12- Theory Tofd Fcb

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  • TOFDTime of Flight Diffraction

  • TOFD TechniqueOverviewCalibrationApplicationsSummary & Limitations

  • TOFD OverviewForward Diffraction TechniqueFirst described by Silk in 1977Using diffracted signals from crack tipsTraditional grey scale TOFD presentation

  • 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

  • Diffraction

    Modification or deflection of sound beamSound striking defect causes oscillationEnds of defect become point sourcesNot related to orientation of defectWeaker signal than reflected needs higher gainSharp defects provide best emittersTips signals are located accuratelyTime of flight of tip signals used to size

  • Diffraction

  • WavesFLAWDiffractedwavesDiffractedwavesIncidentwaveReflectedwaveAll directions

    Low energy

    Independent of incidence angle

  • Conventional Use of DiffractionTip diffraction method (satellite-pulse observation technique)TOF, Angle and velocity Height

  • SignalsSignals ReceivedLateral waveSubsurfaceBack-wall echoMode converted (shear wave) echoDefine top and bottom of partNote phase change

  • Basic Principles of the TOFD Technique

  • TOFD: Typical SetupTransmitterReceiver

  • A-Scan SignalsTransmitterReceiver

  • Some Typical DefectsUpper surface breaking crackBack wall breaking crackHorizontal planar defect

  • Upper Surface Breaking CrackTransmitterReceiverNo Lateral wave

  • Back Wall Surface Breaking CrackTransmitterReceiver

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

  • Data VisualizationAmplitudeTimeTimeOne A-scan picture is replaced by one gray-coded line

  • Data VisualizationD-scan

  • Calibration ToolsA-scanD-scanc

  • Measurement ToolsA-scanD-scan

  • Defect Position InfluenceTransmitterReceiverSSdt0t0

  • Defect Position UncertaintyTransmitterReceiverSSt2t1In 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.

  • Transverse ScanUpper surfaceBack-wallB-scanLateralwaveThis type of scan yields a typical inverted parabola

  • What do TOFD scans really look like?TOFD images show the lateral wave and backwall, plus SW signals after and reflections from all defects

    Source: Ginzel


    Incomplete Penetration


    Lack of Fusion

  • 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.

  • Recommended SolutionTOFD: YESBUT: do not forget the good things offered by the standard Pulse-Echo techniqueSOLUTION: do both TOFD and PE simultaneously, without reducing the scan speed

  • Typical Requirements for TOFD and PE ApplicationsSmall, lightweight, 1 to 16 channelsPE and TOFD softwareLateral wave straighteningReal-time averagingMulti-channel data acquisition and displayLinearization for true depth on flat or cylindrical surfacesProcessing (data compression,..)

  • CalibrationA-scanD-scancPCS, Thickness, velocity, Probe delay, Lateral wave or Back wallTypical multi-channel UT instrument is very user friendly and guides you with a software Wizard

  • Weld 1(PL4882)

  • Weld 1 (plate 4882) with Pulse-EchoTOFD and PE clearly show the defects embedded in this weld:Lack of Fusion (root)Lack of Sidewall FusionPorosityToe Crack

  • Parallel ScanD-scan

  • Recommended SolutionThe system allows for simultaneous acquisition and analysis (inTomoview only) of TOFD and PE

  • Linearized Lateral WaveTransmitterReceiverCouplant thickness variation Change in time of flight

  • Linearized Lateral WaveTransmitterReceiverLateral waveMisalignment variations Change in time of flight

  • Linearized Lateral WaveTransmitterReceiverSmall mechanical variations of probe separation Change of time of flight

  • TOFD AdvantagesExcellent 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-echo

  • TOFD LimitationsDead zone of ~3mm at outer surfacePotential dead zone at inner surfaceProne to noiseOver emphasizes some benign defects, e.g. porosity, laminations, interlamellar LoFNot easy to interpret

  • A Few Final Words on TOFDIs the best defect sizing technique available when correctly set-upUse in conjunction with pulse-echo for code and PoD reasons

  • SignalsLateral WaveBack-wall Echo

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

  • Calculator

  • CalculatorNote volume coverage less than ideal at this PCS (missing upper third). -12dB beam transmit only used for coverage calculation7MHz 100mmPCS 1.5 cycles assumed

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

  • 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

  • TOFD - SAFT Pre-SAFT processingPost-SAFT Processing

  • Limitations of TOFDCannot detect all defectsLimited coverage results from two potential dead zonesDead zone near the surface as a result of the lateral waveDead zone at the backwallresulting from the width of the backwall reflection

  • Other Typical Defects

  • Near Surface CrackThe crack blocks the Lateral WaveAnd the lower tip appears on the A-scan21

  • Incomplete Root Penetration21Note the two signals from the top & bottom1234

  • Lack of Root PenetrationNote the inverted phase between LW and defect123

  • Lack of Fusion - Side WallNote the two signals from the top & bottom1234

  • PorosityPorosity may image in many forms whether individual or cluster12

  • Transverse CrackIn the LW we can observe the wide beam effect on the crack1234123

  • Concave Root Distortion of back-wall echo123

  • Lack of Fusion - Interpass

  • Choosing an AngleOptimum Upper tip q 64Optimum Lower tip q 68From Charlesworth & TempleAngle selected is a compromise for depthMay require selecting several zones for best results

  • Diffracted rather than reflected signalsLongitudinal wavesB-scan type imaging (side view)Accurate sizing capability (height)Fast scanningInterpretation of defectsLess sensitive to defect orientationTOFD Advantages

  • Blind area - near surface, backwall Weak signalsFlaw classification limitationInterpretation of defectsSensitive to grain noise TOFD Limitations

  • Codes and StandardBritish StandardEuropeanASME

  • TOFD Guides DevelopedBS 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.

  • ASME Adaptations to TOFDASME 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

  • Root CrackTOFD techniqueRoot crack defectX-Ray

  • PorosityTOFD techniquePorosity defect

  • Slag inclusionTOFD techniqueSlag inclusion defect

  • Lack of Root FusionTOFD techniqueLack of root fusion defect

  • Root ConcavityTOFD techniqueRoot Concavity defect

  • Incomplete Root PenetrationTOFD techniqueIncomplete root penetration defect

  • Over PenetrationTOFD techniqueOver penetration defect

  • Irregular Root PenetrationTOFD techniqueIrregular root penetration defect

  • Excess Weld CapTOFD techniqueExcess weld cap defectX-Ray

    Traditional TOFD grey scale display with A-ScanTime-of-Flight Diffraction (TOFD) Diagram and inspection results for weld inspection using TOFD.TOFD is a powerful technique, allowing efficient and fast inspection along with very accurate sizing of flaws. TOFD is an amplitude-independent flaw sizing method, providing excellent sizing even in the presence of noise. This technique has many advantages:Wide coverage area using a pair of transducers Accurate flaw sizing; amplitude-independent Sizing technique using time-of-flight information On-line volume inspection, provides very fast scanning Setup independent of weld configuration Very sensitive to all kinds of defects No sensitivity to defect orientation Combined TOFD Display with Pulse/Echo DisplayCombined TOFD and pulse echo While TOFD is a very powerful and efficient technique, it suffers from limited coverage resulting from two dead inspection zones. The first dead