magnetic analysis 5 (1)

8/10/2019 Magnetic Analysis 5 (1)

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Roderic K. Stanley, NDE Inspection Consultants,Houston, Texas

5C H A P T E R

Magnetic LeakageField Measurements 1


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Magnetic particle testing is not an isolatedtechnical discipline. It is a combination of two distinct nondestructive testingtechniques: magnetic flux leakage testingand visual testing. The basic principle of the magnetic particle technique is tomagnetize an object to a flux density thatcauses magnetic flux leakage from adiscontinuity. Powdered ferromagneticmaterial is then passed through theleakage field and those particles held overthe discontinuity are visually interpretedby the operator.

From a theoretical point of view, theonly difference between magnetic fluxleakage testing and magnetic particletesting is the use of iron or iron oxides asa sensor. In effect, magnetic particles maybe considered a commonly used form of sensor for the detection of magnetic fluxleakage (sometimes called stray fields ).

The key to ideal magnetic particletesting is to provide the highest sensitivityto the smallest discontinuities by a carefulcombination of: (1) applied magnetic fieldintensity H (t ), (2) flux density B(t ) inthe test object, (3) particle size andapplication method and (4) optimalviewing conditions. In order to do this,experiments are necessary with all of theparameters. The best combination is then

chosen for a particular application.Writers of specifications have oftenover-generalized this empirical process inorder to provide the magnetic particle testoperator with a set of rules that govern allsituations. This generalization can lead toinappropriate specifications for certainmagnetic particle tests.

There are many forms of magnetic fieldsensors, including the hall element, themagnetodiode, the ferroprobe and thesensor coil. 2,3 Tape recorder heads aremagnetic sensors, as are the triaxial fluxgate magnetometers that are orbitedabove the Earth to detect very small

changes in magnetic fields. The purposeof this chapter is to provide details aboutthe use of sensors in measuring anddetecting fields for magneticnondestructive tests.

Induction of Magnetic FluxLeakageThe essence of all magnetic flux leakagetesting is to induce a magnetic flux

density B

(t ) around a discontinuity. The

flux density may or may not be timedependent but should be at such a levelthat some of the flux is displaced by thediscontinuitys higher magneticreluctance. The displaced flux is forcedout of the objects surface into thesurrounding environment (air or water),where it can be detected (Fig. 1). 4

Magnetizing CurrentTo induce flux leakage, magnetizingcurrent can be passed through the testobject by direct contact. This iscommonly done but because of thedanger of arc burns, it is not alwaysrecommended (Fig. 2). Insulated currentcarrying rods or cables may be used, bypassing them through holes in the testobject. Other alternatives are the use of coils to carry the current around the testobject and the use of electromagnets orpermanent magnets applied to the testobject. 5

When current is present, there is anassociated field intensity H (t ) that raiseslocalized areas to various flux density B (t )values, based on the BH properties of thetest material. Figure 1 shows a computersimulation of field lines in and above a

material at some value below magneticsaturation, as can be seen from thebending of the field lines under thediscontinuity. 4

140 Magnetic Testing

PART 1. Fundamentals of Magnetic FluxLeakage Fields

FIGURE 1. Field lines around, through andabove discontinuity (an oblique slot), ascomputed by finite element computer model. The magnetic flux leakage field isasymmetric. 4


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Effect of Flux Leakage onFalse IndicationsIn a magnetic particle test, it is importantto raise the field intensity and fluxdensity in the object to a level thatproduces magnetic flux leakage sufficientfor holding particles in place overdiscontinuities. On the other hand,excessive magnetization causes particles tostick to minor surface leakages not causedby discontinuities.

If such surface leakage occurs (Fig. 3)and attracts large numbers of particles,the result is a false indication and the testobject is said to be over magnetized forthis inspection. It may then be necessaryto verify the test results with anothernondestructive testing method. Such falseindications may result from localpermeability changes which are caused bylocal stresses in the test object. In somecases, the magnetic flux leakage fieldmight be caused by a subsurface material

discontinuity and it may not be possibleto distinguish the cause of the leakagewithout the use of additionalnondestructive testing technology.

One way around this problem of excessive magnetization is to localizemagnetizing fields at the object surface.This can be done using alternating currentfields and the corresponding skin effect.As a rule, skin depth (also called standard depth of penetration ) for 60 Hz alternatingcurrent fields in steel is typically about1 mm (0.04 in.), depending on thepermeability and electrical conductivity of the test object. The field intensity falls toe1 of its surface value, or 37 percent, atthis depth. At two skin depths, fieldintensity falls to e2 or 13 percent of itssurface value. Magnetic flux leakage from

discontinuities depends on the value of H (t ) and, in turn, on how large a B(t )value the field intensity causes around thediscontinuity.

Why Particle IndicationsFormSurface-breaking discontinuities bestdetected by magnetic particle tests arethose that expel the optimal magneticflux leakage for the technique. To gain aclearer insight of this, it is necessary tounderstand three sets of variables: (1) howdiscontinuity parameters affect theexternal magnetic flux leakage, (2) howmagnetic field parameters affect theexternal flux leakage field and (3) how thesensor reacts to passing through suchfields.

Discontinuity ParametersThe discontinuity characteristics that arecritical to the formation of magneticparticle indications include depth, width,and angle to the object surface. Theeffects of discontinuity width on thetopography of the magnetic flux leakagefield have been described in what mightbe termed classical approaches 68 wherethe discontinuity may be replaced byarrays of poles. Higher ambient fieldintensities or flux densities are includedwithin such models by increasing the poledensities that give rise to the magneticflux leakage fields. More recently,computer models have been developed 4,9to explain how magnetic flux leakage

fields are related to discontinuityparameters (Fig. 1 is an example of suchwork).

In cases where the discontinuity isnarrow and surface breaking (seams, laps,quench cracks and grind tears), themagnetic flux leakage field near themouth of the discontinuity is highlycurved (Fig. 4). The activating fieldintensity may be quite small (a fewamperes per meter) or, after saturating thetest object, inspection can be performedwith the resulting residual induction.

In the case of subsurfacediscontinuities (inclusions and

141Magnetic Leakage Field Measurements

FIGURE 2. Micrograph of typical arc burn onsurface of steel pipe, caused by directcontact magnetization.

Legend A. Air.B. Molten metal, solidified.C. Steel, burnt and recrystallized.D. Steel not burnt.

A

B

C

DFIGURE 3. Minor surface flux leakage fromvariations in local magnetic permeabilitymay be source of false test indications.


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laminations), the magnetic flux leakagefield at the inspected surface (Fig. 5) ismuch less curved. Relatively high valuesof field intensity and flux density withinthe object are required for testing. Thislack of leakage field curvature is due tosaturation and greatly reduces theparticles ability to stick to suchindications.

Magnetic Field ParametersThe properties of the magnetic field thatmost affect flux leakage include the fieldintensity, local BH properties and theangle to the discontinuity opening. Theleakage fields ability to attract magneticparticles is determined by severaladditional factors. These include (1) themagnetic forces between the magneticflux leakage field and the particle,(2) image forces between a magnetizedparticle and its magnetic image in thesurface plane of the test object,(3) gravitational forces that may act

to pull the particle into or out of themagnetic flux leakage field and (4) surface

tension forces between the particle vehicleand the object surface (wet methodtests). 10

Some of these forces may in turn varywith: discontinuity orientation; theEarth's gravitational field; particle shapeand size (in effect, with particle effectivepermeability); and with the particlescontaining medium.

The magnetic force F m (N) that holds asingle particle to a magnetic flux leakagefield is determined by the vector relationin Eqs. 1a and 1b. Strictly speaking, themagnetostatic force F m acts on a magneticdipole with a spatially constantmoment m (such as a magnetic particle ora current carrying loop with a lengthmuch smaller than the scale over whichthe field is varying) in a magnetic leakagefield H :

(1a)

where H is the ambient leakage field

intensity (Am1

) and H

is the gradientof the field (Am 2).In the special circumstance where the

particles moment is directly proportionalto the magnetic leakage field, Eq. 1a canbe rewritten:

(1b)

where K is a mathematical constant(Nm 3A2).

It can be seen from Eq. 1 that magneticforce F m depends on local field intensity H and how it changes over the length of theparticle H . For surface discontinuities,

H is large (because the field is highlycurved), while H itself need not be large.For subsurface discontinuities, H isrelatively small and H itself must be raisedto compensate for the small change.Unfortunately, raising H will also raisesurface noise.

In other forms of magnetic flux leakagetesting, the flux density is raised to ahigher level than is common withmagnetic particle testing and nonrelevantindications (noise) are in some wayrecognized. For example, the signals thatnoise induces in flux sensitive detectorsmay be filtered out. Magnetic flux leakagetesting is therefore not limited by ahuman inability to distinguish real fromapparent discontinuities. It is limited byan electronic inability to perform thesame function.

F K m = ( ) H H

F mm = H


0.5 mm(0.02 in.)

FIGURE 4. Highly curved magnetic field fromnarrow, surface breaking discontinuity.

FIGURE 5. Effects of induction on flux lines inpresence of discontinuity: (a) compression of

flux lines at low levels of induction arounddiscontinuity, so that no surface flux leakageoccurs; (b) lack of compression at highinduction, showing some broad surfacemagnetic flux leakage.

Flux leakage

(a)

(b)


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Described below are flux sensitive devicesused in magnetic nondestructive testing.The sensors detailed here measure eithermagnetic fields or their gradients.

Research 11 indicates that a lack of discontinuity detection can be blamed onthe magnetizing method, the particlesused, and the capability of the inspector.The important question that must beanswered before beginning any magneticparticle test is: what is the best possiblecombination of magnetizing means, particleshape, type and size, and operator training todetect a discontinuity of a specific size every time? Commonly accepted magnetizationmethods may not always be the best. Fluxmeasurement devices can help providemore accurate information about the testprocedure.

Commonly used magnetic fluxsensitive devices include: (1) a longstraight wire passing through a magneticfield, (2) the search coil, (3) search coilderivatives such as C and E cores, (4) thehall element, (5) the magnetodiode,(6) the ferroprobe and (7) the flux gatemagnetometer. For sensors in categories1 through 3, the output signal dependson some form of time variation for theambient field intensity. Sensors incategories 4 through 7 are not time

dependent.A long straight wire passing througha magnetic field is not used fornondestructive testing, but it is a crucialconcept for understanding the signaldeveloped in coil sensors as they passthrough magnetic flux leakage patterns.

Voltage Developedbetween Ends of StraightWireAs shown in Fig. 6, two conducting wires

PQ and RS are placed at right angles to amagnetic field (shaded area) of constantflux density B directed toward the reader.Let another free wire AA' be moved toposition CC' , a distance x away. The areaswept out by the wire is then:

(2)

where dA is the area swept out by themoving wire (square meters), L is thelength of the wire between PQ and RS

(meters) and x is the distance betweenposition AA' and CC' (meters). Themagnetic flux interrupted by the wire is:

(3)

where B is the magnetic flux density(tesla), n is the unit vector for the area dAand is the interrupted magnetic flux(weber). The two equations together give:

(4)

Faradays law of induction states thatan electromotive force e will be inducedin the wire and its magnitude is given bythe relation:

(5)

This is the rate at which the magneticflux is cut. Eliminating the flux betweenEqs. 4 and 5, and taking the componentof B perpendicular to ndA gives:

(6)

Finally, since dxdt 1 is actually thevelocity v of the wire, the inducedelectromotive force becomes:

(7a) e BLv =

e BLdxdt

=

e d

dt =

= L n x B

= B And

dA L x=


PART 2. Flux Sensitive Devices

FIGURE 6. Wire cutting magnetic fluxbetween A-A and C-C .

A C

L

S

Q

C A

x

v = dx (dt )1

P

R


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As an example calculation, consider atruck traveling north at 100 kmh 1. If thelength of the trucks axle is 2 m (6.6 ft)and the vertical component of the Earthsmagnetic field intensity is 3 10 5Wbm 2 (0.3 G), then from Eq. 7 theelectromotive force between the ends of the axle is:

(7b)

This example indicates the magnitudeof voltages induced when metal objectsmove in relatively small magnetic fields.As another example, compute theelectromotive force generated between theends of a 10 mm long wire when movingat 500 mms 1 through a field of 1.6 10 3 Wbm 2 (16 kG).

(7c)

It is unusual for B to be at right anglesto and under such circumstances a moregeneral form of Eq. 7 is required:

(8)

where B is the vector cross product of

the wire velocity and the flux densitythrough which it passes. Numerically, thisis ( B ) sin , where is the smaller anglebetween and B . The integral is takenalong the length of the wire because thelocal value of B (through which eachsegment of wire is passing) may vary.

In magnetic nondestructive testing,wires in the form of coils are moved in acontrolled fashion over a test surface sothat the value of is known and Eq. 8 canthen be written as:

(9)

where B is the perpendicular componentof the magnetic field, such as themagnetic flux leakage field shown inFig. 1.

The tangential flux density Bt plays norole in the development of theelectromotive force in the conductorbecause sin is zero for this fieldcomponent.

The electromotive force developedbetween A and A' appears across PR(Fig. 6) and can be measured with a

sensitive voltmeter. No current flows if P and R are not connected. Furthermore, inthe general case of conductor motionthrough magnetic fields, the variation of B along the conductor must be known sothat the integral of Eq. 9 can becomputed.

Example of a Straight Wire Signal

The electromotive force generated in theleading edge of the coil shown in Fig. 7 isdeduced from the perpendicular fieldcomponent of a tight crack. The simplestapproximation for this magnetic fluxleakage field is: 6,7

(10)

and:

(11)

where Bg is the flux density deep withinthe discontinuity (weber per squaremeter); Lg is the width of thediscontinuity (meter).

The origin of coordinates is the mouthof the discontinuity. If the length L of thewire is parallel to and shorter than the

B B L x

x y =

+

g g

2 2

B B L y

x y t

g g=

+ 2 2

e v B dl=

e = ( ) v B dl

e = ( ) ( ) ( )=

1 6 10

0 01 0 5

8 1

3 2

1

.

. .

Wb m

m m s

00 6 V

e = ( ) ( )

3 10 2

100 00060 60

5 2

2

Wb m m

sm

== 1 7 10 3. V


FIGURE 7. Parallel and perpendicular coils cutting magnetic flux leakage fields from discontinuity at speed v. For discontinuity fields longer than coil, coil outputs are as givenin Legend for E and E .

LegendB y = leakage as measured in vertical directionB = magnetic flux leakage (T)

E = parallel coil output, where E (B y 1 B y 2)Lv E = perpendicular coil output, where E (B y 1 B y 2)Lv H c = liftoff measured to center of coil (m)h = constant sensor liftoff (m)L = coil length into page (m)v = speed of coil relative to test object (ms 1)X = horizontal axis Y = vertical axis

Y

Perpendicular coil

Parallel coil

B y 1

B y 1

B y 2

h H c

v

B y 2

X


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discontinuity opening, then theelectromotive force developed betweenthe ends of the wire is taken from Eqs. 10and 11 as:

(12)

In traditional magnetic flux leakagetesting equipment, the value of

y is

maintained at some constant value h(liftoff of the sensor). The form of theelectromotive force is shown in Fig. 8 forincreasing value of liftoff h. From Eq. 12,the magnitude of the electromotive forceis shown to depend linearly on (1) thevalue of Bg Lg (the magnetomotive force of the discontinuity), (2) L (the length of thewire, provided that the aforementionedconditions are met) and (3) (the relativevelocity between the object and theconductor).

The dependence on liftoff can be seenby differentiating Eq. 12 for the turningpoints ( xo = h) and using these values tocompute the swing e in e. The result is:

(13)

In this field approximation, the swingin voltage as the conductor passesthrough the magnetic flux leakage field isinversely proportional to the liftoff.

Simple Pickup CoilsFigure 7 shows two commonly usedpickup coils: parallel and perpendicular.In some cases, the turns of these coils arewound onto small blocks of ferrite toincrease the value of B above its air value( B is the flux leakage componentperpendicular to the test surface). Air core

coils are discussed below.

Perpendicular CoilWith a one-turn coil passing at speedthrough the same magnetic flux leakagefield as above, the signal electromotiveforce is the difference between the twoelectromotive forces developed in thebranches:

(14)

where h1 and h2 are the liftoffs of the twobranches.

If the coil has N turns and a width of 2b, then h1 = H c + b and h2 = H b (theliftoff H c is measured to the center of thecoil). The electromotive force thenbecomes:

(15)

The results of varying h and b areshown in Fig. 9 where the electromotiveforce is similar in form to that of thestraight wire. The turning points in theelectromotive force are given by thesolution to Eq. 16.

(16)

For example, when b = 0.5 H c, then theturning points are xo = 0.74 H and 2 xo(the distance between the turning points)is 1.49 H c. The swing in signal e is difficultto compute in closed algebraic form.

Parallel CoilWhen the coil is oriented so that one setof wires follows another, then the outputsignal is the difference between the

x H H b b H b

02

4 2 4 2 2 22

3=

( ) + +( )c c c

e B L LNvx

x H b

x H b

= + +( )

+ ( )

g g

c

c

1

1

2 2

2 2

e B L Lv

x

x h x h

= ( )

+

+

g g

1 1

212 2

22

=e B L Lv

hg g

e B L Lv x

x y =

+

g g

2 2


FIGURE 8. Electromotive force developedbetween ends of conductor passing atconstant speed through leakage field such

as Eqs. 11 and 12.

Legende = electromotive force signal (V)x = lateral distance (meter)

Smallliftoff

Large liftoff

e

2x o

x o x ox

e


8/18

signals developed in the leading andtrailing branches:

(17)

Using Eq. 11 for the leading andtrailing edge fields ( B L and B T),substitute xL = x b and xT = x + b for theleading and trailing edge distances from

the center of the coil:

(18)

The form of Eq. 18 is shown in Fig. 10.The dashed lines are voltages induced inthe leading and trailing edges. The solidline is their difference or the form of theelectromotive force. The signal consistsof a major peak at x = 0 and two smaller

side peaks. The roots of Eq. 11 occur atxo = (h2 + b2)0.5 so that the distancebetween the points at which e = 0 isgiven:

(19)

The maximum value of the coil signaloccurs at x = 0 and is proportional tob(h2 + b2).1 Differentiation with respect

to b indicates that the coil signal ismaximized when b = h. Thus when thehalf-width of the coil is equal to theliftoff, the coil output voltage ismaximized with respect to the magneticflux leakage from the discontinuity. Thisargument also indicates that this type of coil discriminates against relatively longrange material surface noise such as mightbe caused by local permeability variations.

Ferrite Cores in CoilsFerrites are useful in pickup coils becausethey not only provide support for the wireturns but they also amplify the fluxdensity through the coil windings by avalue equal to the effective permeabilityof the ferrite.

For small pieces of ferrite (Fig. 11)where the dimensional (length to depth)ratio is small, the effective permeability of the ferrite may vary from the low teens tothe thousands. The advantage of usingferrite occurs not only in this

amplification but also in the fact thatferrites have very low electricalconductivities, minimizing detrimentaleddy current effects in them.

Electronic Considerations for CoilVoltagesIt is essential that pickup coils are used togenerate voltages and not currents. Once

2 2 2 2x h bo = +

ebNB L Lv

h b x

x b h x

=

+ +( ) +

2

2 2 2

2 2

g g

( ) +b h2 2

e B B Lv = L T


FIGURE 9. Form of voltage signal developedin perpendicular coil when passing atconstant speed through magnetic leakage

field. 2

Legendb = half of coil width (m)e = signal (V)

H c = liftoff (m) measured to center of coilx = lateral distance (m)

e b = 0.5 H c

b = 0.25 H c

b = 0.15 H c

xH c14 3 2 1

1 2 3 4

FIGURE 10. Electromotive force induced inparallel coil by passing it through magnetic

flux leakage field such as in Eq. 18. Coilpotential (volts) is difference betweenleading and trailing edge signals.

Legende = signal (V)x = lateral distance (m)

Raw signal fromleading edge of coil

Raw signal fromtrailing edge of coil

Coil motion

e

x o x o x


9/18

a current is allowed to flow in a coil, itcreates its own magnetic field, one thatcan interfere with the field underinvestigation. The output of such coils istherefore generally fed to an operationalamplifier of high impedance.

Coil Applications and DerivativesExamples of coils as detectors for

magnetic flux leakage are presented in theNondestructive Testing Handbook onelectromagnetic testing. Such coils canbe connected in series adding, seriesopposing (a figure eight), overlapping andmany other configurations.

Search coils are often wound on ferritecores to increase the flux through them(Fig. 11 shows two commonconfigurations). A detailed discussionof such sensors is given in theelectromagnetic testing volume.

Hall Element SensorsHall elements are slices of semiconductormaterial. When a current is passedthrough them and they are placed in amagnetic field, then a voltage developsacross two of the faces of the element.The voltage is proportional to themagnetic flux density B .

A solid state tesla meter is made up of the electronic components needed to

supply current to a hall element, to detectand measure the resulting voltage and tothen convert it to the measured fieldvalue.

Theory of Hall Element OperationElectrically conducting solids are almosttransparent to the flow of conductionelectrons because the ions in the element

lattice do not deflect conduction electronsas might be expected from a typicalbilliard ball model. As current is fed intoone end of an element (Fig. 12), electronsare deflected toward one side of theelement, in accordance with the lorentzforce F :

(20)

where B is applied flux density (tesla), E iselectric field intensity (volt per meter) onthe particle, e is electronic charge(coulomb) and is velocity of the particle(meter per second).

The term B is a vector crossproduct and is itself a vector at rightangles to both and B. Its directiondetermines which side of the element theelectrons are deflected toward. The theoryof solid state physics provides a voltage V hacross the element: 2

(21)

where b is the thickness of the element inthe direction of the magnetic field(meter), Bz is the component of the

applied field at right angles to the current(webers per square meter), I is the appliedcurrent (amperes) and Rh is the hallcoefficient (A 1s1).

In general, if the element is placed atan angle to the field B, such that Bz = Bcos , then the cosine of the angle mustbe found. Normally, the crystal is rotated

V R IB

bhh z=

F E B= + ( )e v


FIGURE 11. Ferrite cored magnetic fluxleakage detector coil systems:(a) configuration one; (b) configuration two.

Legendd = depth (meter)f = winding

= length (meter)

Ferrite

Input

f o 2f o

d 2

2f o 3 to 5 mmf o

(a)

(b)

FIGURE 12. Magnetic flux, drive current andhall effect voltage relationship (see Eqs. 20and 21).

B z

F

I b

X Y

Z

V h


10/18

until the maximum tesla meter reading isfound. At that point, = 0 becausecos = 1.

The value the hall coefficient Rh isdetermined by the interaction of chargecarriers with the crystal lattice. In a metalelement, it would be given by:

(22)

where e is the charge on the electron(1.6 10 19 C) and n is the electronconcentration.

Metals do not make good hall sensorsbecause their hall coefficients are too low.As can be seen in Eq. 21, the larger thehall coefficient, the larger the hall voltage.Investigations of hall coefficients formany substances have shown thatcombinations of elements from groups IIIand V of the periodic table give thehighest hall voltages and have the leastsensitivity to changes in temperature.Also, the charge carrier for these groups is

more likely to be a hole rather than anelectron.

Excitation of Hall ElementsWhere contacts occur between twodissimilar metals such as the current andvoltage attachments on the hall crystal,thermoelectric electromotive forces aregenerated.

If direct current is used to excite thecrystal, the voltage read by circuitryfollowing the voltage contacts is the sumof the hall voltage and the thermoelectricvoltage. For this reason, hall elementcrystal excitation is usually performedwith 25 to 350 mA alternating current.

Manufacture of Hall ElementsBulk hall elements are generally bismuthdoped semiconductors such as indiumantimonide (InSb). These are produced bysolid state crystal growth technology, cut

into small rectangular blocks and havecurrent and voltage leads attached beforebeing encapsulated. Typical sizes are assmall as 0.8 mm (0.03 in.) long by0.4 mm (0.015 in.) wide by 0.5 mm(0.02 in.) thick. 5

Vapor deposited hall elements havebeen reported for use in the testing of ballbearings by the magnetic fluxtechnique. 12 In this application, bismuthwas evaporated onto an aluminasubstrate. A newer development is tocombine the hall sensor, its power supplyand an amplifier on one chip. Figure 13shows configurations of typical hallsensors and their specifications.

Applications of Hall ElementsHall elements are used with tesla metersor other devices to detect or measuremagnetic fields. Typical configurations areshown in Fig. 14. In Fig. 14a, the hallsensor is held a fixed distance from acurrent carrying wire and the tesla meter

measures the field intensity created by thecurrent. In the case of pulsed currents, thepeak current can be measured with a peakreading tesla meter.

In Fig. 14b, a ferrite ring is added tomeasure small fields or currents. The highpermeability of the ferrite aids in creatinga high B value in the vicinity of thesensors active area. Figure 14c, 14d and14e show combinations of hall elementsand ferrite flux concentratorconfigurations used in magnetic fluxleakage testing.

The level of external field just outside apartially demagnetized material may bestbe measured with a hall element meter.Figure 15 shows an inspector checking theexternal field level with a tesla meter afterpartial demagnetization of the test object,a 270 mm (10.75 in.) diameter steel tube.

Crossed hall elements can also be used.Such configurations are used to checkwelds or to reconstruct the total fieldfrom the measured components. 13

MagnetodiodesThe magnetodiode is a solid state devicewhose resistance changes with fieldintensity. The device consists of positiveand negative zones within asemiconductor, separated by a region of material that has been modified to createa recombination zone (Fig. 16). Itsfrequency response is flat from directcurrent to 3 kHz and the device is stablewithout temperature dependence from10 to 50 C (15 to 120 F).

Rneh

= 1


FIGURE 13. Typical hall element probes:(a) flat; (b) axial.

2 5 mm (0.08 0.2 in.)

Aluminum holder

0.64 2 mm (0.025 0.08 in.)

Brass tube (nonmetallic optional)

(a)

(b)


11/18

Applications of MagnetodiodesMagnetodiodes have been used fordetecting magnetic flux leakage fromdiscontinuities in tubes. 3 The magneticflux leakage is excited by alternatingcurrent electromagnets arranged to detecteither internal or external surfacebreaking discontinuities. The systemillustrates the general principles of magnetic flux leakage testing. 1

Sensors are connected differentially toeliminate signals from the applied fieldand from relatively long range variationsin surface field intensity. This system andmagnetic flux leakage systems like it areused to rapidly evaluate the surfacecondition of tubes and can detect tight

discontinuities with a depth of only0.1 mm (0.004 in.).

Magnetic particle testing is often usedto inspect such tubes but while it isextremely sensitive to outer surface, tightdiscontinuities, its use for inner surfacediscontinuities requires a viewing device.

FerroprobesFerroprobes (also called foerster microprobes ) take many forms but for thepurposes of nondestructive testing theygenerally consist of cylindrical orrectangular ferrite upon which one or twocoils are wound (Fig. 11).

Flux gate magnetometers are used todetect small changes in the Earthsmagnetic field. As might be used bygeophysical prospectors, these devicesconsist of ferrite rings carrying many coilconfigurations.

Both of these devices are based on thesame physical laws as a tape recorder head

or any other ferrite cored magnetic fieldpickup. The difference between the two isthat ferroprobes are activated at highfrequency.

Typically, one coil is excited withalternating current at a frequency f . Thevoltage induced in a second coil atfrequency 2 f is then detected. Thissecondary signal carries informationabout the scanned magnetic flux leakagefield. Figure 17 is an example of thetangential magnetic flux leakage fieldtaken with such a probe over an angle slotin residual induction at a liftoff of 1 mm(0.04 in.). 8

Ferrite cores might be solid or hollow,to reduce eddy currents in the ferrite.


FIGURE 14. Hall element configurations:(a) sensor at fixed distance from wire;(b) ferrite core; (c) free standing fluxconcentrator; (d) symmetrically positionedcontacting concentrator; (e) asymmetric

contacting concentrator.

Legend= hall element

I = current (A)

I

I

(a)

(b)

(c)

(d)

(e)

FIGURE 15. Checking external field level withtesla meter after partial demagnetization.


12/18

Large Volume MagneticField Indicators 14

Bulk Field IndicatorsThe field measurement systems discussedabove are designed and used for theassessment of magnetic leakage fields

from material discontinuities. In all cases,the active sensing area of such a deviceis very small. In the case of the hallelement, which is rectangular in shape, itis possible to integrate the field over theactive area of the hall crystal, and socompensate for it. Then, by takingmeasurements at controlled distancesabove a magnetized surface, it is possibleto extrapolate the field values to that atthe surface. Once this is done, theelectromagnetic boundary conditionsindicate the magnetic field intensity justinside the surface.

The following text relates to the

detection or measurement of magneticfields over much larger areas, since theactive area of the sensor is larger than thatused for leakage field testing. Theinstruments are handheld, movingmagnet sensors used to measure bulkexternal fields at a relatively high liftoff from the test object. They are often usedas a practical check on the externaldemagnetization state of an object. Theiruse to detect magnetic field intensitywithin coils should be discouraged,since the coil field may remagnetize themoving magnetics. Note that thesedevices do not measure leakage fieldsfrom discontinuities.

A bulk magnetic field indicator can beused to measure the value of a uniformmagnetic induction field in air. Becausethe relative magnetic permeability of airis 1, this reading is also the numericalequivalent of the magnetic field intensityof air.

The magnetic field indicator is alsoused to determine the existence of amagnetic field external to a ferromagneticobject. To do this, the indicator isoriented against the objects surface andmoved to the position that gives themaximum external field reading.

Bulk Field Indicator Construction

Many magnetic field indicators are round,about 64 mm (2.5 in.) in diameter withthicknesses of 13 to 25 mm (0.5 to 1 in.).The indicators commonly used forchecking external field levels aftermagnetic particle tests have a range of about 1 to 2 mT (10 to 20 G) in divisionsof 0.05 to 0.1 mT (0.5 to 1 G). Positivereadings are north and negative readingsare south (Fig. 18).

A key component of the magnetic fieldindicator is a small movable field sensingmagnet. The magnet is mounted so it isfree to rotate. Its angular deflection isshown by the movement of a pointer.

A second key component is a fixedpermanent magnet. Its magnetic fieldintensity limits the useful range of theunit by providing a restraining force toprevent the sensing magnet from rotatingfreely. With no external magnetic field,these two magnets stay antiparallel toeach other and the pointer remains in a


40 degrees

M

a g n e t

i c f l u x

d e n s i

t y B

x ,

m T ( G )

FIGURE 17. Tangential magnetic flux leakage fields in saturated residual induction over 40 degree slot. 8

Legend A. Experimental data at 1 mm (0.04 in.) liftoff.B. Model with increased charge on acute face of slot.

0.4 (4)

0.2 (2)

0

0.2 (2)

0.8 (8) 4 0 4 8

(0.16) (0.16) (0.32)

Distance X , mm (in.)

B

B

A

A

Negative zone

Recombination zone

Positivezone

H H +

FIGURE 16. Diagram of magnetodiode,showing positive and negative zones inintrinsic semiconductor material. Changes inmagnetic field intensity H perpendicular tosurface alters resistance in recombinationzone.


13/18

neutral position, registering a zero reading(Fig. 18b).

The field indicator is designed so thatthe net magnetic field from these twomagnets is weak outside the device.Placing the indicator on an unmagnetizedobject does not induce poles on the objectsufficient for causing inaccurate meterreadings.

In principle, magnetic field indicators

could use a coiled spring instead of acalibrated magnet to return the pointer tozero once the external field is removed.However, slight changes in the sensingmagnets intensity would then require

recalibration of the unit. Also, a stronglypoled sensing magnet, when placed veryclose to an unmagnetized object, couldinduce localized poles and causeinaccurate readings.

Principles of Field IndicatorOperationA fixed magnet inside the devices

housing sets up a reference magnetic field.A small movable sensing magnet ismounted inside this field. If a nearbyobject also sets up a magnetic field in thesame area, the field sensing magnetrotates into a direction parallel to theresulting, combined magnetic field.

The instruments pointer is attached tothe sensing magnet and correspondinglyrotates into a direction perpendicular tothe resulting field. If the external fieldchanges polarity, the pointer rotates inthe opposite direction and the readingsalgebraic sign changes. If the magneticfield indicator is rotated through

180 degrees about its pointers zerodirection, there is no algebraic signchange in the reading (the scale is alsorotated 180 degrees).

However, in practice a reverse of polarity or rotation of the indicator oftenproduces a change in a readingsmagnitude unless the external field isperpendicular to the reference field.

Calibration and Use of FieldIndicatorsGenerally, there are two ways to calibratemagnetic field indicators. Thesecalibration methods provide two distinctways of using the devices.

The most common calibration methodcorrelates the angular deflection of theindicators pointer with the magnitude Bof a uniform external field whosedirection is parallel to the zero directionof the pointer. To measure a uniform field,the field indicator is positioned so thatthe zero direction of the pointer is parallelto the field.

Used in this manner, the field B froman external object is perpendicular to thereference field B* inside the magnetic fieldindicator. Or as depicted in Fig. 19:

(23)

For a small deflection, B* may beconsidered uniform and a large indicatesa relatively strong B. To keep within apractical calibrated scale when isbetween +45 and 45 degrees, themeasured field must be weaker than thereference field ( B less than B*).

In many applications, B may not beperpendicular to B*. For example, the

B B= * tan


FIGURE 18. Typical magnetic field indicator:(a) photograph; (b) diagram. Distance frompivoting point of sensing magnet to closestpoint on edge of casing is about 18 mm(0.75 in.). Size, shape, material, magnetic

field intensity and relative positions of sensing magnet and reference magnet varywith manufacturer.

(a)

(b)

Field indicator

Planview

Sideview

S N S N

N S

S N


14/18

direction of B may be unknown or thefield could be nonuniform. In a case likethis, the magnetic field indicator ispositioned against the objects surface andoriented in such a way that thedirectional marking on the devices casingis near to and perpendicular to theobjects surface. Because the field is notnecessarily normal to the objects surface,the reading can be less or greater than theactual value, depending on whether thefield makes an acute or obtuse angle withthe reference field ( and in Fig. 20).

The alternate way of calibratingcorrelates the uniform field value B withthe maximum deflection of themagnetic field indicators pointer. Thevalue of is obtained by orienting theinstrument inside the field B. As shown inFig. 21, the field vector B changes itsdirection relative to the field vector B*and their resulting vector traces out acircular path of radius B centered at thetip of B*.

For B less than B* and for a maximum

deflection , the resulting field vector istangential to this circular path. It followsthat the field B is actually parallel to thepointer (Fig. 21). When B B*, Eq. 24 isvalid.

(24)

Note the differences between Eqs. 23and 24. When measuring very weak fields,these two calibration methods are aboutthe same and the pointers angulardeflection is approximately linearwith the uniform fields magnitude( = B B* = for a small B B*). In general,for the same uniform field B:

(25)

As a consequence of this inequality, thescale of the second type of calibration isgenerally wider if marked on the same arcinside the same magnetic field indicator.When using an instrument that iscalibrated in the second way, the unit isoften rotated to verify that the readingsare maximized. Sometimes this isinconvenient for objects with complicatedgeometry because a rotation of the devicemay move its sensing magnet away fromthe area of interest.

If magnetic field indicators of the firsttype are used as if they had the secondtype of calibration (maximizing theirreadings by rotation) then the resultingmaximum values are actually greater thanthe true field values. Sometimes, in thisway, an estimate can be made for the sizeof a uniform or nearly uniform field, eventhough its direction is unknown.

B B= * sin


FIGURE 19. Pointer deflection in firstcalibration type. B is perpendicular to B * andtan = B B *1.

Pointersinitial/zeroposition

B *

Pointersnew positiondue to B

Sensingmagnetsinitial position

Sensing magnetsnew position due to B

FIGURE 20. Pointer deflection and whenB is not perpendicular to B * ( < < ).

B

B *

B

FIGURE 21. Pointer deflection in second calibration type. B and the pointer are parallel and is at maximum ( B * > B and > ).

Pointersinitial/zeroposition

Pointers newposition parallel to B

Sensing magnetsinitial position

Sensing magnetsnew position

B *


15/18

When measuring a nonuniform field,the reading of a magnetic field indicator isat best the average field value over thearea covered by the sensing magnet. Forexample, assume that the flux lines froman external magnetic source are almostparallel with the special directionalmarking on the magnetic field indicator.The two ends of the magnetic fieldindicators field sensing magnet may still

experience deflection forces of differentmagnitudes because of different fieldvalues and the resulting pointer deflectionis an average of the two field values.

Measuring of Residual FieldsThe primary function of a field indicatoris to measure the external magnetic fieldintensity close to an object, but not everykind of residual magnetism can bedetected by these instruments.

In a circularly magnetized object,where residual flux lines arecircumferential and form closed loopsinside the material, the induction issignificantly different from zero but thefield may not produce poles outside theobject. Such a circular field does notproduce significant readings in magneticfield indicators. Circularly magnetizedobjects do not attract or deflect magneticmaterials during normal use unless surfacediscontinuities occur in the magnetizedobject, producing strong external poles.

Consider a cylinder with flat ends thathas been longitudinally magnetized. Amagnetic field indicator is placed againstthe cylinders end surface and thedirectional marking on the indicatorscasing is lined up with the length of thetest object. The indicator is alignednormal to the cylinders surface and areading is taken.

If a demagnetization procedure hasbeen properly performed, the indicatorreading will be 0.1 mT (1 G) or less.Similar readings obtained on the side of the cylinder (with the directional markingperpendicular to the side surface) shouldbe about zero. Sometimes, if residualmagnetism is high, a nonmagnetic spaceris placed between the object and themagnetic field indicator and relativereadings are obtained.

For a cylinder that is large compared to

the indicators size, measurements madeat the center of the end surface are closeto the actual values immediately beneaththe surface of the object. The reason forthis accuracy is that the magnetic fluxesimmediately inside and outside thecylinders end are perpendicular to theend surface and the perpendicularcomponent of the magnetic inductionfield across the boundary surface iscontinuous, according to electromagneticfield theory.

However, the same accuracy is notpossible for geometries with sharp cornersor for objects that are small compared tothe size of the indicator. In theseinstances, the measured field may not beuniform (the direction and density of theflux lines vary across a small distance) anda tangential component exists. It isknown from electromagnetic field theorythat the tangential component of

magnetic induction may not becontinuous when crossing the boundarysurface between two media of differentpermeabilities. Therefore, the magneticinduction inside and outside the objectmay not be the same.

When the test object has an irregularshape or the residual field readings arelarge, one way to test external magnetismis to scan the entire surface of the objectwith a magnetic field indicator. Maximumreadings occur at locations wheresignificant external poles exist.

At such maximum reading locations,the field indicator can be used to

determine if the field is normal to theobjects surface. The field indicator ispositioned against the object and orientedwith its directional marking normal to thesurface. The device is rotated through180 degrees about the directional marking(normal to the objects surface). Duringrotation, variations of the indicatorreading are noted.

If the rotation does not affect theindicator reading, then the reading is thetrue field value and the true field isnormal to the objects surface at thislocation. If the field is not perpendicularto the objects surface, it is likely that, at acertain time during rotation, the actualfield vector will have no projection alongthe direction of the indicators referencefield, and the reading at that time will beexactly the component of the fieldnormal to the objects surface. In otherwords, the normal component of the fieldat this location will be no greater than thelargest value observed in the rotation.More precisely, it is in between thereadings obtained at the beginning and atthe end of the rotation and is no greaterthan the average of these two values. If the same value appears twice duringrotation, then it must be the normalcomponent of the field.

In most applications, the purpose of external residual field measurement is toensure that the objects are free of magnetic poles that detract fromserviceability. The exact value of theexternal residual magnetism may not becritical, so long as it is lower than a limitpredetermined by the users empiricaldata.



16/18

Checking Indicator ReadingAccuracyIf inconsistent results occur in matchingmagnetic field indicators, it is likely thatsome of the devices are malfunctioning.Certain indicator malfunctions are easy todetect, such as an imbalanced or damagedpointer, or mechanical failures at thesupport of the sensing magnet and

pointer assembly. High mechanical impactor sudden exposure to a strong magneticfield are among the less obvious causes forerratic readings.

A magnetic field indicator can alsobecome inaccurate if its magnetization ischanged by exposure to a strong directcurrent or a decaying alternating currentfield. If the fixed reference magnetsbecome partially demagnetized, the unitcan give readings much larger than agood units results (smaller B* in Eq. 23).If an indicators magnetic components aretotally demagnetized, its pointer may notreturn to the zero position, remainingvirtually anywhere on the scale.

Two different field indicators may givedifferent results at the same location onthe test object. However, these differencesalone do not indicate that one of the fieldindicators is malfunctioning. The sensingmagnets of different devices may havedifferent sizes and their location insidethe units may be different. They maytherefore not be measuring the field atexactly the same location. In addition,reference fields inside the units may alsodiffer.

In a highly nonuniform field, readingsmay not vary in the same ratio as varyingmeasurement locations. As a result, it maynot be possible to verify the accuracy of amagnetic field indicator by comparing itsreadings to a known reference unit (otherfield measurement devices may be equallyinaccurate in the nonuniform field). Thebest way to test the accuracy of aparticular device is to perform referencecomparisons in uniform or nearlyuniform magnetic fields.

To set up a uniform magnetic field forcalibration, a helmholtz coil may be used.This device contains two parallel coilsseparated at a distance equal to theirradius and connected in series addingmode. In about 30 percent of the volume

between the two coils, there is a veryuniform magnetic field parallel to theiraxes. The field value can either bemeasured with an appropriate meter orcalculated from the coils dimensions andthe value of applied direct current (inEq. 26, x R1 is 0.5 and Bo is replacedwith 2 Bo).

In addition to the helmholtz coil orcommercial calibration fixtures, anapproximately uniform magnetic fieldmay be established using a large direct

current coil. Over a small distance alongthe coils axis, a magnetic field can beconsidered nearly uniform. As examples,Table 1 shows a set of magnetic fieldvalues for a five-turn coil of 300 mm(12 in.) diameter carrying 1500 A directcurrent. The table values were calculatedusing the following equations.

(26)

where Bo is the magnetic field at thecenter of the coil (millitesla), R is coilradius (meters) and x is distance (meters)from the center of coil along the axis.

(27)

where I is applied direct current(amperes), N is number of turns in thecoil and o is the permeability constant(4 10 7).

Sometimes, the Earths magnetic fieldcan indicate a meters accuracy: theEarths field is about 0.05 mT (0.5 G). If the indicators accuracy is within0.03 mT (0.3 G), then with propernorth/south and horizontal orientations,the device should be able to register anapproximate reading of the Earths field,provided there are no other magneticobjects nearby.

The best magnetic field indicators areprecision calibrated. Their accuracy mayalso be less susceptible to the influence of a strong magnetic field. In someapplications, less costly magnetic fieldindicators may be used to domeasurements. Precision calibrated unitsare then used as reference standards,verifying the readings of the less costlydevices. Periodically, the reference devicesare returned to the manufacturers forcalibration.

B NI

Ro o=

2

B B

x R

=+

o

12 3 2/


Table 1. Magnetic flux densities forfive-turn coil carrying direct current,compared with linear distance from coilcenter.

Distance fromCoil Center Measured Value_______________ ________________

m (ft) mT (G)

0 0 31.0 (309.0)0.45 (1.5) 1.0 (9.8)0.9 (3.0) 0.14 (1.4)1.0 (3.3) 0.1 (1.0)


17/18

Use of Magnetic Field IndicatorA magnetic field indicator is a convenientlow cost tool for measuring the residualexternal field intensity of ferromagneticobjects. To measure a uniform field,indicators are often calibrated in a waythat requires the operator to align aspecial directional marking (line or arrowon the devices casing) with the fieldsknown direction.

For an external flux measurement, thefield indicator is positioned against theobject with its directional marking near toand perpendicular to the objects surface.This positioning is based on the fact thatflux lines are expected to be perpendicularto the objects surface at the location of significant poles.

In cases where the field direction isuncertain, the indicator may be rotatedabout its directional marking, which is inturn positioned normal to the objectssurface. The rotation moves through180 degrees to get a maximum reading.The component of the magnetic fieldnormal to the objects surface is no greater

than the maximum value registeredduring rotation and no greater than theaverage of the readings at the beginningand the end of rotation. If the rotationdoes not affect the reading, then the fieldis perpendicular to the objects surfaceand the reading is the fields true value.

For a nonuniform field, the reading of the magnetic field indicator is an averagevalue (at the spot where the indicators

field sensing magnet is located).Good magnetic field indicators havesound mechanical supports for theirreading pointers and these supportscannot be easily damaged. Their magneticcomponents cannot be easilydemagnetized by strong external fields.Also, they do not induce significantmagnetic poles on the objects they test.

For an accurate calibration of a fieldindicator, a uniform magnetic field maybe provided by a helmholtz coil. For aquick check of calibration, variousapproximate, uniform field values alongthe axis of a large direct current coil may

be used.



18/18

1. Stanley, R.K. and L.[C.] Wong.Chapter 7, Magnetic Leakage FieldMeasurements. Nondestructive Testing

Handbook , second edition: Vol. 6, Magnetic Particle Testing . Columbus,OH: American Society forNondestructive Testing (1989):p 179-198.

2. Bray, D.E. and R.K. Stanley.Nondestructive Evaluation: A Tool for

Design, Manufacturing and Service .Baton Rouge, LA: CRC Press (1994).

3. Nondestructive Testing Handbook ,second edition: Vol. 4, Electromagnetic Testing: Eddy Current, Flux Leakage and

Microwave Nondestructive Testing .Columbus, OH: American Society forNondestructive Testing (1986).

4. Hwang, J.H. Defect Characterization by Magnetic Leakage Fields . Dissertation.Fort Collins, CO: Colorado StateUniversity (1975).

5. Stanley, R. Basic Principles of Magnetic Flux Leakage InspectionSystems for the Evaluation of OilCountry Tubular Goods.

Electromagnetic Methods of Nondestructive Testing . New York, NY:Gordon and Breach (1985): p 97-150.

6. Foerster, Friedrich. NondestructiveInspection by the Method of Magnetic

Leakage Fields: Theoretical andExperimental Foundations of theDetection of Surface Cracks of Finiteand Infinite Depth. Defektoskopiya .Vol. 11 (1982): p 3-25.

7. Foerster, Friedrich. On the Way fromKnow How to Know Why in theMagnetic Leakage Field Method of Nondestructive Testing (Part 1).

Materials Evaluation . Vol. 43, No. 10.Columbus, OH: American Society forNondestructive Testing (September1985): p 1154. Part 2, Vol. 43, No. 11.(October 1985): p 1398.

8. Zatsepin, N. and V. Shcherbinin.Calculation of the MagnetostaticField of Surface Defects: Part 1, FieldTopography of Defect Models andPart 2, Experimental Verification of the Principal TheoreticalRelationships. Defektoskopiya . No. 5(1966): p 50-65.

9. Heath, S.E. Residual and Active Magnetostatic Leakage Field Modelling .Master of Science thesis. Fort Collins,CO: University of Colorado (1984).

10. Swartzendruber, L. Magnetic Leakageand Force Fields for Artificial Defectsin Magnetic Particle Test Rings.

Proceedings of the Twelfth Symposium onNDE. San Antonio, TX: SouthwestResearch Institute (1970).

11. Skeie, K. and D. Hagemaier.Quantifying Magnetic ParticleInspection. Materials Evaluation .Vol. 46, No. 6. Columbus, OH:American Society for NondestructiveTesting (May 1988): p 779.

12. Beissner, R., G. Matzkanin and C.Teller. NTIAC-80-1, NDE Applications of

Magnetic Leakage Field Methods: A Stateof the Art Survey . San Antonio, TX:Southwest Research Institute (1980).

13. Hall Effect Transducers: How to ApplyThem as Sensors. Freeport, IL:

MicroSwitch Company (1982).14. Wong, L.C. Magnetic Field Indicator:Principles and Use. Materials

Evaluation . Vol. 46, No. 6. Columbus,OH: American Society forNondestructive Testing (May 1988):p 749-754.

References

magnetic analysis 5 (1)

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