Time-of-flight diffraction (TOFD) method of ultrasonic testing is an effective and reliable weld testing procedure. Due to the use of difffracted signals as opposed to reflected signals it is a valuable complement to PAUT or as a standalone method in certain applications.
The Time of Flight Diffraction (TOFD) 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:
Detecting cracking and sizing cracks as well as other planar defects, e.g. lack of fusion
In-service defect monitoring and detecting manufacturing defects
Weld examination - for pressure vessels, piping and storage tanks and spheres; in-service and new construction examinations
Flaw growth monitoring and discrimination of defects between weld overlay, clad and base metal
Measuring wall loss at welded joints damaged by preferential corrosion/root erosion
Time of Flight Diffraction UT testing advantages:
Very fast and effective scanning welded joints (new construction and in service)
Position and size data for every flaw can be compared for repeat scans of the same areas to track flaw growth or corrosion rates
Can also be used for in-service defects, such as cracking, corrosion, erosion, etc.
Measuring wall loss at welded joints is typically more effective with TOFD than with angle-beam pulse-echo methods
Limitations of Time of Flight Diffraction:
Weld must be accessible from both sides
Gathers and displays information in a way that requires experience to interpret
Not always code accepted as a stand-alone inspection technique