Time-of-Flight Diffraction (TOFD) Technique

The TOFD method represents a significant departure from conventional amplitude-based ultrasonic techniques. It is based on the physical phenomenon that when an ultrasonic wave strikes the sharp edge of a discontinuity, such as the top or bottom tip of a crack, the tip acts as a secondary source, radiating sound in all directions. This radiated energy is known as a diffracted wave.

In a typical TOFD setup, a transmitting probe sends a wide beam of ultrasound into the material, and a receiving probe, positioned a fixed distance away on the same surface, listens for the signals. The first signal to arrive at the receiver is the ‘lateral wave,’ which travels just beneath the surface directly from the transmitter. If a flaw is present, faint diffracted signals from its upper and lower extremities will arrive later. The time difference between the arrival of the lateral wave and the diffracted signals is precisely measured.

Using simple geometry and the known velocity of sound in the material, the time-of-flight data can be converted into highly accurate measurements of the flaw’s depth and through-wall height. A key advantage of TOFD is that its sizing accuracy is largely independent of the flaw’s orientation and the signal amplitude, overcoming major limitations of pulse-echo methods. This makes it one of the most reliable techniques for sizing critical planar defects like cracks and lack of fusion.