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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"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.
"Meanwhile, the concept of metamaterials has been extended to acoustic and elastic media. Much effort has been focused on negative index of refraction     , sub-wavelength imaging  , and transformation acoustic       . Recently, acoustic ZIM have also drawn intense attention and various schemes have been proposed to realize them, such as acoustic waveguides loaded with membranes and/or Helmholtz resonator   , coiling up space with curled channels  , and 2D acoustic crystals with Dirac-like cones    . "
[Show abstract][Hide abstract] ABSTRACT: We theoretically investigate the unusual transmission property of in-plane
elastic waves through systems of isotropic elastic zero-index-metamaterials
(ZIM) slab embedded with solid defects. Both double-zero-index-metamaterials
(DZIM) and single-zero-index-metamaterials (SZIM) are examined. Particularly,
mode conversion does not occur when either longitudinal (P) wave or transverse
(S) wave is scattered by the defects in a specific type of DZIM possessing near
zero reciprocal of shear modulus and near zero mass density . Thus, P wave and
S wave can be controlled independently by simply adjusting the parameters of
the defects to achieve high transmission (cloaking) and total reflection
(blocking). And an analytic model is proposed to explain the mechanisms of the
total reflection. On the other hand, enhanced transmission is observed for the
SZIM systems. Our analysis shows that the enhanced transmission is due to
resonant modes arisen in the defects. In addition, we suggest a two-dimensional
(2D) phononic crystal (PC), which has effective double-zero-index, to provide
the possibility of achieving the intriguing transmission properties
"This model provided richer applications than other model reported so far but the microstructure is more complex due to the lack of symmetry. In this letter, we follow the same concept  to propose an alternative design of anisotropic elastic metamaterials with simpler constituents but better functionality. Numerical method is performed to analyze the effective parameters and wave propagation characteristics through the text. "
[Show abstract][Hide abstract] ABSTRACT: In this paper, we introduce an alternative model of elastic metamaterial by following the conceptual design of hybrid elastic solid reported by Lai and colleges . The proposed model is comprised of build-in resonant microstructures which are made of three kinds of conventional materials. Under specific incident frequencies, it behaves seemingly as a medium with negative mass density and elastic modulus so that a few novel wave propagation properties can be observed. We utilize finite element simulation to analyze the effective material parameters as well as the wave transmission properties. Comparing to the literatures, the proposed model appears larger band of practical application, and furthermore the required material is reduced which may be more easy to fabricate.