Improved laser Ultrasonics NDT through simultaneous quasi-monochromatic and pulsed Rayleigh waves generation

Laser Ultrasonics is a cutting-edge NDT solution offering a completely non-contact approach that overcomes the limits of classical ultrasonic methods. It provides high-precision spatial resolution while adapting seamlessly to complex geometries and harsh environments. Most commonly in NDT, a thermoelastic line source is used to generate surface acoustic waves. The main goal of this study is to optimize defect detection capabilities by enhancing the generation efficiency of the thermoelastic sources. To this end and to the best of the authors' knowledge, a pulsed Nd:YAG laser beam is combined with a cost-effective and practical wavefront splitting device to generate, for the first time, simultaneous quasi-monochromatic and pulsed Rayleigh waves on an aluminum sample. This device enabled the generation of both broadband low-frequency and easily adjustable narrowband high-frequency surface waves. These ones were detected using a Mach-Zehnder interferometer. Finite element simulations revealed the spectral content of the excited waves as a function of various spatial distributions of the thermoelastic sources. These results were confirmed experimentally with strong agreement and subsequently applied to the study of defects with subwavelength depths. It is clearly established that this technical implementation improves defect detection and sizing while retaining the advantages of conventional thermoelastic sources.