Time of Flight Diffraction (ToFD)

The Time of Flight Diffraction (ToFD) method of ultrasonic inspection is an accurate, efficient and safe method of detecting volumetric defects in welds or base material. 

ToFD enables crack sizes to be measured accurately, so that fitness for service decisions about critical equipment can be made cost-effectively.

SubjectDetails
PrinciplesTOFD uses the time of flight of an ultrasonic pulse to determine the position of a reflector instead of amplitude as in traditional ultrasonic methods. Amplitude can be less reliable in cases where the orientation of the crack is not ideal.
In a TOFD system, a pair of probes sit on opposite sides of a weld. One of the probes emits an ultrasonic pulse that is picked up by the probe on the other side. In undamaged pipe, 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. When a crack is present, there is a diffraction of the ultrasonic wave from the tip(s) of the crack. Using the measured time of flight of the pulse, the depth of a crack tip can be calculated automatically by simple trigonometry. This method can be even more reliable than Radiographic testing of a weld.
How and where is it used?A very sensitive and accurate method for nondestructive testing of welds for defects. Crack sizes can be measured more accurately.
Applicable
Codes/Standards
A computerized and automated system for weld inspection.
Probes are mounted on a buggy that travels along a weld, recording data as it moves.
Form of report or deliverable?
Advantages
LimitationsTOFD has two blind zones where it is not sensitive to defects, hence TOFD ought always to be supplemented by a conventional pulse-echo examination of the near and far walls.
Alternatives
Certifications of technicians performing the work?
Rope access applicable?
Yes