12- theory tofd fcb

TOFDTime of Flight Diffraction

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TOFD Technique Overview Calibration Applications Summary & Limitations

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TOFD Overview Forward Diffraction

Technique First described by Silk

in 1977– Using diffracted signals

from crack tips

Traditional grey scale TOFD presentation

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Advantages of TOFD Technique

– Wide coverage area using a pair of transducers – Accurate flaw sizing; amplitude-independent – Sizing technique using time-of-flight information – On-line volume inspection - very fast scanning – Setup independent of weld configuration – Sensitive to a variety of defects – No sensitivity to defect orientation – Amplitude-insensitive - acoustical coupling less critical

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Diffraction

– Modification or deflection of sound beam– Sound striking defect causes oscillation– Ends of defect become point sources– Not related to orientation of defect– Weaker signal than reflected – needs higher gain– Sharp defects provide best emitters– Tips signals are located accurately– Time of flight of tip signals used to size

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Diffraction

Each point of the defect generates new elementary spherical waves called diffraction

The incoming wave vibrates the defect.

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Waves

FLAW

Diffractedwaves

Diffractedwaves

Incidentwave

Reflectedwave

All directions

Low energy

Independent of incidence angle

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Slot or crack

Conventional Use of Diffraction Tip diffraction method (satellite-pulse observation technique)

Time

Amplitude

2

2

Tip diffraction1

1

Corner reflection

TOF

Angle

TOF, Angle and velocity Height

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Signals Signals Received

– Lateral wave– Subsurface– Back-wall echo– Mode converted (shear wave) echo

Define top and bottom of part Note phase change

Basic Principles of the TOFD Technique

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TOFD: Typical Setup

Transmitter Receiver

Lateral wave

Upper tip

Lower tip

Back-wall reflection

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A-Scan SignalsTransmitter Receiver

Lateral wave

LW

Upper tip Lower tip

Back-wall reflection

BW

Some Typical DefectsUpper surface breaking crack

Back wall breaking crackHorizontal planar defect

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Upper Surface Breaking CrackTransmitter Receiver

Crack tip

Back-wall reflection

BW

Lateral wave is blocked

No Lateral wave

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Back Wall Surface Breaking CrackTransmitter Receiver

Lateral wave

LW

Tip

Back wall echo blocked

No back wall echo

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Horizontal Planar Defect(Lack of Inter-Run Fusion, Laminations)

Transmitter ReceiverLateral wave

LW

Back wall reflection

BW

Reflection echo

Reflected signal

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Data Visualization

White+

Black-

Amplitude

Time

Time

One A-scan picture is replaced by one gray-coded line

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Data Visualization

D-scan

Uppersurface

Back wall

A-scanLW

BW

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Calibration Tools A-scan

D-scan

PCS

Tc

LW BW

Parameters:PCS, Thickness, velocity, Probe delay, Lateral wave or Back wall

Not all of the parameters need to be known

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Measurement Tools A-scan

D-scan

Cursors

t1 t2

l

Pt1,t2 d1, d2 and h are automatically calculated

d1d1h

Build-in calculator

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Defect Position Influence

Transmitter ReceiverS S

d

t0 t0

x

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Defect Position Uncertainty

Transmitter ReceiverS S

t2t1

Constant timelocus

(t1+t2=ct)

dmin dmax

In practice:Maximum error on absolute depth position lies below 10 %.Error on height estimation of internal (small) defect is negligible.Caution for small defects situated at the back wall.

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Transverse Scan

Upper surface

Back-wall

B-scan

Lateralwave

This type of scan yields a typical inverted parabola

Time will be minimum when probes are symmetrically positioned over the defect

04/17/23 Source: Ginzel

What do TOFD scans really look like?

Lack of Fusion

Porosity

Incomplete Penetration

Slag

TOFD images show the lateral wave and backwall, plus SW signals after and reflections from all defects

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What do TOFD scans really look like?

Lateral wave is clearly seen in a good TOFD scan. Typically used for calibration.

On clean material, defects show up well.

Backwall is always strong. Watch for perturbations.

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Recommended Solution

TOFD: YES BUT: do not forget the good things offered by the

standard Pulse-Echo technique SOLUTION: do both TOFD and PE

simultaneously, without reducing the scan speed

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Typical Requirements for TOFD and PE Applications

Small, lightweight, 1 to 16 channels PE and TOFD software Lateral wave straightening Real-time averaging Multi-channel data acquisition and display Linearization for true depth on flat or cylindrical

surfaces Processing (data compression,..)

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Calibration A-scan

D-scan

PCS

Tc

LW BW

PCS, Thickness, velocity, Probe delay, Lateral wave or Back wall

Typical multi-channel UT instrument is very user friendly and guides you with a software Wizard

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Weld 1(PL4882)

Toe crack

Lack of side wall fusion

Lack of root fusion

Porosity

12.5 mm

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Weld 1 (plate 4882) with Pulse-Echo

TOFD and PE clearly show the defects embedded in this weld:

•Lack of Fusion (root)

•Lack of Sidewall Fusion

•Porosity

•Toe Crack

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Parallel Scan

D-scan

Uppersurface

Back-wall

A-scanLW

BW

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Recommended Solution

The system allows for simultaneous acquisition and analysis (inTomoview

only) of TOFD and PE

TOFDPE 45 SW PE 60 SW

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Linearized Lateral Wave

Transmitter Receiver

Lateral wave

Couplant thickness variation Change in time of flight

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Linearized Lateral Wave

Transmitter Receiver

Lateral wave

Misalignment variations Change in time of flight

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Linearized Lateral Wave

Transmitter Receiver

Lateral wave

Small mechanical variations of probe separation Change of time of flight

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TOFD Advantages Excellent PoD for mid-wall defects Good detection of mis-oriented defects Can characterize surface-breaking defects Excellent sizing for defects in transverse TOFD mode,

especially with signal processing Tolerable sizing for defects in linear mode Works very well in conjunction with pulse-echo

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TOFD Limitations Dead zone of ~3mm at outer surface Potential dead zone at inner surface Prone to “noise” Over emphasizes some benign defects, e.g.

porosity, laminations, interlamellar LoF Not easy to interpret

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A Few Final Words on TOFD Is the best defect sizing technique available when correctly

set-up Use in conjunction with pulse-echo for code and PoD

reasons

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SignalsLateral Wave

Back-wall Echo

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Calculator

TOFD probe separation can be calculated with basic mathematical formula or Excel calculator tools

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Calculator

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Calculator

-13.6

-28.9

-55

-45

-35

-25

-15

-5

5

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

X-axis (mm.)

Z-a

xis

(mm

.)

Note volume coverage less than ideal at this PCS (missing upper third). -12dB beam transmit only used for coverage calculation

7MHz 100mmPCS 1.5 cycles assumed

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Calculator

Increase refracted angle to 65° improves coverage without compromising resolution (for these specific conditions) PCS remains the same.

-6.6

-23.3

-55

-45

-35

-25

-15

-5

5

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

X-axis (mm.)

Z-a

xis

(mm

.)

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Flaw Tip

Flaw lengths parallel to the surface can be measured from the TOFD image by fitting hyperbolic cursors similar to SAFT correction but SAFT post-processes the data

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TOFD - SAFT

Pre-SAFT processing – Post-SAFT Processing

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Limitations of TOFD Cannot detect all defects Limited coverage results from two potential

dead zones– Dead zone near the surface

» as a result of the lateral wave

– Dead zone at the backwall» resulting from the width of the backwall reflection

Other Typical Defects

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Near Surface Crack

The crack blocks the Lateral WaveAnd the lower tip appears on the A-scan

21

1

2

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Incomplete Root Penetration

21

Note the two signals from the top & bottom

12

3

4

1 2 3 4

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Lack of Root Penetration

Note the inverted phase between LW and defect

1

2

3

1

23

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Lack of Fusion - Side Wall

Note the two signals from the top & bottom

1

2

3

4

1

2

3

4

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Porosity

Porosity may image in many forms whether individual or cluster

12

3

1

2

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Transverse Crack

In the LW we can observe the wide beam effect on the crack

1

2

3

4

1

2

3

1

2

3

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Concave Root

Distortion of back-wall echo

1

23

1

2

3

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Lack of Fusion - Interpass

1

2

3

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Choosing an Angle» Optimum Upper tip q ≈

64°» Optimum Lower tip q ≈

68°

From Charlesworth & Temple

Angle selected is a compromise for depth

May require selecting several “zones” for best results

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Diffracted rather than reflected signals Longitudinal waves B-scan type imaging (side view) Accurate sizing capability (height) Fast scanning Interpretation of defects Less sensitive to defect orientation

TOFD Advantages

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Blind area - near surface, backwall Weak signals Flaw classification limitation Interpretation of defects Sensitive to grain noise

TOFD Limitations

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Codes and Standard British Standard European ASME

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TOFD Guides Developed BS 7706 (1993) Guide to calibration and

setting-up of the ultrasonic time-of-flight diffraction (TOFD) technique for detection, location, and sizing of flaws. British Standards Institute 1993.

pr EN 583-6 (1995) Nondestructive testing- ultrasonic examination - Part 6: Time-of-flight diffraction technique as a method for defect detection and sizing.

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ASME Adaptations to TOFD

ASME VIII Code Case 2235 (2000 Edition) Ultrasonic examination to be in accordance

with ASME Section V, Article 4 “Alternatively, for techniques that do not use

amplitude recording levels, …”. This has opened the door for TOFD to be

used on Section VIII pressure vessels

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Root CrackTOFD technique Root crack defect

X-Ray

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PorosityTOFD technique Porosity defect

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Slag inclusionTOFD technique Slag inclusion defect

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Lack of Root FusionTOFD technique Lack of root fusion defect

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Root ConcavityTOFD technique Root Concavity defect

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Incomplete Root PenetrationTOFD technique Incomplete root penetration defect

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Over PenetrationTOFD technique Over penetration defect

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Irregular Root PenetrationTOFD technique Irregular root penetration defect

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Excess Weld CapTOFD technique Excess weld cap defect

X-Ray