Laser generation/EMAT (electromagnetic acoustic transducer) reception ultrasonic inspection technique shows a great promise in the field of Non-Destructive Testing of welds due to its non-contact nature. However, the broadband nature of laser generation and the dispersive characteristic of Lamb waves make the laser generated ultrasonic signals in thin structures extremely complicated. In order to ease the interpretation of received signals, it is desirable to investigate the interactions of different structural features with Lamb waves and find their related signatures in signal, which offers the potential to detect different types of weld defects using a single test procedure. This work proposed a technique based on 2-D Fourier Transformation to investigate the laser generated Lamb waves experimentally and to find the most sensitive predictors in EMAT received signals corresponding to a specific structural feature of interest. The amplitudes of different Lamb wave modes in the wave-number/frequency domain provide a wealth of information. The demonstration of the technique was carried out on aluminum plates with isolated rectangular notches of different depths. The procedure introduced here is general which can be employed in other applications of Non-Destructive Testing.
Laser generation of Rayleigh and Lamb waves for ultrasonic testing
