Abstract
Gas-Coupled Laser Acoustic Detection (GCLAD) is an ultrasonic, non-contact detection technique that has been recently proven to be applicable to the inspection of mechanical components. GCLAD response raises as the intersection length between the probe laser beam and the acoustic wavefront propagating in the air increases; such feature differentiates the GCLAD device from other optical detection instruments, making it a line detection system rather than a point detector. During the inspection of structures mainly extending in two dimensions, the capability to evidence presence of defects in whichever point over a line would enable moving the emitter and the detector along a single direction: this translates in the possibility to decrease the overall required time for interrogation of components compared to point detectors, as well as generating simpler automated monitoring layouts. Based on this assumption, the present study highlights the possibility of employing the GCLAD device as a line inspection tool. To this end, preliminary concepts are provided allowing maximization of the GCLAD response for the non-destructive testing of components which predominantly extend in two dimensions. Afterwards, the GCLAD device is employed in pulse-echo mode for the detection of artificial defects machined on a 12 mm-thick steel plate: the GCLAD probe laser beam is inclined to be perpendicular to the propagation direction of the airborne ultrasound, generated by surface acoustic waves (SAWs) in the solid which are first reflected by the defect flanks and subsequently refracted in the air. Numerical results are provided highlighting the SAW reflection patterns, originated by 3 mm deep surface and subsurface defects, that the GCLAD should interpret. The subsequent experimental campaign highlights that the GCLAD device can identify echoes associated with surface and subsurface defects, located in eight different positions on the plate. B-scan of the component ultimately demonstrates the GCLAD performance in accomplishing the inspection task.
Highlights
The application of non-contact ultrasonic excitation and detection techniques are standard practice in the non-destructive inspection of mechanical components, structures and systems
Gas-Coupled Laser Acoustic Detection (GCLAD) is an optical, non-contact ultrasonic detection technique unestablished in the field of non-destructive inspections, yet potentially interesting for its features as a line inspection technology
The GCLAD technique is based on the deviation and displacement sustained by a laser beam when crossing a pressure field in the air, attributable to an ultrasonic wave refracted in the air from the surface of a component under examination
Summary
The application of non-contact ultrasonic excitation and detection techniques are standard practice in the non-destructive inspection of mechanical components, structures and systems. OBD is a point detection technique, and its response strongly depends on the quality of the piece surface reflecting the laser beam, effectively limiting its use in cases of industrial interest Another optical technique is the Gas-Coupled Laser Acoustic Detection (GCLAD), whose physical principle leverages on the deviation of a probe laser beam propagating in air or water, in correspondence of areas with variations in the refractive index[25]. The objective of the present work is to pinpoint the characteristics of the GCLAD technique in line inspections, useful for speeding up the ultrasonic monitoring processes of wide surfaces For this purpose, application for SAW detection and identification of surface defects on an element extending in one dimension (i.e., a bar) is first illustrated, both in pitch-catch and pulse-echo mode. The effectiveness of the technique is highlighted in detecting subsurface d efects[33], introduced in the same plate
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
