Experimental study of anisotropic and dispersive effects on carbon-epoxy composite using laser generated ultrasound

New contactless techniques using laser-generated ultrasound are applied to the inspection of composite materials. Experimental results are obtained with a 1-μs pulse dye laser excitation and a piezoelectric detection on half cylindrical composite samples. The measurements are taken in two different lay-up design composites (carbon/epoxy: unidirectional and cross-ply 0°/45°/90°/-45°). Angular directivity patterns of quasi-longitudinal, quasi-shear, and shear waves are presented. Velocities of the different types of waves simultaneously generated are compared to the phase and group velocities computed using Christoffel equations and a hexagonal mode. It is shown that with this point-source measurement technique the wavefront arrival times agree with the group velocity rather than with the phase velocity. At higher frequencies, a large dispersive effect for the longitudinal and shear elastic waves propagating along the plies of the cross-ply composite is emphasized. Experiments performed in the frequency range of 0.5-4 MHz show velocity variations from 5600 to 2200 m/s (3600-1600) for longitudinal (shear) waves