Time-of-Flight Diffraction Measurement Using Laser Ultrasound

We developed a new time-of-flight diffraction (TOFD) method based on laser ultrasound, which can use not only longitudinal waves but also shear waves. This method is more advantageous than the conventional TOFD method, because it enables us to obtain flaw depths accurately without knowing the longitudinal or shear wave velocity in the specimen or the distance between the transmitter and receiver, which are not easily obtained in complex-shaped objects. We applied the proposed method to estimate slit depths in aluminum alloy plates. The times of flight of the lateral wave, diffracted waves and the mode-converted shear wave at the flaw tip were measured and used to estimate the slit depth. As a result, an accurate estimation of flaw depth was achieved.