Thermo-Elastic Model of Epicenter Displacement by Laser Pulse Irradiated on Metallic Surfaces

In recent years, there is a much interest in developing of nondestructive testing (NDT) systems using the pulse-echo laser ultrasonics. The key idea is to combine a low-power and short-pulsewidth laser excitation with a continuous sensing laser; and use a scanning mechanism, such as five degrees-of-freedom (5DOF)-axis robot, laser mirror scanner, or motorized linear translation or rotation scanner stage, to scan the combined beam on the structure. In order to optimize the parameters of the excitation laser, a realistic theoretical model of the epicenter displacement in thermo-elastic regime is needed. This paper revisits and revises the study of Spicer and Hurley (1996, "Epicentral and Near Epicenter Surface Displacements on Pulsed Laser Irradiated Metallic Surfaces," Appl. Phys. Lett., 68(25), pp. 3561-3563) on thermo-elastic model of epicenter displacement with two new contributions: first, we revised Spicer's model to take into account the optical penetration effect, which was neglected in Spicer's model; and second, the revised model was used to investigate the effect of laser rise time and beam size to the epicenter displacement. We showed that a pulse laser with short rise time generates an equivalent surface displacement with a pulse laser with long rise time, except a "spike" at the beginning of the epicenter waveform; also when the laser beam size increases, the epicenter displacement decreases. These two conclusions were then validated by experiments.