Time-of-flight diffraction ultrasonics

Bray and Stanley (1997) summarized TOFD as tip-diffraction techniques which utilized the principle that the tips of a crack when struck by a wave will diffract the signals back to the other location on the surface.

[3] 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.

In undamaged pipes, the signals picked up by the receiver probe are from two waves: one that travels along the surface and one that reflects off the far wall.

Using the measured time of flight of the pulse, the depth of a crack tips can be calculated automatically by simple trigonometry.

Calibration blocks with side drilled holes as shown in Reference[4] and ISO 10863 used to validate the "dead zone" and sizing accuracy.

A TOFD setup with transmit and receive probes. In this case the receive probe sees four indications: one from the lateral wave that has travelled along the upper surface, one from the wave that has reflected off the far surface, and two from the defect in the test object.
Typical TOFD data, created by aligning the data traces from the above figure vertically and colour-coding them for amplitude. The defect or discontinuity creates a characteristic parabolic indication, due to the apparent change in depth as the probes travel.
Manually-guided TOFD probes