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Time of Flight Diffraction (TOFD) technique of ultrasonic testing (UT) is a sensitive and accurate method for non-destructive testing of welds for defects. TOFD is widely used in the oil and gas, petrochemical and power generation industries and is more reliable than traditional radiographic, pulse echo manual UT and automated UT weld testing methods.

TOFD systems use a pair of ultrasonic probes sitting on opposite sides of a weld-joint or area of interest. The 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.

One of the limitations of TOFD is the ‘dead zone’ created by the lateral wave signal just below the inspection surface (or OD surface in case of the pipe). The dead zone is approximately 5mm and there is no flaw detection in this zone unless a raster TOFD method is used which means moving the probes in and out from the weld. Alternatively, by combining TOFD and phased array methods, allows for the PA probes to cover the dead zone to create an increase in the POD (Probability of Detection) to improve the chances of finding defects.