Laser-Generated Surface Acoustic Wave Technique for Crack Monitoring – A Review

Abstract

In this paper, the principle of surface acoustic wave techniques and their application to the monitoring of cracks are presented and compared to other classic non-destructive techniques. A practical classification of methods regarding the excitation and detection of surface acoustic waves is enumerated, among them, laser-generated surface acoustic wave technique is carefully analyzed as a prospective technique, and two important detection methods using piezoelectric and light deflection are described. Then, the strategies and variables used in crack monitoring based on laser-generated surface acoustic wave technique are reviewed. To achieve the goal of quantitative detection of cracks, most researchers use numerical models and experiments to characterize main crack features. Discussions and prospective approaches for further quantitative monitoring of cracks are provided.

Use of Surface Acoustic Waves for Crack Detection on Railway Track Components—Laboratory Tests

“…These waves may also be termed quasi-Rayleigh waves. A promising method for permanent monitoring of components with regard to surface cracks is to employ surface acoustic waves for crack detection and characterization [3]. To this end, surface acoustic waves are excited at one point of the structure, and the transmitted waves are detected at another point.…”

“…To this end, surface acoustic waves are excited at one point of the structure, and the transmitted waves are detected at another point. The detected signal changes if a crack grows between the excitation and the detection points, since the crack acts as an obstacle in the wave path [3,4]. Possible techniques for the excitation of surface acoustic waves are electromagnetic acoustic transducers (EMAT) [5], piezoelectric transducers equipped with wedge or comb adaptors [6], or pulsed laser beams [3].…”

Simulation study of melanoma detection in human skin tissues by laser-generated surface acoustic waves

“…[6][7][8] When a pulse laser beam impinges on the skin, a part of the energy is scattered, but a significant part is converted into heat. With this conversion process, elastic waves are mainly caused by local thermal expansion during temperature rise and spreading in the skin surface.…”

“…Under this excitation mode, called thermo-elastic mechanism, the energy of the generated surface acoustic waves (SAW) is concentrated at the depth of approximately one wavelength of the elastic wave below the surface. 8,9 Since the characteristics of SAW propagation are directly related to the properties of the surface material, the changes that happened during the propagation process can be used as a detection index, reflecting the local differences of tissue's mechanical behavior, which may be a useful indicative of the presence of skin diseases, as Fig. 1 shows.…”

Simulation study and guidelines to generate Laser-induced Surface Acoustic Waves for human skin feature detection

“…This technique has been promisingly used in recent years due to its high accuracy and non-contact testing potential of layered and thin film materials [3][4][5]. Since some characteristics of the SAW propagation, such as the phase velocity, are directly related to the properties of the tested material, surface wave changes happened during the propagation process can be used to characterize the tissue properties, which may be a useful testing tool in biological and biomedical applications [6][7][8].…”