Low-frequency and second-harmonic guided-wave features for disbond assessment in adhesively bonded structures

Structural adhesive bonding offers significant advantages over conventional fastening methods, including weight reduction, improved stress distribution, and enhanced fatigue performance. However, interfacial defects such as disbonds can seriously compromise structural integrity. This study numerically and experimentally investigates nonlinear guided-wave responses associated with disbonds in an aluminum–epoxy–aluminum bonded plate, with particular emphasis on the low-frequency (LF) component and second-harmonic generation (SHG). Three-dimensional time-domain finite element simulations and corresponding experiments were performed on intact and disbonded specimens, and the received signals were processed using a phase-reversal technique with filtered RMS-based nonlinear indicators. The results show that the LF component exhibits cumulative growth with propagation distance and is less affected by attenuation than the SHG component. Both simulations and experiments confirm that disbonded specimens produce stronger nonlinear responses than intact specimens, while intact specimens still exhibit a smaller cumulative LF component due to background material nonlinearity. The relative performance of the nonlinear indicators depends strongly on excitation frequency: at 500 kHz, SHG provides clearer discrimination between intact and disbonded joints, whereas at 1000 kHz, the LF component shows greater sensitivity to disbond size and better persistence over distance. These results highlight the importance of frequency-tailored selection of nonlinear guided-wave features for practical disbond detection and characterization in adhesively bonded structures.