TOFD(Time of Flight Diffraction)

Principle and Method

TOFD is one of the methods of ultrasonic inspection. The use of ultrasonic waves is the same, but ordinary UT uses reflected waves, but TOFD is an inspection method that uses diffraction waves to measure the length and depth, height of the discontinuity.
In the case of welds and fault-free sound areas, there is only a lateral wave reflected from the surface and a back-wall echo reflected from the bottom.
There is no signal between these two signals.
However, if there is an inclusion inside, the ultrasonic wave will be diffracted on the discontinuous line as shown in the figure. The time at which this diffraction signal arrives at the receiving probe is calculated to determine the location of the discontinuity.