Dottorato in INGEGNERIA STRUTTURALE E GEOTECNICA

Titolo della tesi: Experimental Investigations into Wave Propagation in Metamaterials

This study presents a comprehensive experimental and numerical investigation into the design, fabrication, and dynamic characterization of acoustic metamaterials. The metamaterial architectures investigated here incorporate embedded resonators arranged in periodic and quasi-periodic configurations to manipulate elastic wave propagation through local resonance effects. Samples are fabricated using stereolithography and fused deposition 3D printing, ensuring high geometric accuracy and repeatability, while their dynamic response is measured using 3D laser scanning vibrometry (Polytec PSV-500-3D), which provides full-field, non-contact measurements of velocity field. The study focuses on honeycomb lattices with distributed resonators, analyzed under piezoelectric and electrodynamic excitations. Distinct vibration bandgaps are experimentally identified, with frequencies and bandwidths that show excellent agreement with theoretical predictions obtained through the Plane Wave Expansion (PWE) method and finite element simulations in COMSOL Multiphysics. The integration of multi-frequency resonators within the honeycomb structure broadens the attenuation range, while the spider-web configuration exhibits multiple low-frequency bandgaps resulting from coupling between radial and circumferential vibration modes. Further investigations on 3D-printed spider-web resonator metamaterials reveal nonlinear behaviors and sensitivity to geometric imperfections. The baseline design of honeycombs with spider web-like resonators exhibits three distinct bandgaps and a hardening-type response under increasing excitation levels, whereas the imperfect variant shows detuned resonators and softening-type nonlinearity, leading to tunable bandgap behavior. Both designs display edge wave phenomena near the transition between acoustic and optical modes, confirmed through eigenvalue analyses. Complementary studies extend the investigation to electrospun membrane-based metamaterials and aperiodic Penrose lattices, which reveal disorder-induced localization and hybrid bandgap mechanisms emerging from quasi-periodicity and multi-scale interference. Overall, this work establishes a rigorous experimental framework for the study of wave propagation in periodic, nonlinear, and aperiodic resonant metamaterials. The findings demonstrate how geometry, nonlinearity, and structural imperfections jointly govern wave behavior, offering design principles for next-generation metamaterials in vibration suppression and noise control.