We illustrate the design of acoustic metasurfaces based on geometric tapers and embedded in thin-plate structures. The metasurface is an engineered discontinuity that enables anomalous refraction of guided wave modes according to the Generalized Snell’s Law. Locally-resonant geometric torus-like tapers are designed in order to achieve metasurfaces having discrete phase-shift profiles that enable a high level of control of refraction of the wavefronts. Results of numerical simulations show that anomalous refraction can be achieved on transmitted anti-symmetric modes (A0) either when using a symmetric (S0) or anti-symmetric (A0) incident wave, where the former case clearly involves mode conversion mechanisms.
We use a recently developed class of metamaterials based on geometric inhomogeneities to design acoustic lenses embedded in thin-walled structural element. The geometric inhomogeneity is based on the concept of Acoustic Black Hole (ABH) that is an exponential taper fully integrated in the supporting structure. The ABH is an element able to bend and, eventually, trap acoustic waves by creating areas with carefully engineered phase velocity gradients. Periodic lattices of ABHs are first studied in terms of their dispersion characteristics and then embedded in thin-plate structures to create lenses for ultrasonic focusing and collimation. Numerical simulations show the ability of the ABH lens to create focusing and collimation effects in an extended operating range that goes from the metamaterial to the phononic regime.
In this paper, we present an approach to the generation of steerable ultrasonic beams in structures for possible application to damage detection. The proposed approach is based on the design of embedded acoustic lenses that exploit fundamental principles of wave propagation in acoustic metamaterials. In particular, the lens design relies on the concept of acoustic drop-channel where multiple waveguides can be coupled and selectively activated by simply tuning the frequency of the excitation. Numerical analyses will show that this design allows generating highly directional excitation by using a single ultrasonic transducer. Plane Wave Expansion and Finite Difference Time Domain methods are used to evaluate the dispersion characteristics of the metamaterial lens as well as to simulate the transient response when the lens is embedded in a plate-like structure. The lens design is then experimentally validated on an aluminum plate where the lens is implemented by through the thickness notches. The overall performances are estimated by reconstructing the dynamic displacement field via Laser Vibrometry.