tofd presentation
TRANSCRIPT
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““Application of TimeApplication of Time--OfOf--Flight Flight Diffraction (TOFD) for Weld Diffraction (TOFD) for Weld
InspectionsInspections””Michael Moles
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Presentation OutlinePresentation Outline
HistoryWhat is TOFD?Advantages of TOFDLimitations of TOFDTypical imagesDefect depth measurementCodesSome TOFD examplesSummary
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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
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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)
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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.
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TOFD: Typical SetupTOFD: Typical Setup
Transmitter ReceiverLateral wave
Upper tip
Lower tip
Back-wall reflection
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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.
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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.
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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.
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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.
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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)
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Defect DepthDefect Depth
Transmitter ReceiverS S
d
t0 t0
( ) 220
2
22
Sttcd −−•
=
Signals NOT linear; either correct, or remember
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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.
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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
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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
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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
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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
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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
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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
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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.
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Example Example -- PorosityPorosity
12
3
Multiple small reflectors, each with hyperbolic tails. Usually can characterize, but sizing difficult.
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Example Example –– Transverse CracksTransverse Cracks
1
2
3
Transverse cracks are rare, and similar to porosity
No perturbation of lateral or backwall
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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.
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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
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TOFD EquipmentTOFD Equipment
New portable system reduces TOFD cost significantlyNeeds encoded data e.g. handscanner or automated system
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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
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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!
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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”.
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Thank youThank you
Any questions?