Thermoelastic modeling of laser-generated ultrasound for nondestructive materials testing.

摘要

In recent times Q-switched lasers have been investigated as an ultrasound source for nondestructive testing (NDT) of materials and manufacturing processes. Q-switched lasers can generate large amplitude ultrasound waves in the thermoelastic regime, but the signals are broadband in nature and are susceptible to noise interference. It may also be desirable to measure the ultrasonic waves with narrowband sensors, but this is problematic for broadband signals. Narrowband ultrasound waves can be generated using temporally modulated laser pulses, which can overcome the problems of noise interference and difficulties in selecting narrowband receiving transducers.; The purpose of this work was to develop a comprehensive numerical model of laser generated ultrasound for isotropic materials in the thermoelastic regime, using temporally modulated laser pulses. Model development included a study of macroscopic heat conduction models. Physical mechanisms of longitudinal wave generation were also investigated. The model was then used to conduct a comprehensive analysis of temporally modulated laser sources of ultrasound. This research contains one of the most complete studies of the frequency content of laser generated ultrasound waves to date, for both Q-switched and modulated laser pulses.