., mm, see Sections 5.5 and 5.8.4.- Distance between the coil of a surface probe and

oample=It is a measure of coupling between probe andsample.

MAGNETIC FLUX - <f> , webers, see Section 9.3.,

MAGNETIZING FORCE - H, amperes/metre, see Section 9.3.Magnetic field intensity.

MAGNETIC FLUX DENSITY - B, tesla, see Section 9.3.

MODULATION ANALYSIS - An instrumentation method whichseparates dosirable from undesirablefrequency signals from the modulatingenvelope of the carrier frequency signal.Test sample must move at constant speed.

NOISE - Any undesired signal that obscures the signal ofinterest.It might be electrical noise or a signal fromspecimen dimensional or property variations .

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NULL BALANCE - see Section A.2.1.

OHM'S LAW - Electromotive force across a circuit is equal tothe current flowing through the circuitmultiplied by the total impedance of thecircuit.

OPERATING POINT - see Sections3.5, 5.6 and 7.3.3.- Point on the impedance diagram that specifies the

normalized inductive reactance and resistance ofa coil.

OSCILLATOR - The electronic unit in an eddy currentinstrument that generates alternating probeexcitation current.

PARAMETER - A material property or instrument variable.

PERFORMANCE STANDARD - Also referred to as ReferenceStandard.A test standard used to qualify and calibrate atest system for a particular test.

PERMEABILITY (MAGNETIC) - p(mu), henry/metre; see Sections 2.4and Section 9.3.or pr> dimensionlesg, relative magneticpermeability.

- Ratio between flux density, B, and magnetizingforce,H. Permeability describes the intrinsicwillingness of a material to conduct magneticflux lines.

PHASE LAG - $(beta), radians or degrees; see Section 2.4.- A lag in phase (or time) between the sinusoidal

currents flowing at the surface and those belowthe surface.

PHASOR - see Section 3.3.- A vector describing sinusoidal signals; it hasboth amplitude and phase.

PRIMARY FIELD - The magnetic field surrounding the coil dueto the current flowing through it.

PROBE - Eddy current transducer.

REFERENCE COIL - Coil which enables bridge balancing inabsolute probes. Its impedance is close to testcoil impedance but does not couple to testmaterial.

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RESONANCE - See Sections4.3, 5.9 and 7.2.5. A circuit havingan inductor and capacitor connected in series orparallel. When inductive reactance equalscapacitive reactance the circuit is tuned or inresonance.

RESISTANCE - R, ohms; see Section 3.2.- The opposition to the flow of electricalcurrent.

- Applies to DC and AC.

RESISTIVITY - p , microhm-centimetre; see Sections 2.4 and9.2

- Reciprocal of conductivity (p -I/a ) .

SATURATION (MAGNETIC) - A condition where incrementalmagnetic permeability of a ferromagnetic materialbecomes 1.0.

SECONDARY FIELD - The magnetic field produced by induced eddycurrents.

SEND-RECEIVE - See Sectiona3.4, 4.5 and 5.4. The variationsin the test object which affect current flowwithin the test object can be detected byobserving their effect upon the voltage developedacross a secondary receive coil.

SIGNAL - A change in eddy current instrument output voltage;it has amplitude and phase. r«

SIGNAL-TO-NOISE RATIO - Ratio between defect signal amplitude a

and that from non-relevant indications. Minimumacceptable ratio is 3:1. T

SKIN DEPTH - See depth of penetration. I

SKIN EFFECT - See Section 2.4. -r- A phenomenon where induced eddy currents are Irestricted to the surface of a test sample.Increasing test frequency reduces penetration.

SURFACE PROBE - See Chapters5 and 6. --- A probe for testing surfaces, which has a

finite coverage. The coil is usually ~\pancake in shape. '

TEST COIL - Coil coupled to test material. It senses —geometric, electric and magnetic changes in [test material.

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VOLTAGE - V, volts, see Section 3.3.- Electric potential or driving force for current.- Output signal from an eddy current instrument.

VOLTMETER - The instrument used to measure voltage.

VECTOR - see Sectioi. 3.3.- A quantity having amplitude (magnitude) and

direction. Normally represented as a line whoselength represents the quantity's magnitude andthe angular position the phase (relative to somereference).

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10.2 REFERENCES

1. H.S. Jackson, "Introduction to Electric Circuits", 2ndedition, Prentice-hall, Inc., Englewood Cliffs, NewJersey (1965).

2. C.V. Dodd, "The Use of Computer-Modelling for EddyCurrent Testing", Research Techniques in NDT, Vol. Ill,edited by R.S. Sharpe, Academic Press Ltd., London,p. 429-479 (1977).

3. H.L. Libby, "Introduction to ElectromagneticNondestructive Test Methods", Wiley-Interscience, NewYork (1971).

4. "Nondestructive Testing Handbook", Vol. II, edited byR.C. McMaster, Ronald Press, New York, p. 36.1-42.74(1963).

5. "Eddy Current Testing, Classroom Training Handbook",General Dynamics/Convair Division, San Diego, California(1979). CT-6-5 Second Edition.

6. W.J. McGonnagle, "Nondestructive Testing", 2nd edition,Gordon and Breach, New York, p. 346-390 (1961).

7. F.R. Bareham, "Choice of Frequency for Eddy Current TubeTesting", British J. Applied Physics, 11, 218-222(1960).

8. V.S. Cecco, "Design and Specifications of a HighSaturation Absolute Eddy Current Probe with InternalReference", Materials Evaluation, 21» 51-58 (1979).

9. J. Stanley, "Electrical and Magnetic Properties ofMetals", American Society for Metals, Metals Park, Ohio(1963).

10. H.V. Pellegrini, "Assessing Heat Damage in AluminumAlloys with an Eddy Current Testing Technique", MetalsProgress, XiJ_, 60-63 (1980).

11. ASME Boiler and Pressure Vessel Code, Section V, Article8, Appendix 1, "Eddy Current Examination Method forInstalled Non-Ferromagnetic Steam Generator HeatExchanger Tubing" (1978).

12. "Nondestructive Inspection and Quality Control", MetalsHandbook, Vol. 11, 8th edition, American Society forMetals, Metals Park, Ohio, p. 75-92 (1976). |f

I13. R. Hochschild, "Electromagnetic Methods of Testing

Metals", Progress in Nondestructive Testing, Vol. 1,MacMillan Co., New York, p. 59-109 (1959). IT

If

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10.3 INDEX

Absolute Probe - 56,105-109Alternating Current - 8,16,21-23Anomaly - 98Bridge - 34-37Bridge Balance - 34-37Calibration Standard - 101-103,125Capacitive Reactance - 20Characteristic or Limit Frequency - 120-125,128Characteristic Frequency Ratio - 120-125,130Characteristic Parameter - 55,74-76,87,88,120Circumferential Coil - 105,109,125Conductivity - 11,163-166Coupling - 25,29,55,107,113Current - 5-10,21-23Defect - 55,65,66,78,89-97,101,131-147,178Depth of Penetration (Standard) - 13,17,18,79Differential Probe - 57-58,105-109Direct Current - 21,22Discontinuity - 188Eddy Currents - 6-18,59,60,109,110,132Eddy Current Method (Testing) - 1,19,55,89,98,131,164Edge Effect - 81Effective Depth of Penetration - 14Encircling Probe (Coil) - 105,113,116,120,151End Effect - 189Excitation Coil - 6,11,45,67Faraday's Law - 9,17,49,60,69,116Faraday, M. - 2Farad - 20Feed-Through Coil - 189Ferrite - 41,189Ferromagnetic - 10,30,98,168Fill-Factor - 29,113-115,150Flaw - 189Forster - 3,120Foucault Currents Method - 189Frequency - 5,8,13,17,72,120,123,124,129,130Frequency (Angular) - 8Frequency Response - 53Hall Detector - 6,33,46,181Henry - 19Hysteresis (B-H curve) - 169-172IACS - 163-166Impedance - 8,9,20,25,32Impedance Diagrams - 25-31Impedance Method - 24,33Inductance - 19,61,62,63,110,111Inductive Reactance - 20,27,69,176Inductor - 19

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I11Internal Probe (Coil) - 106

Lena's Law - 9,23Lift-off - 43-47,84 *Limit Frequency - 120-125 gMagnetic Field - 6-7Magnetic Flux - 7-10 --Magnetic Flux Density ~ 7,168,169 IMagnetic Permeability - 11,13,71,72,94,98,99,149,150,168-174 •Magnetic Saturation - 168-171,178-184Magnetizing Force - 168,171 1Modulation Analysis - 50 [|Noise - 34,37,41,50,87,161,190Non-ferromagnetic - 10,98,151 r|Null Balance (Bridge Balance) - 34-36 IIOersted - 6,8Ohm's Law - 8,17,61,117Operating Point - 27-29,31,76,98,99,120-122,133,150 IOscillator - 5,33,34,43 !lParameter - 65,191Performance Standard - 191 [1Permeability (Magnetic) - 11,13,71,72,94,98,99,149,150,168-174 [|Phase - 76,78Phase Lag - 2,14-17,78,93 r*Phasor - 21 11Primary Circuit - 8,25Primary Field - 191Probe - 55-60,105-110 {[Receive Coil - 6 , 24 ,67 ,81 llReference Coil - 36 ,56 ,57 ,106Resistance - 19,28-31,131-133 PTR e s i s t i v i t y - 1 3 , 1 7 , 7 1 , 7 2 , 8 0 , 1 0 0 , 1 6 3 - 1 6 8 IIResonance - 3 8 , 3 9 , 8 5 , 8 6 , 1 1 2Saturation (Magnetic) - 171,178-184 cjSecondary Field - 10,191 IjSecondary Voltage - 78 '" '•"Send-Receive - 6,24,33,45-48,81Sensing Coil - 6,24 j"Signal - 192 'Signal-to-Noise Ratio - 63,192Similarity Condition (Law) - 75,122 rr,Sinusoidal - 5,12 ! jSkin Depth - 13,14,17,125Skin Effect - 11 -,Speed of Response - 53 . jStandard Depth of Penetration - 13,14,17,79 'Surface Probe - 55-59Test Coil - 56-57Vector - 23Voltage - 8,9,21,33Voltmeter - 6 ~

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