Topologically customized and surface-mounted meta-devices for Lamb wave manipulation

Lamb waves inside thin-walled structures have received extensive attention due to their great promise in applications such as structural health monitoring. Applications point at the common need for effective conditioning and manipulation of the wave propagation in terms of both frequency content and mode components. In this work, the concept of metamaterials is exploited to construct functional meta-devices (MDs). The MDs are designed to deliver prescribed functionalities after they are surface-mounted onto a structure conveying Lamb waves. To this end, a unified inverse-design scheme based on topology optimization is proposed and applied to achieve multifold functions such as frequency filtering, single-mode transmission and wave filtering at the subwavelength scale. Configuration features of the optimized MDs are extracted to reveal the mechanism governing the generation of broad Bragg scattering bandgaps. Analyses on negative effective mass density and the polarized mode explain the directional locally resonant bandgaps which exhibit strong anisotropic density. A representative MD with a finite number of unit cells is examined through finite element simulations. Temporal signals and their transmission spectra confirm the expected band features. An experiment is carried out to confirm the prescribed wave manipulation functions of the designed MD in terms of achieving selective frequency and wave mode transmission. This work provides a universal approach for topologically customizing MDs for the precise and tactic control of Lamb wave propagation.