Damage visualization using laser-generated residual guided waves with optimization of laser scanning path

Abstract Laser ultrasonic scanning is intensively used in guided wave-based non-destructive testing due to its non-contact scanning, automated scanning path and full wavefield measurement. However, in practical applications, the selected repetition rate of laser generation is limited due to the existence of residual guided waves generated by consecutive pulsed laser impingements, leading to a low scanning speed. To overcome such limitations, the formation of residual wavefield is investigated and an optimization method of laser scanning path is developed to improve the damage visualization using the residual guided waves. First, the energy partition of residual wavefield is investigated based on diffuse wavefield approximation and transient statistical energy analysis. Then, a virtual superposition method is proposed to obtain the synthetic wavefields with different laser repetition rates. The influence of laser repetition rate on the wavefield directivity and energy distribution is analyzed. Moreover, according to the ultrasonic attenuation property of the test specimen, an optimized sub-block raster scanning path is proposed to minimize the spatial misalignment of the energy distribution in residual wavefield. Experiments are conducted on an aluminum alloy plate with multiple artificial notches using a 1-kHz-repetition-rate scanning laser ultrasonic system. Based on the residual wavefield signal energy, damage visualization results before and after laser scanning path optimization are compared. Results show that the proposed method is not only more efficient during the laser scanning process, but also more capable of distinguishing damaged areas from the intact structure.