““Application of TimeApplication of Time--OfOf--Flight Flight Diffraction (TOFD) for Weld Diffraction (TOFD) for Weld

InspectionsInspections””Michael Moles

Presentation OutlinePresentation Outline

HistoryWhat is TOFD?Advantages of TOFDLimitations of TOFDTypical imagesDefect depth measurementCodesSome TOFD examplesSummary

HistoryHistory

Developed in UKAEA Harwell in ~70’sManufactured commercial ZipscanUsed very effectively in nuclear PISC II and DDT trials=> Showed good detection and excellent sizingRecently “adopted” by petrochemical and other industries

DDT Defect Sizing using UltrasonicsDDT Defect Sizing using Ultrasonics

Comparison of measured vs. actual defect sizes: left, all UT techniques; right, TOFD only. (DDT Plate 1)

Source: NDT On-line

What is TOFD?What is TOFD?

Time-Of-Flight Diffraction (TOFD) relies on the diffraction of ultrasonic energies from 'corners' and 'ends' of internal structures (primarily defects) in a component being tested.

TOFD: Typical SetupTOFD: Typical Setup

Transmitter ReceiverLateral wave

Upper tip

Lower tip

Back-wall reflection

TOFD WavesTOFD Waves

The Lateral wave: A sub-near-surface longitudinal wave generated from the wide beam of the transducer.

The Backwall reflection: A longitudinal wave reflected from the back wall

The Reflected wave: A longitudinal wave reflected by a lamellar planar defect

The Tip Diffracted wave: A circular longitudinal (or L-wave) diffracted by the edge of a defect.

Advantages of TOFDAdvantages of TOFD

Good midwall defect detection. Accurate sizing of defects using the time of arrivals of diffracted signals.Defect detection even if defects are mis-oriented or located away from the weld centreline. Very rapid linear scanning (raster scanning not required)Non-amplitude scanning and detection.Set-up independent of weld configuration.

Limitations of TOFDLimitations of TOFD

Dead zone at top surface (OD).Dead zone at bottom surface (ID).Sensitive to very small defects with a risk of false calls if not combined with pulse echo.Analysis can be difficult; specialistinterpretation required.Some sizing errors possible from lateral position of defect.Low signal-to-noise ratio.

Alternative Diffraction ArrangementsAlternative Diffraction Arrangements

Shear-shear diffractionLongitudinal-shear diffractionSingle transducer diffraction (called “back diffraction” or the “tip echo method” in Japan)Twin transducer TOFD with both transducers on the same side of the defect/weld.Complex inspections, e.g. nozzles.

Typical TOFD DisplayTypical TOFD Display

•Gray scale and rf for phase info.

•OD and ID visible

•Defects detectable in middle

•L-wave display only (usually)

Defect DepthDefect Depth

Transmitter ReceiverS S

d

t0 t0

( ) 220

2

22

Sttcd −−•

=

Signals NOT linear; either correct, or remember

Defect Analysis with CursorsDefect Analysis with Cursors

A-scan

D-scan

Cursors

t1 t2

l

Pt1,t2 ⇒ d1, d2 and h are automatically calculated

d1d1h

Build-in calculator

Use cursors on top and bottom of defect (note phase changes)

Use calculator or math for analysis.

TOFD CodesTOFD Codes

Two European “guidelines”: BS7706 and EN583_6.ASTM E-2373-04 now availableASME working on TOFD codes – due out soonASME TOFD Interpretation Manual (Nonmandatory App. N) due out very soon.ASME CC 2235 permits TOFD

Examples Examples –– Inside SurfaceInside Surface--breaking Defectbreaking Defect

Transmitter ReceiverLateral wave

LW

Tip

Back wall echo blocked

No back wall echo

1

2

3

No, or reduced, backwall echo

Signal from tip for detection and sizing

Example Example –– NearNear--Surface Breaking DefectSurface Breaking Defect

Transmitter Receiver

Crack tip

Back-wall reflection

BW

Lateral wave is blocked

No Lateral wave

1

2

Lateral wave blocked

Sizing by measuring crack tip

Example Example –– Midwall DefectMidwall Defect

12

3

4

No break in lateral wave or backwall

Top and bottom signals visible (if defect deep enough)

Can measure lengths using hyperbolic signals

Typical Defect for TOFDTypical Defect for TOFD

TOFD is very “strong” on midwall defects where pulse echo is weak

Strengths of TOFD complement PE, and vice versa

Example Example –– Lack of Root PenetrationLack of Root Penetration

1

23

•Sometimes see break in backwall signal; should see perturbation

•Defect can be sized using time-of-arrival

•Similar to other root defects

Example Example –– Lack of Sidewall FusionLack of Sidewall Fusion

12

3

4

•Should see no perturbations in lateral wave or backwall

•In this case, top signal is “buried” in lateral (OD) wave

•Can size if signals clear.

Example Example -- PorosityPorosity

12

3

Multiple small reflectors, each with hyperbolic tails. Usually can characterize, but sizing difficult.

Example Example –– Transverse CracksTransverse Cracks

1

2

3

Transverse cracks are rare, and similar to porosity

No perturbation of lateral or backwall

Example Example –– Interpass Lack of FusionInterpass Lack of Fusion

Transmitter ReceiverLateral wave

LW

Back wall reflection

BW

Reflection echo

Reflected signal

12

3

Strong signal, but defect benign. Cannot size depth.

TOFD TOFD –– What can go wrong?What can go wrong?

TOFD scans can go wrong, just like any NDE inspection. Some reasons:– Incorrect gain levels (too high or too low)– Incorrect gate positions– Incorrect transducer separation– Poor coupling– Noise interference

TOFD EquipmentTOFD Equipment

New portable system reduces TOFD cost significantlyNeeds encoded data e.g. handscanner or automated system

TOFD AdvantagesTOFD Advantages

Excellent PoD for mid-wall defectsGood detection of mis-oriented defectsCan characterize surface-breaking defectsExcellent sizing for defects in transverse TOFD mode, especially with signal processingTolerable sizing for defects in linear modeWorks very well in conjunction with pulse-echoRapid (and relatively low cost) inspections

TOFD LimitationsTOFD Limitations

Dead zone of ~3mm at outer surfacePotential dead zone at inner surfaceProne to “noise”Over emphasizes some benign defects, e.g. porosity, laminations, interlamellar LoFHard to interpret

Finally we have a TOFD code!

A Few Final Words on TOFDA Few Final Words on TOFD

Good for detection, especially midwallMuch the best defect sizing technique available when correctly set-upAlways use in conjunction with pulse-echo for code and PoD reasonsWell worth including in inspections, even with “company specs”.

Thank youThank you

Any questions?