The Time Of Flight Diffraction (TOFD) is a conventional NDT method.
The TOFD method typically uses a pair of conventional probes located on each side of a zone of interest (weld) (see pictures below). The probes are mounted on a scanning device that maintain a constant distance between the two probes.
TOFD inspection uses the “lateral wave echo” directly traveling from one probe to the user as reference for the surface, and the back-wall echo, indirectly traveling from one probe to the other via reflection off the back-wall, as reference for the back-wall.
On a “defect free” specimen, no indications above a selected threshold should be present in between these two echoes. On a “flawed” specimen, the TOFD inspection should show indications in between the lateral wave and the back-wall echoes. When a crack is present, there is a diffraction of the ultrasonic wave from the tips of the crack. Using the measured time-of-flight of the pulse, the depth of a crack tips can be calculated automatically by simple trigonometry.