G. K. Hu

An elastic metamaterial with simultaneously negative mass density and bulk modulus

In this letter, an elasticmetamaterial which exhibits simultaneously negative effective mass density and bulk modulus is presented with a single unit structure made of solid materials. The double-negative properties are achieved through a chiralmicrostructure that is capable of producing simultaneous translational and rotational resonances. The negative effective mass density and effective bulk modulus are numerically determined and confirmed by the analysis of wave propagation. The left-handed wave propagation property of this metamaterial is demonstrated by the negative refraction of acoustic waves.

Ultrathin low-frequency sound absorbing panels based on coplanar spiral tubes or coplanar Helmholtz resonators

Performance of classic sound absorbing materials strictly depends on their thickness, with a minimum of one-quarter wavelength to reach full sound absorption. In this paper, we report ultrathin sound absorbing panels that completely absorb sound energy with a thickness around one percent of wavelength. The strategy is to bend and coil up quarter-wavelength sound damping tubes into 2D coplanar ones, and embed them into a matrix to form sound absorbing panel. Samples have been designed and fabricated by 3D printing. Efficacies of sound absorption by these panels were validated through good agreement between theoretical analysis and experimental measurements.

Experimental study of an adaptive elastic metamaterial controlled by electric circuits

The ability to control elastic wave propagation at a deep subwavelength scale makes locally resonant elastic metamaterials very relevant. A number of abilities have been demonstrated such as frequency filtering, wave guiding, and negative refraction. Unfortunately, few metamaterials develop into practical devices due to their lack of tunability for specific frequencies. With the help of multi-physics numerical modeling, experimental validation of an adaptive elastic metamaterial integrated with shunted piezoelectric patches has been performed in a deep subwavelength scale. The tunable bandgap capacity, as high as 45%, is physically realized by using both hardening and softening shunted circuits. It is also demonstrated that the effective mass density of the metamaterial can be fully tailored by adjusting parameters of the shunted electric circuits. Finally, to illustrate a practical application, transient wave propagation tests of the adaptive metamaterial subjected to impact loads are conducted to valida...

Effective Dynamic Properties and Multi-Resonant Design of Acoustic Metamaterials

In the study, a retrieval approach is extended to determine the effective dynamic properties of a finite multilayered acoustic metamaterial based on the theoretical reflection and transmission analysis. The accuracy of the method is verified through a comparison of wave dispersion curve predictions from the homogeneous effective medium and the exact solution. A multiresonant design is then suggested for the desirable multiple wave band gaps by using a finite acoustic metamaterial slab. Finally, the band gap behavior and kinetic energy transfer mechanism in a multilayered composite with a periodic microstructure are studied to demonstrate the difference between the Bragg scattering mechanism and the locally resonant mechanism.

Microstructural designs of plate-type elastic metamaterial and their potential applications: a review

Elastic metamaterials are of growing interest due to their unique effective properties and wave manipulation abilities. Unlike phononic crystals based on the Bragg scattering mechanism, elastic metamaterials (EMMs) are based on the locally resonant (LR) mechanism and can fully control elastic waves at a subwavelength scale. Microstructural designs of EMMs in plate-like structures have attracted a great deal of attention. In this paper, the recent advances in the microstructural designs of LR-based EMM plates are reviewed. Their potential applications in the fields of low frequency guided wave attenuation, wave manipulation and energy trapping at a subwavelength scale, and structural health monitoring are discussed.

A single-phase elastic hyperbolic metamaterial with anisotropic mass density

Wave propagation can be manipulated at a deep subwavelength scale through the locally resonant metamaterial that possesses unusual effective material properties. Hyperlens due to metamaterials anomalous anisotropy can lead to superior-resolution imaging. In this paper, a single-phase elastic metamaterial with strongly anisotropic effective mass density has been designed. The proposed metamaterial utilizes the independently adjustable locally resonant motions of the subwavelength-scale microstructures along the two principal directions. High anisotropy in the effective mass densities obtained by the numerical-based effective medium theory can be found and even have opposite signs. For practical applications, shunted piezoelectric elements are introduced into the microstructure to tailor the effective mass density in a broad frequency range. Finally, to validate the design, an elastic hyperlens made of the single-phase hyperbolic metamaterial is proposed with subwavelength longitudinal wave imaging illustrated numerically. The proposed single-phase hyperbolic metamaterial has many promising applications for high resolution damage imaging in nondestructive evaluation and structural health monitoring.

An adaptive metamaterial beam with hybrid shunting circuits for extremely broadband control of flexural waves

A great deal of research has been devoted to controlling the dynamic behaviors of phononic crystals and metamaterials by directly tuning the frequency regions and/or widths of their inherent band gaps. Here, we report a new class of adaptive metamaterial beams with hybrid shunting circuits to realize super broadband Lamb-wave band gaps at an extreme subwavelength scale. The proposed metamaterial is made of a homogeneous host beam on which tunable local resonators consisting of hybrid shunted piezoelectric stacks with proof masses are attached. The hybrid shunting circuits are composed of negative-capacitance and negative-inductance elements connected in series or in parallel in order to tune the desired frequency-dependent stiffness. It is shown theoretically and numerically that by properly modifying the shunting impedance, the adaptive mechanical mechanism within the tunable resonator can produce high-pass and low-pass wave filtering capabilities for the zeroth-order anti-symmetric Lamb-wave modes. These unique behaviors are due to the hybrid effects from the negative-capacitance and negative-inductance circuit elements. Such a system opens up important perspectives for the development of adaptive vibration or wave-attenuation devices for broadband frequency applications.

Optimization on microlattice materials for sound absorption by an integrated transfer matrix method

Materials with well-defined microlattice structures are superlight, stable, and thus bear great potential in sound absorption. An integrated transfer matrix method (TMM) is proposed to evaluate the sound absorbing efficiency of these lattice materials, in which a massive number of micropores are densely placed. A comparison between integrated TMM and conventional TMM reveals that the proposed approach offers better predictions on sound absorption of microlattice. This approach is then employed to optimize the microlattice material to determine the best pore and porosity that lead to maximum absorbing efficiency capability and minimum required thickness to attain a target sound absorption.

Analytical formulation of a discrete chiral elastic metamaterial model

By embedding appropriately designed chiral local resonators in a host elastic media, a chiral metamaterial with simultaneously negative effective density and bulk modulus can be achieved. In this work, an two dimentional (2D) ideal discrete model for the chiral elastic metamaterial is proposed. The discrete dynamic equation is derived and then homogenized to give the continuous description of the metamaterial. The homogenization procedure is validated by the agreement of the dispersion curve of the discrete and homogenized formulations. The form of homogenized governing equations of the metamaterial cannot be classified as a traditional Cauchy elastic theory. This result conforms the conscience that the Cauchy elasticity cannot reflect the chirality, which is usually captured by higher order theory such as the non-centrosymmetric micropolar elasticity. However when reduced to a (2D) problem, the existing chiral micropolar theory becomes non-chiral. Based on reinterpretation of isotropic tensors in a 2D case, we propose a continuum theory to model the chiral effect for 2D isotropic chiral solids. This 2D chiral micropolar theory constitutes a hopeful macroscopic framework for the theory development of chiral metamaterials.

Particle focusing in a microchannel with acoustic metafluid

This work proposed a method of particle focusing by acoustic waves in a microfluidic channel with meta-structures. The channel was first filled by homogeneous metafluid possessing negative bulk modulus or density, mechanism and efficacy of particle focusing in such channel have been studied. Then as a realization, a structural microchannel composed of acoustic resonant elements has been proposed, which generated similar acoustic field gradient as that in homogeneous metafluid. Accordingly, particle movements in the structural microchannel were investigated and particle focusing was also achieved. The proposed particle focusing method is independent on the type of incident wave and microchannels size.