12- Theory Tofd Fcb

Download 12- Theory Tofd Fcb

Post on 01-Dec-2015

526 views

Category:

Documents

4 download

Embed Size (px)

DESCRIPTION

TOFD

TRANSCRIPT

<ul><li><p>TOFDTime of Flight Diffraction </p></li><li><p>TOFD TechniqueOverviewCalibrationApplicationsSummary &amp; Limitations</p></li><li><p>TOFD OverviewForward Diffraction TechniqueFirst described by Silk in 1977Using diffracted signals from crack tipsTraditional grey scale TOFD presentation</p></li><li><p>Advantages of TOFD Technique </p><p>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 </p></li><li><p>Diffraction</p><p>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</p></li><li><p>Diffraction</p></li><li><p>WavesFLAWDiffractedwavesDiffractedwavesIncidentwaveReflectedwaveAll directions</p><p>Low energy</p><p>Independent of incidence angle</p></li><li><p>Conventional Use of DiffractionTip diffraction method (satellite-pulse observation technique)TOF, Angle and velocity Height</p></li><li><p>SignalsSignals ReceivedLateral waveSubsurfaceBack-wall echoMode converted (shear wave) echoDefine top and bottom of partNote phase change</p></li><li><p>Basic Principles of the TOFD Technique</p></li><li><p>TOFD: Typical SetupTransmitterReceiver</p></li><li><p>A-Scan SignalsTransmitterReceiver</p></li><li><p>Some Typical DefectsUpper surface breaking crackBack wall breaking crackHorizontal planar defect</p></li><li><p>Upper Surface Breaking CrackTransmitterReceiverNo Lateral wave</p></li><li><p>Back Wall Surface Breaking CrackTransmitterReceiver</p></li><li><p>Horizontal Planar Defect(Lack of Inter-Run Fusion, Laminations)TransmitterReceiver</p></li><li><p>Data VisualizationAmplitudeTimeTimeOne A-scan picture is replaced by one gray-coded line</p></li><li><p>Data VisualizationD-scan</p></li><li><p>Calibration ToolsA-scanD-scanc</p></li><li><p>Measurement ToolsA-scanD-scan</p></li><li><p>Defect Position InfluenceTransmitterReceiverSSdt0t0</p></li><li><p>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.</p></li><li><p>Transverse ScanUpper surfaceBack-wallB-scanLateralwaveThis type of scan yields a typical inverted parabola</p></li><li><p>What do TOFD scans really look like?TOFD images show the lateral wave and backwall, plus SW signals after and reflections from all defects</p><p>Source: Ginzel</p><p>Slag</p><p>Incomplete Penetration</p><p>Porosity</p><p>Lack of Fusion</p></li><li><p>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.</p></li><li><p>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</p></li><li><p>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,..)</p></li><li><p>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</p></li><li><p>Weld 1(PL4882)</p></li><li><p>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</p></li><li><p>Parallel ScanD-scan</p></li><li><p>Recommended SolutionThe system allows for simultaneous acquisition and analysis (inTomoview only) of TOFD and PE</p></li><li><p>Linearized Lateral WaveTransmitterReceiverCouplant thickness variation Change in time of flight</p></li><li><p>Linearized Lateral WaveTransmitterReceiverLateral waveMisalignment variations Change in time of flight</p></li><li><p>Linearized Lateral WaveTransmitterReceiverSmall mechanical variations of probe separation Change of time of flight</p></li><li><p>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</p></li><li><p>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</p></li><li><p>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</p></li><li><p>SignalsLateral WaveBack-wall Echo</p></li><li><p>CalculatorTOFD probe separation can be calculated with basic mathematical formula or Excel calculator tools</p></li><li><p>Calculator</p></li><li><p>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</p></li><li><p>Calculator Increase refracted angle to 65 improves coverage without compromising resolution (for these specific conditions) PCS remains the same.</p></li><li><p>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</p></li><li><p>TOFD - SAFT Pre-SAFT processingPost-SAFT Processing</p></li><li><p>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</p></li><li><p>Other Typical Defects</p></li><li><p>Near Surface CrackThe crack blocks the Lateral WaveAnd the lower tip appears on the A-scan21</p></li><li><p>Incomplete Root Penetration21Note the two signals from the top &amp; bottom1234</p></li><li><p>Lack of Root PenetrationNote the inverted phase between LW and defect123</p></li><li><p>Lack of Fusion - Side WallNote the two signals from the top &amp; bottom1234</p></li><li><p>PorosityPorosity may image in many forms whether individual or cluster12</p></li><li><p>Transverse CrackIn the LW we can observe the wide beam effect on the crack1234123</p></li><li><p>Concave Root Distortion of back-wall echo123</p></li><li><p>Lack of Fusion - Interpass</p></li><li><p>Choosing an AngleOptimum Upper tip q 64Optimum Lower tip q 68From Charlesworth &amp; TempleAngle selected is a compromise for depthMay require selecting several zones for best results</p></li><li><p>Diffracted rather than reflected signalsLongitudinal wavesB-scan type imaging (side view)Accurate sizing capability (height)Fast scanningInterpretation of defectsLess sensitive to defect orientationTOFD Advantages</p></li><li><p>Blind area - near surface, backwall Weak signalsFlaw classification limitationInterpretation of defectsSensitive to grain noise TOFD Limitations</p></li><li><p>Codes and StandardBritish StandardEuropeanASME</p></li><li><p>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.</p></li><li><p>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</p></li><li><p>Root CrackTOFD techniqueRoot crack defectX-Ray</p></li><li><p>PorosityTOFD techniquePorosity defect</p></li><li><p>Slag inclusionTOFD techniqueSlag inclusion defect</p></li><li><p>Lack of Root FusionTOFD techniqueLack of root fusion defect</p></li><li><p>Root ConcavityTOFD techniqueRoot Concavity defect</p></li><li><p>Incomplete Root PenetrationTOFD techniqueIncomplete root penetration defect</p></li><li><p>Over PenetrationTOFD techniqueOver penetration defect</p></li><li><p>Irregular Root PenetrationTOFD techniqueIrregular root penetration defect</p></li><li><p>Excess Weld CapTOFD techniqueExcess weld cap defectX-Ray</p><p>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 zone is near the surface, as a result of the lateral wave; the second is at the backwall, resulting from the width of the backwall reflection.R/DTech's equipment allows inspections simultaneously combining TOFD with conventional pulse echo. Pulse echo complements TOFD and covers the dead zones.TOFDTime-of-flight diffraction (TOFD) technique is an ultrasonic NDT technique which was first described by Silk (1977). This method relies on the diffraction of ultrasonic energies from 'corners' and 'ends' of internal structures (primarily defects) in a component being tested. Some recent crack sizing results performed with ultrasonic methods on a thick-walled pressure vessel weld demonstrated that it is uncertain if TOFD is a reliable method for detection of cracks or sharp grooves at the inner wall of vessels or piping. Whether or not it can be used to replace more established conventional ultrasonic or radiography methods, TOFD testing has become a hot topic, but so far, the worldwide debate has not reached a consensus. TOFD does have its limitations if used for defect detection - the technique cannot detect all defects. But there is no doubt of its potential for use as an adjunct to other techniques' weakness in the area of probability of detection. (ndt.net encyclopedia) Time-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 One-line volume inspection, provides very fast scanning Setup independent of weld configuration Very sensitive to all kinds of defects No sensitivity to defect orientation Amplitude-insensitive, acoustical coupling less critical Combined 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 zone is near the surface, as a result of the lateral wave; the second is at the backwall, resulting from the width of the backwall reflection.R/DTech's equipment allows inspections simultaneously combining TOFD with conventional pulse echo. Pulse echo complements TOFD and covers the dead zones.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 One-line volume inspection, provides very fast scanning Setup independent of weld configuration Very sensitive to all kinds of defects No sensitivity to defect orientation Amplitude-insensitive, acoustical coupling less critical </p><p>In practice however, the maximum error that will ocur on the absolute depth measurement will be limited. Due to the beam spread limitations of the probe the examined zone is limited and the maximum error in this zone will never be higher than about 10 %.</p><p>The defect height estimation on the other hand will still be accurate since the height is calculated on the basis of two depth measurements.The absolute errors on the depth measurement are almost canceled by this process.</p><p>A problem can arise for small defects situated at the back-wall. Due to the explained phenomenon they can appear deeper then they really are and thus become lost in the back-wall signal.</p><p>This is one of the major problems of longitudinal scans and explains the need for tranverse scans, which will be explained later on. But, since the defect is initially not detected, the complete weld should be scanned transversally. And this annulates of course one of the major advantages of the TOFD technique, which is a high inspection speed. However, we will see later that a combination of the TOFD technique with the standard pulse-echo technique can efficiently resolve this, and other shortcommings of TOFD.For the parallel scan, which is across the weld section, the path to the defect tip varies with the probe position because of the changing of the path lengths. This variation generates an inverted parabola curve which is typical for TOFD parallel scans.</p><p>The path to the defect reduces to a minimum when the probes are positioned symmetrically over the defect. This allows for an accurate measure of the defects depth as discussed earlier.</p><p>The parallel scan allows also to determine the lateral position of the defect. The minimum wave path indicates the place were both probes are symmetrical positioned over the defect. At this location the defect lies thus straight in the middle of both probes. For this purpose the software enables the drawing of locus curves that will be shown during the demo.</p></li></ul>