LASER-GENERATED ELASTIC-WAVES IN CARBON-EPOXY COMPOSITE

New contactless techniques using laser-generated ultrasound have been applied to the inspection of composite materials. Transient elastic waves were launched thermoelastically in half cylindrical composite samples using a long pulse dye laser or a Q-switched Nd-YAG laser. The waves were detected with piezoelectric transducers or with an optical heterodyne interferometer. The measurements have been carried out on two different lay up design composites (carbon/epoxy: unidirectional and cross-ply 0-degrees/45-degrees/90-degrees/ - 45-degrees). Quasi-longitudinal, quasi-shear, and shear bulk waves and also head waves are clearly discerned in the stacking of a large number of waveforms. Velocities of the different types of waves simultaneously generated are compared to the phase and group velocities computed using Christoffel equations and an hexagonal model. It is shown with this point-source measurement technique that the wavefront arrival times agree with the energy velocities rather than with the phase velocities. A pronounced anisotropy is observed in the amplitudes of the wave arrivals. Angular directivity patterns of quasi-longitudinal, quasi-shear, and transverse bulk waves were plotted. Simulations have shown that this phenomenon can be ascribed to the self-focusing effect whereby, in anisotropic materials, the energy flow is much more concentrated in some direction than others.