1. Hybrid metamaterials combining pentamode lattices and phononic plates

In this work, we propose a design strategy for hybrid metamaterials with alternating phononic plates and pentamode units that produce complete bandgaps for elastic waves. The wave control relies on the simultaneous activation of two scattering mechanisms in the constituent elements. The approach is illustrated by numerical results for a configuration comprising phononic plates with cross-like cavities. We report complete bandgaps of tunable width due to variations of geometric parameters. We show that the wave attenuation performance of the hybrid metamaterials can be further enhanced through implementation of lightweight multiphase material compositions. These give rise to efficient wave attenuation in challenging low-frequency regions. The proposed design strategy is not limited to the analyzed cases alone and can be applied to various designs of phononic plates with cavities, inclusions or slender elements.

This research is done in collaboration with Dr. P. Galich from Technion in Haifa, Israel; Dr. F. Bosia from the University of Turin, Italy; Prof. N. Pugno from the University of Trento, Italy, and Dr. S. Rudykh from the University of Wisconsin-Madison, U.S.A.

2. Accordion-like metamaterials with tunable ultra-wide low-frequency band gaps. (link)

Composite materials with engineered band gaps are promising solutions for wave control and vibration mitigation at various frequency scales. Despite recent advances in the design of phononic crystals and acoustic metamaterials, the generation of wide low-frequency band gaps in practically feasible configurations remains a challenge. Here, we present a class of lightweight metamaterials capable of strongly attenuating low-frequency elastic waves, and investigate this behavior by numerical simulations. For their realization, tensegrity prisms are alternated with solid discs in periodic arrangements that we call 'accordion-like' meta-structures. They are characterized by extremely wide band gaps and uniform wave attenuation at low frequencies that distinguish them from existing designs with limited performance at low-frequencies or excessively large sizes. To achieve these properties, the meta-structures exploit Bragg and local resonance mechanisms together with decoupling of translational and bending modes. This combination allows one to implement selective control of the pass and gap frequencies and to reduce the number of structural modes. We demonstrate that the meta-structural attenuation performance is insensitive to variations of geometric and material properties and can be tuned by varying the level of prestress in the tensegrity units. The developed design concept is an elegant solution that could be of use in impact protection, vibration mitigation, or noise control under strict weight limitations.

This research is done in collaboration with Dr. A. Amendola and Prof. F. Fraternali from the University of Salerno, Italy; Dr. F. Bosia from the University of Torino, Italy; Prof. N.M. Pugno from the University of Trento, Italy, and Prof. C. Daraio from California Institute of Technology, Pasadena, U.S.A.

3. Tuning frequency band gaps of tensegrity mass-spring chains with local and global prestress (link)

This work studies the acoustic band structure of tensegrity mass-spring chains, and the possibility to tune the dispersion relation of such systems by suitably varying local and global prestress variables. Building on established results of the Bloch–Floquet theory, the paper first investigates the linearized response of chains composed of tensegrity units and lumped masses, which undergo small oscillations around an initial equilibrium state. The stiffness of the units in such a state varies with an internal self-stress induced by prestretching the cables forming the tensegrity units, and the global prestress induced by the application of compression forces to the terminal bases. The given results show that frequency band gaps of monoatomic and biatomic chains can be effectively altered by the fine tuning of local and global prestress parameters, while keeping material properties unchanged. Numerical results on the wave dynamics of chains under moderately large displacements confirm the presence of frequency band gaps of the examined systems in the elastically hardening regime. Novel engineering uses of the examined systems are discussed.

This research is done in collaboration with Dr. A. Amendola and Prof. F. Fraternali from the University of Salerno, Italy; Dr. F. Bosia from the University of Torino, Italy; Prof. N.M. Pugno from the University of Trento, Italy, and Prof. C. Daraio from California Institute of Technology, Pasadena, U.S.A.

4. Coupling local resonance with Bragg band gaps in single-phase mechanical metamaterials (link)

Various strategies have been proposed in recent years in the field of mechanical metamaterials to widen band gaps emerging due to either Bragg scattering or to local resonance effects. One of these is to exploit coupled Bragg and local resonance band gaps. This effect has been theoretically studied and experimentally demonstrated in the past for two- and three-phase mechanical metamaterials, which are usually complicated in structure and suffer from the drawback of difficult practical implementation. To avoid this problem, we theoretically analyze for the first time a single-phase solid metamaterial with so-called quasi-resonant Bragg band gaps. We show evidence that the latter are achieved by obtaining an overlap of the Bragg band gap with local resonance modes of the matrix material, instead of the inclusion. This strategy appears to provide wide and stable band gaps with almost unchanged width and frequencies for varying inclusion dimensions. The conditions of existence of these band gaps are characterized in detail using metamaterial models. Wave attenuation mechanisms are also studied and transmission analysis confirms efficient wave filtering performance. Mechanical metamaterials with quasi-resonant Bragg band gaps may thus be used to guide the design of practically oriented metamaterials for a wide range of applications (source).

This research is done in collaboration with Dr. M. Miniaci from University of Le Havre, France, Dr. F. Bosia from University of Turin, Italy, and Prof. N. Pugno from University of Trento, Italy.