Time of Flight Diffraction(TOFD)

TOFD UT 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

DiffractionModification or deflection of sound beamSound striking defect causes oscillationEnds of defect become point sourcesNot related to orientation of defectWeaker signal than reflectedSharp 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

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

SignalsAmplitudeTimeOne A-scan picture is replaced by one gray-coded lineA-scan

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

CalibrationPlace the TOFD configuration on the calibration block

CalibrationAdjust probe distance to be 2 times the part thickness

CalibrationUnder Probe/Part >Select>WedgeSelect Wedge

CalibrationUnder Probe/Part >Select>WedgeSelect Probe

TOFD SetupEditing probe properties

CalibrationUnder UT menu>Mode >TOFD

CalibrationUnder UT General>Set Gain>Set Range>

CalibrationIf there is a lot of noise, averaging can be used UT>Receiver >Averaging

Scanning

Save OptionsSave TOFD setup

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

Near Surface CrackThe crack blocks the Lateral Wave and 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 Charles worth & TempleAngle selected is a compromise for depthMay require selecting several zones for best results

Traditional TOFD grey scale display with A-Scan

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 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 Display

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.

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.

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

Put the probes on a calibration block.The distance between the exit point of each probe (Separation) is twice the thickness of the part to inspect.In Probe/Part>Select>SelectTx/Rx, select the correct wedge and probe in Select probe and Select Wedge. If the probe or wedge is not in the list, create it, refer to the following procedures: How to define a probe and How to define a wedge.Go in UT>PulserTx/Rx mode> and choose TOFD. The ruler unit will become s instead of mm.In UT>General>set the Gain> -the Start and the Range of the A-Scan to see correctly the Lateral wave and the Backwall. If there is a lot of random noise, the Averaging can be used. To do it, go in UT>Receiver>Averaging.Save TOFD setup Put the probes on a calibration block.The distance between the exit point of each probe (Separation) is twice the thickness of the part to inspect.In Probe/Part>Select>Select Tx/Rx, select the correct wedge and probe in Select probe and Select Wedge. If the probe or wedge is not in the list, create it, refer to the following procedures: How to define a probe and How to define a wedge.Go in UT>PulserTx/Rx mode> and choose TOFD. The ruler unit will become s instead of mm.In UT>General>set the Gain> -the Start and the Range of the A-Scan to see correctly the Lateral wave and the Backwall. If there is a lot of random noise, the Averaging can be used. To do it, go in UT>Receiver>Averaging.Save TOFD setup Put the probes on a calibration block.The distance between the exit point of each probe (Separation) is twice the thickness of the part to inspect.In Probe/Part>Select>SelectTx/Rx, select the correct wedge and probe in Select probe and Select Wedge. If the probe or wedge is not in the list, create it, refer to the following procedures: How to define a probe and How to define a wedge.Go in UT>PulserTx/Rx mode> and choose TOFD. The ruler unit will become s instead of mm.In UT>General>set the Gain> -the Start and the Range of the A-Scan to see correctly the Lateral wave and the Backwall. If there is a lot of random noise, the Averaging can be used. To do it, go in UT>Receiver>Averaging.Save TOFD setup Put the probes on a calibration block.The distance between the exit point of each probe (Separation) is twice the thickness of the part to inspect.In Probe/Part>Select>SelectTx/Rx, select the correct wedge and probe in Select probe and Select Wedge. If the probe or wedge is not in the list, create it, refer to the following procedures: How to define a probe and How to define a wedge.Go in UT>PulserTx/Rx mode> and choose TOFD. The ruler unit will become s instead of mm.In UT>General>set the Gain> -the Start and the Range of the A-Scan to see correctly the Lateral wave and the Backwall. If there is a lot of random noise, the Averaging can be used. To do it, go in UT>Receiver>Averaging.Save TOFD setup Put the probes on a calibration block.The distance between the exit point of each probe (Separation) is twice the thickness of the part to inspect.In Probe/Part>Select>SelectTx/Rx, select the correct wedge and probe in Select probe and Select Wedge. If the probe or wedge is not in the list, create it, refer to the following procedures: How to define a probe and How to define a wedge.Go in UT>PulserTx/Rx mode> and choose TOFD. The ruler unit will become s instead of mm.In UT>General>set the Gain> -the Start and the Range of the A-Scan to see correctly the Lateral wave and the Backwall. If there is a lot of noise, the Averaging can be used. To do it, go in UT>Receiver>Averaging.Save TOFD setup

Put the probes on a calibration block.The distance between the exit point of each probe (Separation) is twice the thickness of the part to inspect.In Probe/Part>Select>SelectTx/Rx, select the correct wedge and probe in Select probe and Select Wedge. If the probe or wedge is not in the list, create it, refer to the following procedures: How to define a probe and How to define a wedge.Go in UT>PulserTx/Rx mode> and choose TOFD. The ruler unit will become s instead of mm.In UT>General>set the Gain> -the Start and the Range of the A-Scan to see correctly the Lateral wave and the Backwall. If there is a lot of noise, the Averaging can be used. To do it, go in UT>Receiver>Averaging.Save TOFD setup

Put the probes on a calibration block.The distance between the exit point of each probe (Separation) is twice the thickness of the part to inspect.In Probe/Part>Select>SelectTx/Rx, select the correct wedge and probe in Select probe and Select Wedge. If the probe or wedge is not in the list, create it, refer to the following procedures: How to define a probe and How to define a wedge.Go in UT>PulserTx/Rx mode> and choose TOFD. The ruler unit will become s instead of mm.In UT>General>set the Gain> -the Start and the Range of the A-Scan to see correctly the Lateral wave and the Backwall. If there is a lot of noise, the Averaging can be used. To do it, go in UT>Receiver>Averaging.Save TOFD setup

Put the probes on a calibration block.The distance between the exit point of each probe (Separation) is twice the thickness of the part to inspect.In Probe/Part>Select>SelectTx/Rx, select the correct wedge and probe in Select probe and Select Wedge. If the probe or wedge is not in the list, create it, refer to the following procedures: How to define a probe and How to define a wedge.Go in UT>PulserTx/Rx mode> and choose TOFD. The ruler unit will become s instead of mm.In UT>General>set the Gain> -the Start and the Range of the A-Scan to see correctly the Lateral wave and the Backwall. If there is a lot of noise, the Averaging can be used. To do it, go in UT>Receiver>Averaging.Save TOFD setup

Put the probes on a calibration block.The distance between the exit point of each probe (Separation) is twice the thickness of the part to inspect.In Probe/Part>Select>SelectTx/Rx, select the correct wedge and probe in Select probe and Select Wedge. If the probe or wedge is not in the list, create it, refer to the following procedures: How to define a probe and How to define a wedge.Go in UT>PulserTx/Rx mode> and choose TOFD. The ruler unit will become s instead of mm.In UT>General>set the Gain> -the Start and the Range of the A-Scan to see correctly the Lateral wave and the Backwall. If there is a lot of noise, the Averaging can be used. To do it, go in UT>Receiver>Averaging.Save TOFD setup

Put the probes on a calibration block.The distance between the exit point of each probe (Separation) is twice the thickness of the part to inspect.In Probe/Part>Select>Select Tx/Rx, select the correct wedge and probe in Select probe and Select Wedge. If the probe or wedge is not in the list, create it, refer to the following procedures: How to define a probe and How to define a wedge.Go in UT>Pulser Tx/Rx mode> and choose TOFD. The ruler unit will become s instead of mm.In UT>General>set the Gain> -the Start and the Range of the A-Scan to see correctly the Lateral wave and the Backwall. If there is a lot of noise, the Averaging can be used. To do it, go in UT>Receiver>Averaging.Save TOFD setup