SAW propagation on cylindrical parts using laser-ultrasonics: application to crack detection

We applied the laser-ultrasonic technique for detecting surface breaking slots in steel or duraluminurn cylinders. Surface acoustic waves (SAW) are launched by a pulsed YAG laser focused along a line. The radial component of the displacement is measured by a heterodyne optical interferometer. In the first experiment, we study the evolution of Rayleigh waveforms with the angle θ between the source and the detection point. Near the source, the Rayleigh pulse is monopolar (positive), it becomes bipolar at θ = 35° and monopolar (negative) at θ= 90°. After many turns of propagation, the Rayleigh waveform undergoes another modification due to the variation of the phase velocity versus frequency (dispersive effect). A simulation, based on the finite difference method, reproduces waveforms whatever the propagation distance. In a second set of experiments performed for an industrial application, we examine a set of samples comprising cracks with depth ranging from 0.08 to 1 mm. The analysis of the interaction of SAW with cracks shows the modifications of the transmitted Rayleigh wave and the evolution of reflected and converted waves according to the depth of the crack. Simulations are in good agreement with experimental signals.