Ultrasonic evaluation of early-stage hygrothermal aging damage for anisotropic composites based on PA-LCR technology

Fiber reinforced plastic (FRP) composites are prone to hygrothermal aging damage during service, which are generally evaluated in destructive ways. In this study, we propose a nondestructive evaluation method based on critically refracted longitudinal (L_CR) wave using phased array ultrasonic, referred to as PA-L_CR method. This method offers broad adaptability to a wide range of material ultrasonic velocities, eliminating the necessary need to design specific focusing laws for particular fiber angles. Hence, the method can address the challenges of ultrasonic characterization in anisotropy, specifically the significant discrepancy in velocity caused by the orientated fibers and aging conditions. Here, the L_CR wave is flexibly excited and received in 0° and 90° fibers orientations based on one wedge with a fixed inclination angle. The wave amplitude is correlated with the preset velocity and deflection angle, and that under the optimal condition for these two parameters shows good correspondence with the early-stage hygrothermal aging. Additionally, the ultrasonic response mechanism is analyzed based on microstructure evolution in aging progress. The PA-L_CR method is proven to be efficient for damage evaluation of anisotropic composites, and can also enable an in-situ testing for some on-line monitoring of large-scale closed structure.