Assessment of thermal damage in variable-thickness poly-methyl methacrylate plates using non-specular reflection of bounded ultrasonic beams

This paper introduces a novel approach for assessing thermal damage in variable-thickness poly-methyl methacrylate (PMMA) plates through the analysis of non-specular reflection phenomena under critical incidence conditions. The theoretical framework begins with the derivation of the interaction mechanism between a finite-width plane wave and a variable-thickness plate under liquid-solid coupling conditions, followed by a comprehensive analysis of the reflection-to-incident energy ratio with respect to the incident angle. The investigation identifies three distinct critical incident angles corresponding to quasi-S2, quasi-S1, and quasi-A1 guided wave modes. Finite element simulations were conducted to examine the non-specular reflection characteristics of bounded ultrasonic beams. The simulations showed that, at specific critical incident angles, the scattered acoustic fields at the specular receiver exhibit exceptional sensitivity for detecting damage states in variable-thickness PMMA plates. Experimental verification substantiates the efficacy of this non-specular reflection approach for assessing the thermal aging state of variable-thickness PMMA plates. The results indicate that the proposed non-specular detection technique offers superior performance in early-stage thermal damage identification and maintains an enhanced signal-to-noise ratio, thereby demonstrating significant practical applicability.