Time of Flight Diffraction (TOFD)
Time of Flight Diffraction (TOFD) was developed in the 1970’s as a more accurate way to size planar flaws (cracks) for the nuclear industry. Transducer technology advancements have made TOFD an extremely reliable and accurate tool for detection of defects and discontinuities. It is possible to use TOFD techniques on materials ranging from 0.200" through 10.000" in thickness with fast, reliable results.
Time of Flight Diffraction uses two ultrasonic transducers in a send-receive method to introduce ultrasonic sound waves into the test piece for inspection. The transducers produce three different acoustic wave paths, each having it's own arrival time to the receiver probe.
Two distinct longitudinal (L-wave/compression) waves are produced, and from these, shearwave modes are generated. The combination of the two wave modes and the shear waves, along with their distinct sound paths and beam spread spread allow for full volumetric coverage of the test piece between the transmitting and receiving probes. The first L-wave travels between the probes just under the surface of the material and is called the Lateral wave. The second L-wave is angled at the back wall and reflects from the ID surface to the receiving transducer completing a full-V path. The shear waves formed from these L-waves also propagate through the volume of the material. Wave energy that come into contact with any defects in the inspection area is diffracted from the edges of the defect and picked up by the receiving probe.
TOFD is generally used on piping and vessel welds. It is an especially useful tool in finding defects in welds from equipment and piping production, as well as service induced defects like weld root erosion, weld washout due to process, cracking in coke drum welds, as well as localized internal/external corrosion can be detected and measured using the TOFD technique. The TOFD technique can easily be modified to perform inspections on heavy walled reactors (2", 3", even 6" thick) in one pass. Since there is no need to raster the transducers, TOFD is performed in a single parallel line scans which makes data collection fast. Up to 350 linear feet of weld can be inspected a day using this technique and equipment.