The Time of Flight Diffraction (TOFD) UT testing method uses a pair of angle-beam L-wave ultrasonic probes that are used in a pitch-catch configuration with the sound beam passing through the area of interest. A transmitter probe emits an ultrasonic pulse which is picked up by the receiver probe on the opposite side. In an undamaged part, the signals picked up by the receiver probe are the result of multiple different wave energies that were generated by the transmitted beam: one that travels along the surface (lateral wave) and L-wave that reflects off the inside surface (back-wall reflection), and one S-wave that reflects off the inside surface. When a discontinuity such as a crack is present, there is a diffraction of the ultrasonic sound wave from the top and bottom tips of the crack. Using the measured time of flight of the transmitted and diffracted energy responses, the height and depth of the flaw can be calculated.
This ultrasonic UT testing method is commonly performed on welds, weld overlay cladding, piping, pressure vessels, storage tanks, and structural steel. Fabricated vessels and piping can be thoroughly examined for fabrication flaws with Time of Flight Diffraction. Sizing of cracks and welding flaws can be done precisely using the TOFD UT testing method. Time of Flight Diffraction is also effective at measuring the remaining wall of a welded joint that has been damaged by preferential corrosion or root erosion.
Applications for Time of Flight Diffraction:
Here are some Time of Flight Diffraction UT testing advantages:
Limitations of Time of Flight Diffraction: