Non-contact ultrasonic inspection by Gas-Coupled Laser Acoustic Detection (GCLAD)

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.

Quality Control Strategy and Evaluation Algorithm for Noncontact Instrument Testing

“…Goyal et al studied the placement strategy of noncontact sensors for condition monitoring of rotating machine components [7]. Gulino et al used gascoupled laser acoustic testing for noncontact ultrasonic testing [8]. Pelivanov et al used noncontact photoacoustics to identify the molecular fngerprints of nanoparticles in complex media [9].…”

Line Scanning with Gas-Coupled Laser Acoustic Detection

“…The sinusoidal pressure field triggered by an acoustic wave that propagates along the z direction can be expressed as follows (coherently with the definitions provided by the authors in previous works [26,35,36]):…”