Metamaterials can exhibit electromagnetic and elastic characteristics beyond those found in nature. In this work, we present a design of elastic metamaterial that exhibits multiple resonances in its building blocks. Band structure calculations show two negative dispersion bands, of which one supports only compressional waves and thereby blurs the distinction between a fluid and a solid over a finite frequency regime, whereas the other displays 'super anisotropy' in which compressional waves and shear waves can propagate only along different directions. Such unusual characteristics, well explained by the effective medium theory, have no comparable analogue in conventional solids and may lead to novel applications.
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"Much effort has been devoted in recent years to study of the propagation of acoustic/elastic waves in the periodic composites called PCs12345. PCs are artificial media consisting of periodic inclusions in a matrix background with various topologies678910. They can exhibit various special physical properties such as phononic band gaps in which waves are prevented from propagating. "
[Show abstract][Hide abstract] ABSTRACT: Band structures are investigated in two-dimensional phononic crystals (PC) composed of a periodic S-shaped slot in an air matrix with a square lattice. Dispersion relations, pressure fields and transmission spectra are calculated using the finite element method and Bloch theorem. Numerical results show that the proposed PC can yield complete and large band gaps at lower frequency ranges compared with that of the Jerusalem slots in Li et al. (Phys B 456:261–266, 2015) under the same parameter setting of the lattice and outline of the inclusions. The transmission spectrum is verified to be reasonably consistent with the band gaps along the \(\Gamma X\) direction. By analysing the pressure fields of several modes, the resonance modes of cavities within the S-shaped slot structure are found to result in the low-frequency band gaps. The effects of the geometrical parameters on the upper and lower edges of the first and second complete band gap are further studied. Numerical results show that the bandwidth of the first and second band gaps can be modulated over an extremely large frequency range by the geometrical parameters. The properties of the proposed PC have potential for implementation in structures and devices of noise and vibration control, such as noise filters and waveguides.
Full-text · Article · Oct 2015 · Acoustics Australia / Australian Acoustical Society
"Unfortunately, EMM configurations involving geometrically simple resonators have limited applicability, as they offer limited design opportunities to tune (widen and/or shift) the bandgaps – a problem that has been only partially alleviated using geometry or topology optimization. For example, it has been shown that, by embedding multiple heavy inclusions into a rubber matrix, an EMM can engage monopolar, dipolar and quadrupolar resonances associated with different dynamics of the resonating masses . As a result, the EMM can simultaneously feature negative effective mass density and negative effective elastic moduli. "
[Show abstract][Hide abstract] ABSTRACT: In this paper, an elastic metamaterial with multiple dissipative resonators is presented for broadband wave mitigation by properly utilizing interactions from resonant motions and viscoelastic effects of the constitutive material. The working mechanism of the metamaterial to suppress broadband waves is clearly revealed in a dissipative mass-in-mass lattice system through both negative effective mass density and effective metadamping coefficient. Based on the novel metadamping mechanism, a microstructure design of the dissipative metamaterial made of multi-layered viscoelastic continuum media is first proposed for efficient attenuation of a transient blast wave. It is found that the extremely broadband waves can be almost completely mitigated with metamaterials at subwavelength scale. The results of the study could be used in developing new multifunctional composite materials to suppress the shock or blast waves which may cause severe local damage to engineering structures.
No preview · Article · Oct 2015 · Composite Structures
"To obtain low frequency band gaps, the insertion in the microstructure of local resonators generally made of a hand core surrounded by a soft coating has been proved effective. In fact, the locally resonant material may exhibit the emergence of stop bands at frequencies around the natural frequency of the resonator with overall negative mass density and bulk modulus (see for instance Liu et al., 2000, Huang et al., 2009a, b, Lai et al., 2011, Raghavan and Srikantha Phani, 2013, Krushynska et al., 2014). Chiral periodic metamaterials with internal locally resonant structures supporting tunable low-frequency stop bands have been recently proposed by Liu et al., 2011a, Bigoni et al., 2013, and Zhu et al., 2014. "
[Show abstract][Hide abstract] ABSTRACT: A simplified model of periodic chiral beam-lattices containing local
resonators has been formulated to obtain a better understanding of the
influence of the chirality and of the dynamic characteristics of the local
resonators on the acoustic behavior. The simplified beam-lattices is made up of
a periodic array of rigid heavy rings, each one connected to the others through
elastic slender massless ligaments and containing an internal resonator made of
a rigid disk in a soft elastic annulus. The band structure and the occurrence
of low frequency band-gaps are analysed through a discrete Lagrangian model.
For both the hexa- and the tetrachiral lattice, two acoustic modes and four
optical modes are identified and the influence of the dynamic characteristics
of the resonator on those branches is analyzed together with some properties of
the band structure. By approximating the generalized displacements of the rings
of the discrete Lagrangian model as a continuum field and through an
application of the generalized macro-homogeneity condition, a generalized
micropolar equivalent continuum has been derived, together with the overall
equation of motion and the constitutive equation given in closed form. The
validity limits of the micropolar model with respect to the dispersion
functions are assessed by comparing the dispersion curves of this model in the
irreducible Brillouin domain with those obtained by the discrete model, which
are exact within the assumptions of the proposed simplified model.
Full-text · Article · Aug 2015 · International Journal of Solids and Structures