Composite materials characterization using phase velocities measured with laser generated and detected ultrasonic waves.

When a laser beam is focused on the surface of a composite material, anisotropy gives rise to folded ray curves in which the acoustic rays are more concentrated in some directions than in others. In particular, the energy density can be very high at the cuspidal edge. The propagation in such media gives rise to internal diffraction by which waves are observed in directions that are not included in the cusp sector. These waves are not explained by ray theory. The measurement of the stiffness tensor of an anisotropic material by means of laser generated ultrasound is a nontrivial matter for essentially two reasons. First, the recovering of the coefficients from the group velocities is a double iterative numerical process that requires a high accuracy in the velocity measurement. Second, internal diffraction is not taken into account by such an algorithm and it provides undesired velocity data which induces a shift of the identified stiffness coefficients. In this paper, it is shown both numerically and experimentally that phase velocities can be measured using signals generated by a line source. The measurement of the stiffness coefficients from such velocities avoids the aforementioned difficulties. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]