Parametric study of laser induced pulse-echo ultrasound for nondestructive testing applications

Abstract

Laser generation and reception of ultrasound is a frequently used tool for nondestructive testing applications. However, thick parts pose a significant challenge to the laser ultrasonic applications, especially for the methods, which require the ultrasound to travel twice the thickness of the specimen, for example, pulse-echo ultrasonic propagation imaging systems

this is because of the increased attenuation as well as the impact of material anisotropic property for the thick composite cases. In this thesis, we experimentally investigate the effects of laser pulse energy, laser beam size, laser pulse width, and laser incidence angles on the laser-induced pulse-echo ultrasound wave and their interrelationships to the inspection depth. Our experimental result revealing that the strength of the amplitude of the direct and the reflected ultrasound waves of the acquired 1D longitudinal wave is directly related to those parameters and to the inspection depth. The proof-of-concept has been done on composite specimens, where at the same fluence the amplitude of the longitudinal wave increased without causing surface damage to the part under test (thermoelastic regime), by increasing the beam size. The amplitude enhancement would significantly increase the propagation depth and make the full-field pulse-echo ultrasonic propagation imaging system feasible to optimize the thick composite part scanning parameters in the measurement setup in the sense that the generation efficiency improved without causing surface damage to the part under investigation and noise due to the incidence angle minimized. However, in order to keep the same fluence, a larger beam size usually requires a higher laser power, a fact that makes the system a little bit costly.