Eliminating PZT-Induced Harmonics Using Metasurface-Based Filters for Enhanced Nonlinear Guided Wave Structural Health Monitoring

Nonlinear guided wave-based Structural Health Monitoring (SHM) systems provide a powerful approach for the accurate detection and localization of structural damage by using harmonic generation as a key indicator of anomalies. However, distinguishing structural harmonics from those induced by instrumentation, particularly piezoelectric (PZT) actuators, remains a significant challenge. Over time, PZT actuators can develop debonding, leading to contact acoustic nonlinearity (CAN) effects and the generation of spurious harmonics that obscure damage signatures. To address this limitation, a novel metallic ring-shaped metasurface-based filter is proposed, specifically designed to suppress both instrument-induced and PZT-related harmonics. The metafilter's bandgap characteristics were assessed through dispersion curve analysis, while its ability to isolate damage-sensitive harmonics was evaluated using time-domain finite element simulations. Experimental validations were performed on an aluminium plate integrated with a defective PZT patch and a 3D-printed metafilter. Additionally, defect localization accuracy was enhanced through a kernel-based weighting of time-difference-of-arrival (TDOA) residuals across multiple sensor sets, enabling precise spatial mapping. The results demonstrate the efficacy of the proposed metafilter in eliminating harmonics caused by the CAN effect while preserving the integrity of damage-sensitive nonlinear wave components. This study highlights the robustness of the metafilter in improving the precision and reliability of nonlinear guided wave SHM systems, offering significant potential for practical applications across various industries.