Recovering of Phase Velocities Through Anisotropic Plates by a Direct Summation of Signals Generated by Line Sources

The LASER ultrasonic technique is a powerful mean to generate and detect ultrasonic waves at a distance, without any contact to the specimen. However, it is now well known that in anisotropic media, acoustic waves generated by a line or point source propagate at group velocities which are different from the phase ones. The recovering of phase velocities from such generated acoustic waves is a non trivial matter that requires a numerical analysis [1, 2] of a set of measured group velocities. An alternative solution is proposed: the signal such as it is generated by a plane wave refracted through an anisotropic plate is numerically built. For that purpose, a set of signals are recorded by scanning the line source in a direction normal to it with a constant step. Each measured signal is then shift in time with a constant delay. The summation of the obtained signals yields a waveform similar to the one recorded when a plane wave is refracted through the material. The phase velocity can thus be easily obtained in various directions of propagation