Mode excitability and selectivity for enhancing scanning guided wave-based characterization of subwavelength defect

The scanning guided ultrasonic waves (GUWs) have been exploited intensively for characterizing defects or material anomalies at their early stage. Yet, the evaluation of defects in a subwavelength scale, which induces indiscernible disturbance to the linear and nonlinear features of GUWs, remains a daunting task for methods using the scanning GUWs owning to their limitation in terms of sensitivity and applicability. Based on the investigation of effect of defect on the excitability of GUW modes, we establish a framework to distinguish the optimal GUW mode with which the characterization of the small defect with subwavelength size using scanning GUWs can be enhanced. In this framework, the excitation transducer is scanned across the specimen surface to generate probing GUWs. The excitability of each GUW mode is investigated analytically, and the theoretical interpretation of effect of defect on excitation of each GUW mode is interrogated using a reciprocity theorem and Born approximation. In conjunction with the analysis of group velocity of GUW modes, the candidate GUW mode, i.e. mode S₁ at a frequency-thickness product of 3.58 MHz⋅mm, which features an excitability with optimal sensitivity to defect and high practical applicability is selected. A damage index that calibrates the extent of defect-induced variation in the waveform of the selected GUW mode is proposed, with which the subwavelength defects can be characterized accurately and reliably. Numerical and experimental validations are performed, in which the surface and subsurface defects of subwavelength scales are identified and visualized using the selected mode and an imaging algorithm. The results corroborate the effectiveness of the proposed framework for enhancing the characterization of subwavelength defects using modally selective scanning GUWs.