Effective generation of shear horizontal waves from Lamb waves by meta-converters

Shear horizontal (SH) waves in thin-walled structures exhibit many advantages that are conducive to structural health monitoring (SHM) applications. However, their effective excitation is challenging in practice due to the inefficiency of shear-type piezoelectric transducers or the installation inconvenience of electromagnetic transducers. In this study, embracing the concept of metamaterials, we propose an SH wave generation scheme based on the mode conversion from Lamb waves activated by piezoelectric transducers. To this end, a metamaterial-mediated converter (referred to as a meta-converter), mounted on the surface of a host structure, is designed to achieve high-efficiency energy conversion from Lamb waves to SH waves. Targeting different Lamb wave modes in specific frequency bands, a topology optimization approach is developed to design the three-dimensional (3D) architecture of the meta-converters. The efficiency of the conversion is further evaluated in terms of the magnitude of the converted SH waves with respect to the incident Lamb waves. Analyses of band structures and mode shapes elucidate the mechanisms of the mode conversion within the targeted frequency band. Time-domain responses are analyzed to verify the efficient generation of SH waves, which are further illustrated by the visualizations of wave fields and wavenumber-frequency Fourier transform spectra. The robustness of the designed meta-converters is evaluated by examining piezoelectric transducers of different sizes. Finally, representative meta-converters are 3D-printed using the selective laser melting technique and tested for validation. The proposed method offers an alternative and feasible route for SH wave generation and paves a novel way for further SH-wave-based SHM applications.