Laser Ultrasonics: Simultaneous Generation by Means of Thermoelastic Expansion and Material Ablation

Abstract

This paper describes a laser ultrasonic probe capable of generating ultrasound by both thermoelastic expansion and material ablation mechanisms simultaneously. Because bulk wave propagation is centered around a 67° line of sight for thermoelastically generated signals, and near normal (0°) for signals generated by means of ablation, the simultaneous generation by both mechanisms results in a wider range of useful observation angles. The system described uses a Nd:Yag laser with fiber optic delivery and a focusing objective, with an independent receiver, such as an EMAT, interferometer or contact transducer. The optical fiber delivery system allows the probe to be easily positioned (i.e. using a robot), and has the added benefit of allowing a single laser to service several test sites. The focusing objective provides the means for generating ultrasound by either thermoelastic expansion or ablation, or a combination of the two mechanism. The objective also serves to protect the fiber from ablated material and manufacturing contaminants (dust, welding gas, etc.). The resulting system is both physically robust and highly adaptable for a wide range of industrial ultrasonic inspection applications.