Time-of-Flight Diffraction 

TOFD Time-of-Flight Diffraction – is an ultrasonic technique which measures the time of flight of a pulse as it travels from a transmitting probe to a receiving probe. Divergent beams are used and it is necessary to scan the TOFD probe pair over the flaw for the technique to function correctly.
What distinguishes the technique from a standard pitch-catch configuration is the D which stands for diffraction. The technique relies on the detection of the diffracted sound wave which is generated from both the top and bottom edges of a planar defect.

The time of arrival of the diffracted signals from the flaw tips is measured with respect to the probe firing time. Time measurement can be done to great accuracy. The combination of this accuracy and the fact that the scattering source is the flaw tip forms the basis of a highly accurate sizing technique.

The phase of the high-frequency ultrasonic pulse, otherwise referred to as the RF (radio frequency) signal, contains information which allows the top-edge signal to be distinguished from the bottom-edge signal. The diffracted edge wave is a lot lower in amplitude than a signal reflected off the face of a defect and so TOFD inspections are applied at a higher sensitivity than standard pulse echo inspections. As a consequence, the results need careful interpretation and it is normally advisable to digitally collect the signals and use imaging software to allow the analyst to view the data generated by scanning probes over the flaw. 

In analysing TOFD data, the shape of the response signals is at least as important as amplitude in identifying the response signals of interest. The typical TOFD display shows the RF waveform for each probe position combined in a waterfall plot and viewed from the side or front of the component.

For more information see:
'Ultrasonics Part 11. Surface and TOFD techniques', J C Drury, Insight – Non-Destructive Testing and Condition Monitoring, Vol 47 No 9, September 2005.

BS 7706:1993
Guide to calibration and setting-up of the ultrasonic time-of-flight diffraction (TOFD) technique for the detection, location and sizing of flaws.

DD ENV 583-6:2000
Non-destructive testing. Ultrasonic examination. Time-of-flight diffraction technique as a method for detection and sizing of discontinuities

There is a European Technical Specification on TOFD:
CEN/TS 14751:2005, Welding – Use of time-of-flight diffraction technique (TOFD) for examination of welds.

Also, a new Work Item has been started in CEN:
WI 00121471, Non-destructive testing of welds – Time-of-flight diffraction technique (TOFD) – Acceptance levels.

What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: