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““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?
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