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Register a new userOmnidirectional elastic wave attenuation via an isotoxal-star-based auxetic micro-lattice
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This paper introduces a micro-lattice based metamaterial for low frequency wide-band vibration attenuation, that is enabled by engineering the metamaterials building blocks to induce local resonance bandgaps for elastic waves in all directions of propagation. The transmission rate through the proposed structure is examined and strong wave attenuation is demonstrated for a remarkably small number of unit cells. Additionally, it is shown that the bandgaps are tailorable via the geometrical parameters and can be leveraged to design a hybrid metamaterial with an extremely wide bandgap. Alongside being thin, lightweight, and capable of attenuating elastic waves in all directions, the proposed material also possesses the second order functionality of exhibiting a negative Poissons ratio and can pave the way for identifying exotic functional materials.
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We design a two-dimensional ultra-thin elastic metasurface consisting of steel cores coated with elliptical rubbers embedded in epoxy matrix, capable of manipulating bulk elastic wave modes for reflected waves. The energy exchanges between the longitudinal and transverse modes are completely controlled by the inclined angle of rubber. One elastic mode can totally convert into another by the ultra-thin elastic metasurface. The conversion mechanism based on the non-degenerate dipolar resonance is a general method and easily extended to three-dimensional or mechanical systems. A mass-spring model is proposed and well describe the conversion properties. We further demonstrate that high conversion rates (more than 95%) can be achieved steadily for one elastic metasurface working on almost all different solid backgrounds. It will bring wide potential applications in elastic devices.
A numerical solver for the elastic wave eigenmodes in acoustic waveguides of inhomogeneous cross-section is presented. Operating under the assumptions of linear, isotropic materials, it utilizes a finite-difference method on a staggered grid to solve for the acoustic eigenmodes of the vector-field elastic wave equation. Free, fixed, symmetry, and anti-symmetry boundary conditions are implemented, enabling efficient simulation of acoustic structures with geometrical symmetries and terminations. Perfectly matched layers are also implemented, allowing for the simulation of radiative (leaky) modes. The method is analogous to eigenmode solvers ubiquitously employed in electromagnetics to find waveguide modes, and enables design of acoustic waveguides as well as seamless integration with electromagnetic solvers for optomechanical device design. The accuracy of the solver is demonstrated by calculating eigenfrequencies and mode shapes for common acoustic modes in several simple geometries and comparing the results to analytical solutions where available or to numerical solvers based on more computationally expensive methods.
We present a numerical study on an enhanced periodic auxetic metamaterial. Rotating squares mechanism allied to precompression induced buckling give these elastic structures exotic properties. The static properties of the reference structure and the enhanced ones are first compared. After numerical analysis to ascertain the differences between several band calculation methods, we demonstrate the effect of precompression issued stress field on the dispersion diagram of the metamaterial. An optimization study is then performed to assess the potential vibration isolation improvement obtained with the new design. As a result, the bandgaps widths and range are found to be greatly increased by the geometric modifications proposed.
Hysteretic damping is often modeled by means of linear viscoelastic approaches such as nearly constant Attenuation (NCQ) models. These models do not take into account nonlinear effects either on the stiffness or on the damping, which are well known features of soil dynamic behavior. The aim of this paper is to propose a mechanical model involving nonlinear viscoelastic behavior for isotropic materials. This model simultaneously takes into account nonlinear elasticity and nonlinear damping. On the one hand, the shear modulus is a function of the excitation level; on the other, the description of viscosity is based on a generalized Maxwell body involving non-linearity. This formulation is implemented into a 1D finite element approach for a dry soil. The validation of the model shows its ability to retrieve low amplitude ground motion response. For larger excitation levels, the analysis of seismic wave propagation in a nonlinear soil layer over an elastic bedrock leads to results which are physically satisfactory (lower amplitudes, larger time delays, higher frequency content).
A simple procedure is demonstrated for fabrication of waveplates which can be released from substrate by laser cutting. Oblique angle deposition, chemical etching and laser inscription steps were used for the final lift-off and release of micro-waveplates in HCl solution.
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