Crack detection with gas-coupled laser acoustic detection technique

Abstract

Gas-coupled laser acoustic detection (GCLAD) is an unestablished ultrasonic detection technique based on the displacement that a laser beam sustains when intersected by an acoustic wave travelling in a fluid. In the present work, GCLAD for the noncontact detection of ultrasound is applied to identify surface defects on metal plates. Two GCLAD configurations are analysed, each associated with different sensitivities to cracks; specifically, GCLAD exhibits the highest sensitivity when inclined perpendicularly to the ultrasonic propagation direction in the air. The high angular selectivity allows evidencing only specific waves, those directly travelling in the component or reflected by defects. A higher ultrasonic amplitude is highlighted for GCLAD compared to air-coupled probes. The work ultimately demonstrates GCLAD's major advantage: the possibility of instantly performing an inspection over an entire line, rather than on a point as for traditional transducers.

Signal Enhancement in Surface Crack Detection with Gas-Coupled Laser Acoustic Detection

“…In this case, however, attenuation by the coating or painting stratum would necessarily result in a decrease in the ultrasonic wave amplitude [28]. No information is available on how the system would perform in different environments, for instance considering weather influence; currently, even if signal amplitudes are comparable to those from an ACT [37], additional optimizations from an electronics and a sensitivity standpoint are required for the GCLAD to become as reliable as other non-contact detectors like interferome-Fig. 5 Experimental layout for the steel plate inspection; the dashed rectangles represent locations of the artificial cracks ters for inspection practices: the low signal-to-noise ratio is currently the main factor preventing GCLAD application to industrialized contexts.…”

“…If not, the finite beam size reverses the behavior: a higher response for high acoustic wavelengths is observed, the beam diameter being the same [41]. As a non-contact detection device, GCLAD can sense variations in the refractive index associated with ultrasound induced by different types of contact or non-contact sources, like continuous wave lasers [42][43][44][45][46] or contact piezoelectric probes [37].…”

“…This technique is relatively simple from the required instrumentation standpoint, has the advantage of being negligibly influenced by the finishing or reflectance of the component's surface and demonstrates a sensitivity comparable to a Fabry-Perot confocal interferometer [35]. Even if the characteristics of the technique and the suitability for defect detection have been recently demonstrated through the analysis of various experimental configurations by [36,37], the GCLAD device currently fea-tures a low signal-to-noise ratio which limits its application range from an industrial perspective.…”

Non-Contact Ultrasonic Inspection by Gas-Coupled Laser Acoustic Detection (GCLAD)

“…The GCLAD technique has currently few industrial applications and there is no evidence of implementation in NDT processes. The knowledge on the physical principles that regulate the detection, useful for optimizing the GCLAD technique for NDT applications, has recently been deepened in some studies by the authors [27,28]: the GCLAD system can be exploited in various experimental configurations, obtaining the maximum sensitivity in configurations in which the interaction zone between the probe laser beam and the ultrasonic wave refracted in air is maximized; the system also proves to be selective with respect to the mutual orientation of the wavefront and the laser beam, enabling to exclude a large number of uninteresting ultrasonic components from the detected signal. The study also illustrates how the GCLAD system allows detecting Surface Acoustic Waves (SAWs) refracted in the air from any point belonging to a line on the surface.…”

“…Therefore, to obtain the maximum sensitivity in the visualization of the ultrasonic waves, it is convenient to employ an experimental configuration in which the laser beam is parallel to the wavefront refracted in the air. The layout hence enables identification of patterns as-230 sociated with specific acoustic waves, namely the direct (pitch-cath) or the reflected ones (pulse-echo) also in a context of defect detection [28]. For example, Figure 3 depicts the experimental arrangement for detecting a surface wave propagating on a plate with maximum 235 sensitivity.…”