Laser Ultrasonic Measurements for Generation and Detection of Lateral Waves in a Solid for Surface Defect Inspection

The first section of this study is devoted to characterizing the lateral wave generated by a laser along the air-aluminum interface. This includes determining its propagation velocity, analyzing its spectrum, and evaluating the variation of its amplitude in relation to the generation/detection distance (d). The obtained results have shown that the lateral wave propagates at the speed of the longitudinal volume wave, following the d^–n law, where n equals 2.46. Its spectrum exhibits a wide bandwidth, with a cutoff frequency of fc = 3 MHz. The second part of the paper focuses on utilizing these waves for surface defect detection, with a comparative analysis of results obtained with Rayleigh waves. Various tests were conducted to analyze the impact of defects on the lateral wave on transmitted or reflected modes. The outcomes illustrate modifications in the temporal signals and frequency spectra of the lateral wave in the presence of defects.