Time-Of-Flight Diffraction Technique
The diffraction travel time technique measures the time that ultrasonic waves are diffracted at crack tips. It precisely determines flaw heights and depths independent of reflectivity.
Physical-Chemical Mechanism of Action
In the time-of-flight diffraction technique, two probes work opposite each other, one transmitting and one receiving. At the tips of a crack, the ultrasound is not reflected but diffracted, and these diffracted waves travel to the receiver. From the time difference between the signal from the upper and lower crack tip, the device calculates the depth and height of the flaw. Because the technique uses diffraction rather than reflection, it is almost independent of how favorably the flaw face is oriented to the sound beam.
Metallurgical Relevance & Risks in Welding
That's exactly where the advantage lies over the classic pulse-echo technique: a steeply oriented, poorly reflecting crack is still found. That's why the technique is used on highly stressed welds and where assessing crack growth matters. Its limitations are immediately beneath the surface and at the root, because there the transmitter and back-wall signals mask the indication.
Workshop Practice & Instructions
Test frequency, probe angle, and spacing depend on wall thickness and material; a few megahertz is typical. The wavelength isn't set; it results from frequency and sound velocity: at 5 MHz in steel, it's in the range of about one millimeter, and you can't resolve finer than that. Before testing, the device is calibrated on a reference block with known flaws, and the surface must couple evenly. Only qualified personnel may interpret the results.