Mode-mismatching enhanced disbond detection using material nonlinearity in guided waves at low frequency

Disbond defect is a critical concern for the integrity of adhesively bonded engineering structures, and its characterization using ultrasonic guided waves (UGW)-based methods, suffers from the contradiction of multiple wave modes co-excitation at high frequency and poor sensitivity at low frequency. With this concern, this study addresses an investigation of disbond-disturbed nonlinearity in ultrasonic guided waves (UGWs) at low frequency, and on this basis, a novel disbond detection method is developed featuring enhancement by the mode-mismatching between fundamental and second-order harmonic wave (SOHW) at a specific mode-frequency combination. First, when the probing UGWs at low frequency are excited in adhesive bonded waveguide (aluminum-epoxy-aluminum), the SOHW in intact waveguides and the disbond region are scrutinized from analytical and numerical perspectives. It is observed that in the case of mode-mismatching for UGW of a specific mode-frequency combination, SOHW is remarkably suppressed in intact regions, whereby leading to the predominance of the disbond-induced SOHW. Then, on this basis, a nonlinearity-based method using the UGW with specific mode-frequency combination is developed to outstand the disbond defect. Finally, the experimental evaluation of disbonds well validates the effectiveness of the proposed method, in which three disbond defects with various lengths (10 mm, 20 mm, and 40 mm) are identified and quantitatively evaluated using the optimal UGW mode-frequency combination.