Application of the Gas-Coupled Laser Acoustic Detection technique to non-destructive monitoring of mechanical components

Non-contact ultrasonic techniques are fundamental to devise online monitoring systems for moving or difficult to access structures. Gas-Coupled Laser Acoustic Detection (GCLAD) is an unestablished, non-contact detection technology that relies on measuring the deviation affecting a laser beam when travelling across an ultrasonic wavefront propagating in a fluid. The aim of the work is to provide in-depth highlights on the principles on which the technique leverages, with a view towards how several laser beam and ultrasonic wave features reflect on the signal acquired by the GCLAD device. By numerical and experimental approaches, parameters needing to be specifically addressed and suitably set during the investigation phase are highlighted, which enable amplitude maximization of the acquired signal. Specifically, effect of the probe laser beam spot size is thoroughly analyzed, as well as the mutual orientation between the beam and the ultrasonic propagation directions. Three test configurations are lastly proposed, providing different results in terms of GCLAD sensitivity to the acoustic waves; such differences are highlighted by applying the technique to a railway axle on which an artificial crack has been machined, providing a first assessment of the GCLAD capabilities in the non-destructive testing field.