In time-of-flight diffraction (ToFD) systems, a pair of ultrasonic probes are used, sitting on opposite sides of a weld-joint or area of interest. A transmitter probe emits an ultrasonic pulse which is picked up by the receiver probe on the opposite side. In an undamaged part, the signals picked up by the receiver probe are from two waves: one that travels along the surface (lateral wave) and one that reflects off the far wall (back-wall reflection). When a discontinuity such as a crack is present, there is a diffraction of the ultrasonic sound wave from the top and bottom tips of the crack. Using the measured time of flight of the pulse, the depth of the crack tips can be calculated automatically by trigonometry application. This method is even more reliable than traditional radiographic, pulse echo manual UT and automated UT weld testing methods.
ToFD offers great accuracy for measuring the critical through-wall size of crack-like-defects. The accuracy of greater than ±1mm can be obtained in a wide range of material thickness from which pressurised components are constructed.
ToFD was initially developed as a method of accurate monitoring and sizing of through-wall height of in-service discontinuities in the nuclear industry. It has now been independently validated as one of the most effective techniques for locating and sizing discontinuities in ferritic welds.
TWI Resources
TWI has extensive R&D and field experience on ToFD application for defect growth monitoring, inspection of welds in gas storage bullet, nozzle inspection, ToFD application in lieu of radiography testing (RT) for pressure vessel and piping welds, detection of thermal fatigue cracks, inspection of turbine and generator components, in-service inspection for flaw detection and sizing as part of FFS assessment.
TWI acquired the following equipment which is capable of performing ToFD:
TWI has a range of versatile scanners specially designed for manual and mechanical applications on site.
Benefits
The advantages of this technique are;
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Speed of detecting discontinuities
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Accurate positioning and sizing of discontinuities
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Reproducibility and accuracy is proven <0.5mm
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Permanent digital records with graphic images
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Detection is independent of type and orientation of discontinuities
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Compliant to ASME Code Case 2235
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Consistent results between pre- and post-heat treatment
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Consistent results between pre- and in-service inspection
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Accurate methods available for measuring the growth of discontinuities
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Offers a higher probability of detection
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Data is collected in B-scan or D-scan format to improve the detection of discontinuities
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Planar discontinuities, which are not perpendicular to measured surfaces can be detected
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Higher POD which means, improving risk reduction and calculation
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No evacuation of areas during operation because no radiation is used
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Inspection results are immediately available
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Inspection can be performed above 200° C
Applications
For more information about TWI's NDT services, please email contactus@twi.co.uk.