Nonlinear phenomenon in laser-induced finite-amplitude acoustic waves propagating along cracks

The propagation of the longitudinal and surface waves along cracks in the plate was studied. Waves with strain up to 5 x 10 - 3 could be generated by radiation of a Q-switched laser absorbed in a layer of liquid suspended carbon particles deposited on the surface of the plate. The longitudinal and surface Rayleigh waves traveled parallel to and on the fracture surfaces, respectively, and bounced from the free surfaces of the plate. It was observed that the longitudinal echo pulses exhibited shapes, which were dependent on the magnitude of the laser excitation and, thus, on the amplitudes of the laser-generated acoustic waves. At a certain photo-excitation level, the leading tensile part of the first longitudinal echo vanished and reappeared again at higher excitation levels. This effect did not occur in the crack-free areas of the plate and was amplitude-dependent; therefore, it could be attributed to the nonlinear interaction of the longitudinal and surface Rayleigh waves with the crack.