Adiabatic Lamb modes in 3D tapered waveguides: Cut-off effects and ZGV resonances

This paper aims to enhance our understanding of the physical behavior of adiabatic modes in inhomogeneous elastic plates, particularly their remarkable capacity to adapt to gradual perturbations. The study investigates the propagation characteristics of higher-order adiabatic Lamb modes in waveguides with linearly varying thickness, with a focus on the influence of critical thicknesses on their propagation. This is achieved by leveraging the broadband excitation capabilities of a pulsed laser generating higher order Lamb modes to reveal various critical thicknesses, such as the cut-off and Zero-Group Velocity (ZGV) thicknesses. Remarkably, ZGV resonances can be induced at locations well beyond the laser source. Moreover, the mode's behavior is strongly influenced by thickness variations in all directions, imparting the plate an anisotropic-like behavior. Additionally, based on the observed effects, our experimental approach enables precise reconstruction of elastic waveguide profiles in additively manufactured aluminum plates with such thickness variations. The reconstructed profiles show a strong correlation with reference measurements across the scanned area.