060301 TOFD Training

www.zetec.com

Principles and Application Principles and Application of the TOFD UT of the TOFD UT

TechniqueTechnique

Advanced NDT Training Programme,November 2004

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• The Diffraction Phenomenon

• Conventional Use of Diffraction

• Principles of TOFD

• Practical Implementation

• Codes and Standards

• Advantages and Limitations of TOFD

• Zetec Solution

• Demonstration

Time of Flight Diffraction TechniqueTime of Flight Diffraction Technique

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The Diffraction PhenomenonThe Diffraction Phenomenon

CRACK

Diffractedwave

Diffractedwave

Incidentwave

Reflectedwave

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The Diffraction PhenomenonThe Diffraction Phenomenon• Huygens’ principle:

Each point of object act as new source of spherical waves

incoming wave makesobject vibrate

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The Diffraction PhenomenonThe Diffraction Phenomenon

All directions

Low energy

Dependent of incidence angle

CRACK

Diffractedwave

Diffractedwave

Incidentwave

Reflectedwave

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The Diffraction Phenomenon: SummaryThe Diffraction Phenomenon: Summary

• Incident wave reflected wave

• Incident wave diffracted waves emitted by defect

boundaries

• Cylindrical/spherical waves emitted in all directions

• Amplitude typically 10 to 20 dB below specular (direct)

reflection

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• Zetec Solution

• Demonstration


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Slot or crack

• Pulse-echo tip diffraction method (satellite-pulse observation technique)

Time

Amplitude

2

2

Tip diffraction1

1

Corner reflectionTOF

Angle

TOF, ANGLE & VELOCITY HEIGHT

Conventional Use of Diffraction Conventional Use of Diffraction

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• Zetec Solution

• Demonstration


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Transmitter Receiver

Lateral wave

Upper tip

Lower tip

Back-wall reflection

Principles of TOFD Principles of TOFD

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Principles of TOFD : Basic Set-UpPrinciples of TOFD : Basic Set-Up

• 2 probes (transmitter, receiver)

• Wide beam, Long. waves

• Symmetrically to the weld center

• Lateral wave (sub-surface LW)

• Back-wall reflection

• Diffraction signal detection (high receiver sensitivity)

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Transmitter ReceiverLateral wave

LW

Upper tip Lower tip


BW

Principles of TOFD: A-Scan SignalPrinciples of TOFD: A-Scan Signal

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Lateral wave

LW

Upper tip Lower tip


BW+

-

+

-

Principles of TOFD: Phase DifferencesPrinciples of TOFD: Phase Differences

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Principles of TOFD: Principles of TOFD: Flaw Depth MeasurementFlaw Depth Measurement

• Based upon:

• Accurate flight-time measurements

• Simple trigonometric equations

• Achieved by UltraVision software

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S

Initial pulse

Transmitter ReceiverS

d

LW BW

t0 t0

t


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Transmitter ReceiverS S

d

t0 t0

0

22

.2.2

tc

dSt


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c

TStt B

22

0 2.2

LB tt

STSc

.22 22


d

t0 t0

PCS

T

SPCS .2 (Probe Center Separation)

c

Stt L

.2.2 0


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d

t0 t0

220

2

.2.2

Sttc

d


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Transmitter Receiver2S

d1

12 ddh

d2

Since only flight-time measurements are used to calculate the height, very accurate height sizing is possible. In practice, 1 mm accuracy on real cracks is

achievable (0.1 mm on artificial reflectors)

Principles of TOFD: Principles of TOFD: Flaw Height MeasurementFlaw Height Measurement

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Principles of TOFD: Principles of TOFD: Flaw CharacterizationFlaw Characterization

• In most cases, no correlation between amplitude and

importance of flaw

• Typical signature for each flaw type

• Interpretation of phase variations

• (Partial) loss and/or variation of LW, BW : indication for

surface-breaking cracks

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Transmitter Receiver

Crack tip


BW

Lateral wave is blocked

No lateral wave

Near-Surface Breaking Cracks Near-Surface Breaking Cracks

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Transmitter ReceiverLateral wave

LW

Tip

Back-wall echo blocked

No back-wall echo

Far-Surface Breaking Cracks Far-Surface Breaking Cracks

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(lack of inter-run fusion, lamination)(lack of inter-run fusion, lamination)Transmitter Receiver

Lateral wave

LW


BW

Reflected signal

Reflected signal

Horizontal Planar Defects Horizontal Planar Defects

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Principles of TOFD : SummaryPrinciples of TOFD : Summary

• Two probes, pitch & catch configuration

• Longitudinal waves

• Lateral wave (LW), back-wall echo (BW)

• Diffracted signals from defect edges

• Phase difference between tip & bottom signals

• Flaw depth and height are determined with high accuracy, based on

flight-time calculations

• Not based on amplitude

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• Zetec Solution

• Demonstration


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Practical ImplementationPractical Implementation

• General Set-Up

• Probe Considerations

• Data Processing and Presentation

• Manipulator

• Scanning Types

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Position encoder

Magnetic wheels

UT probes

Weld

Z-Scan UT

Scanner

Probes

Z-Scan UT

Practical Implementation: Practical Implementation: General Set-UpGeneral Set-Up

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• Propagation mode and angle

• Time domain resolution

• Beam characteristics

• Synthesis table

Practical Implementation: Practical Implementation: Probe ConsiderationsProbe Considerations

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• Longitudinal Waves :

• Fastest waves, easy interpretation, no confusion with mode

converted waves (SW)

• Relation between signal phase and signal origin (tip, bottom)

• Stronger diffracted signals

Propagation ModePropagation Mode

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• Relation between probe angle and amplitude of

generated diffracted signals

• Precision on flaw height measurement

• Inspected volume coverage

Compromise

In many cases 60 degrees is a good compromise

Probe AngleProbe Angle

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Time Domain ResolutionTime Domain Resolution

• Measurements based on flight-time

• Requirement for short ultrasonic pulses (importance of

UT equipment : probe excitation parameters)

• Higher frequencies than standard UT (pulse-echo)

examinations

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Beam CharacteristicsBeam Characteristics

• Wide beam to cover volume to be inspected

• High frequency small probe aperture lower

sensitivity

Compromise

Aperture

Beam Beam

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Probe Selection TableProbe Selection Table

Wall-thickness

(mm)

Frequency

(MHz)

Diameter

(mm)

Angle

(°)

t 15 7.5 / 10 / 15 6 60 / 70

15 < t 35 7.5 / 10 6 60

35 < t 100 + 5 10 30 / 45 / 60

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• Processing of all non-rectified signals requires powerful

computing capability

• Mass amount of complex signals requires a simple way

to visualize the data

• Calculations require easy to use tools

Practical Implementation: Practical Implementation: Data ProcessingData Processing

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• Huge amount of data

• Need for phase information

Practical Implementation: Practical Implementation: Data ProcessingData Processing

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Practical Implementation: Practical Implementation: PresentationPresentation

White+

Black-

Amplitude

Time

Time

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

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B-scan

Near-surface Back-wall

A-scanLW

BW


38


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• Flaw depth is expressed by a complex mathematical

equation

• Basic tools are needed for

• Initial calibration

• Performing depth and height measurements


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A-scan

B-scan

PCSt0 t0

Tc

LW BW

PCS, thickness, sound velocity, probe delay, lateral wave or back-wall flight-time

Not all parameters have to be known

Practical Implementation: Practical Implementation: CalibrationCalibration

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Cursors

Build-in calculator

t1,t2 d1, d2 and h are automatically calculated

A-scan

B-scan

c

d1d1h

t1 t2

l

P

Practical Implementation: Practical Implementation: Measurement ToolsMeasurement Tools

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Very simple to use

Magnetic wheels

Manual (or motorized)

One axis position encoding

Basically 2 probes, must be able to hold more (PE)

Easy and precise adjustment of probe separation is needed

Position encoder

Magnetic wheels

UT probes

Weld

Practical Implementation: Practical Implementation: ManipulatorManipulator

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• Non-parallel, along defect axis

• Parallel, across defect axis

Practical Implementation: Practical Implementation: Scanning TypesScanning Types

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Weld

Non-parallelscan

Perpendicularto probe beam direction

DetectionInitial sizing

High speed inspection

Most frequently used for weld inspection

Scanning Types: Scanning Types: Non-Parallel ScanNon-Parallel Scan

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• Limitations :

• Defect depth measurement only accurate when probes are

symmetrically positioned with regard to defect

• Uncertainty on lateral position of defect results in height sizing

error

Scanning Types: Scanning Types: Non-Parallel ScanNon-Parallel Scan

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Influence of Defect Position UncertaintyInfluence of Defect Position Uncertainty


d

t0 t0

x

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Influence of Defect Position UncertaintyInfluence of Defect Position Uncertainty


t2t1

Constant timelocus

(t1+t2=ct)

dmin dmax

In practice:Maximum error on absolute depth smaller than 10%Height sizing error on internal (small) defect is negligible.Caution for small defects situated at the back-wall.

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Parallel ScanParallel Scan

Weld

Parallel scan

Parallelto probe beam direction

Accurate sizing and positioning

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Back-wallB-scan

Lateral waveThis type of

scan yields a typical inverted

parabola

Flight-time will be minimal when probes are positioned symmetrically over defect

Parallel ScanParallel Scan

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Parallel Scan : LimitationsParallel Scan : Limitations

• Weld inspection: weld cap often reduces or makes

impossible the extent of the scan.

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Practical Implementation : SummaryPractical Implementation : Summary

• Simple, light weight set-up

• High speed inspection

• L-waves, wide beam, high frequency probes

• Data visualization and analysis tools

• Two scan types : non-parallel, parallel

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• Zetec Solution

• Demonstration


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Codes and standardsCodes and standards

• British Standard

• European Norm

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British StandardBritish Standard

• Guide to calibration and setting-up of TOFD technique,

BS 7706 (1993)

• Detailed document with useful practical guidelines for

setting up TOFD examination

• Guide for interpretation of TOFD data

• Examples of typical weld defects

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CENCEN

• TOFD technique as a method for defect detection and

sizing, CENV 583-6 (1997)

• Preliminary standard

• Recommended probe parameters with regard to different

wall thicknesses (frequency, crystal size, nominal angle)

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• Zetec Solution

• Demonstration


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• Advantages (+) :

• rapid, flexible

• reliable detection of both

volumetric and planar flaws

• amplitude insensitive

• accurate height sizing of flaws (±

1 mm)

• independent of weld configuration

• on-line interpretation, permanent

record

• Limitations (-) :

• “dead zones”

• masking of flaws

• influence of lateral defect position

uncertainty

• some cases require

complementary pulse-echo UT

• strongly attenuating materials?

Advantages and Limitations of TOFDAdvantages and Limitations of TOFD

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A-scan

D-scan

LW BW

Uppersurface

Back-wall

Dead Zones Dead Zones

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• Zetec Solution

• Demonstration


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Zetec SolutionZetec Solution

• TOFD : YES

• BUT : let’s also benefit from the advantages offered by

the standard Pulse-echo (PE) technique

• SOLUTION: perform TOFD and PE simultaneously,

without reducing inspection speed

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The Z-Scan UT system allows for simultaneous

acquisition and analysis of TOFD and PE

TOFDPE 45 SW PE 60 SW


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Z-Scan UT

&

UltraVision

Software

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• Portable system (11.7 kg)

• Multi channel data acquisition and display

• Real-time averaging

• UltraVision software supports simultaneous Pulse-Echo and

TOFD examination

• Calibration for true depth on flat and cylindrical surfaces

• Parabolic cursors for improved length sizing

• Lateral wave straightening and removal for TOFD

• SAFT processing


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• Zetec Solution

• Demonstration


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DemonstrationDemonstration

• General description

• System calibration

• Demonstrations of acquisition and analysis

Documents

060301 TOFD Training