Time of Flight Diffraction (TOFD)
Time of Flight Diffraction (TOFD) method of Ultrasonic Inspection is a very sensitive and accurate method for nondestructive testing of welds for defects. TOFD is a computerised system that was invented in the UK in the 1970s for the nuclear industry by Dr. Maurice Silk. The use of TOFD enabled crack sizes to be measured more accurately, so that expensive components could be kept in operation as long as possible with minimal risk of failure. After many years of development TOFD has now become a well recognised and accepted inspection technique, replacing slower, more expensive and hazardous Radiography techniques.
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.