Propagation Characteristics of Ultrasonic Waves Generated by Phased Array Laser in Coating/Substrate Structure

The phased array laser ultrasonic technology is suitable for non-destructive testing (NDT) of coating/substrate structure due to its excellent ability to generate ultrasonic waves with a high signal-to-noise ratio and flexible regulation. This research focuses on the propagation characteristics of ultrasonic waves generated by phased array laser in the coating/substrate (Ni/Al) structure. Considering the spatiotemporal distribution of phased array laser energy and the boundary conditions at the interface, the finite element model of phased array laser-generated ultrasonic waves in the coating/substrate structure was established. Furthermore, the propagation characteristics of bulk wave and Rayleigh wave were investigated, and the influences of element spacing and time delay of phased array laser on the steering characteristics of bulk wave and tuning characteristics of Rayleigh wave were analyzed. Finally, the accuracy of the simulation was verified through experiments based on the laser ultrasonic detection system. The simulation and experimental results indicate that the phased array laser can enhance the bulk wave at the steering direction, with the best enhancement effect occurring when the steering direction coincides with the maximum radiation direction in the directivity pattern of the laser-generated bulk wave. Besides, phased array laser can enhance certain frequency components of tuned Rayleigh waves by adjusting element spacing or time delay. The enhancement effect on low-frequency components is more significant with a short time delay or large element spacing. In contrast, the enhancement effect on high-frequency components is more significant with a long time delay or small element spacing. The research work in this paper has clarified the propagation characteristics for different modes of ultrasonic waves generated by phased array laser in the coating/substrate structure, which can provide a theoretical basis for subsequent defect detection.