Video visualization of the dispersive behaviour of Rayleigh and Lamb wavetrains by double-pulsed TV holography

Abstract

In this work we present an application of TV holography to the generation of movies showing the propagation of acoustic guided waves in aluminium plates. Each movie shows a wavetrain whose envelope (i.e., the acoustic amplitude) and carrier wave move at the group and phase velocities, respectively. In particular, we use the S0 Lamb mode and the Rayleigh wave to illustrate the behaviour of dispersive and non-dispersive waves. Both wavetrains were generated by means of the classical wedge method and detected with our double-pulsed TV holography system, which renders 2D maps of the instantaneous out-of-plane displacement fields of the plate surface. The snapshots of the movie are obtained from a set of these 2D measurements, taken under repeatability conditions by successively increasing the delay between generation and detection. Then, a processing based on the 3D-FFT is applied to the set; the result is a new set of complex maps that permits to characterize the evolution of the positions of the envelope centre and of a point with a given value of the phase, so that it is possible to compare the phase and group velocities of the wavetrain.