TOFD (Time Of Flight Diffraction)
Time of Flight Diffraction (TOFD) is widely used in the oil and gas, petrochemical and power generation industries and is even 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.