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This paper presents a standardized process for fabricating multifunctional energy storage composites that involves using activated carbon fiber (CF) electrodes and a poly(vinylene fluoride) (PVDF)-...

This paper presents a metamaterial plate (metaplate) consisting of a periodic array of holes on a homogeneous thin plate with slot-embedded resonators. The study numerically proves that the proposed model can generate a complete vibrational bandgap in the low-frequency range. A simplified analytical model was proposed for feasibly and accurately capturing the dispersion behavior and first bandgap characteristics in the low-frequency range, which can be used for initial design and bandgap study of the metaplate. A realistic and practical unit metaplate was subsequently designed to verify the analytical model through finite element simulations. The metaplate not only generated a complete vibrational bandgap but also exhibited excellent agreement in both analytical and finite element models for predicting the bandgap characteristics. This study facilitates the design of opening and tuning bandgaps for potential applications such as low-frequency vibration isolation and stress wave mitigation.

This paper presents a tunable planar auxetic metamaterial (PAM) for controlling and filtering acoustic waves and provides guidelines for bandgap design of the proposed PAMs. Numerical results for deformed and undeformed PAMs were obtained from several finite element analyses based on Bloch–Floquet theory. The acoustic band structures of the PAMs were calculated with periodic boundaries. Tunable bandgaps in certain frequency ranges were generated by various deformations applied to the PAMs. Wave attenuation in experimental transmission loss at specific frequencies was demonstrated, showing favorable agreement with the bandgaps obtained from numerical calculations. Both the numerical and experimental results indicate that the proposed structure demonstrates great tunability and offers significant advantages over the regular materials for controlling sound wave propagation and filtering sound waves within specific frequency ranges.

By combining rotating squares with auxetic properties, we developed a metamaterial transformer capable of realizing metamaterials with tunable functionalities. We investigated the use of a metamaterial transformer-based thermal cloak–concentrator that can change from a cloak to a concentrator when the device configuration is transformed. We established that the proposed dual-functional metamaterial can either thermally protect a region (cloak) or focus heat flux in a small region (concentrator). The dual functionality was verified by finite element simulations and validated by experiments with a specimen composed of copper, epoxy, and rotating squares. This work provides an effective and efficient method for controlling the gradient of heat, in addition to providing a reference for other thermal metamaterials to possess such controllable functionalities by adapting the concept of a metamaterial transformer.

Structures created by incorporating resonating endo-structures within a load bearing exo-structure forbid dynamic disturbances within a specific frequency range from propagating into them without attenuations. Their dynamic behavior can be characterized using a negative effective mass (NM) density. The suitability of such negative effective mass structures (NMS) as infrastructural building-blocks implicitly less susceptible to both harmonic and broadband impact-type loadings is demonstrated. For harmonic loading, an apparent damping coefficient is derived to compare the degree of attenuation achieved in the wholly elastic NMS to an “equivalent” conventionally damped structure. Parametric studies were used to design and construct a low frequency vibration isolator with tip-loaded cantilever beam resonators that evinced 98% payload isolation at resonance. It was found that the higher the stiffness of the host structure, the narrower the isolation bandwidth is in the vicinity of the resonance frequency. Under impact loading, using a numerical optimization procedure, it was established that resonator frequencies toward the higher end of the incoming spectrum gave better reduction in transmitted peak stress. Compact and efficient resonators were constructed using plate springs with chemically etched reentrant patterns and machined titanium resonator masses. Tests performed using an impact pendulum on a resonator stack attached to a transmission bar, substantiated a peak stress reduction of about 60% and filtering of the resonator frequencies in the transmitted spectrum.

Modifying the geometry of a bonded area improves the strength of adhesively bonded joints. In this study, the joint performance of an adhesively bonded joint with nonflat interfaces was experimentally and numerically evaluated. Various parameters of embedded reinforcements were studied. A new fracture mechanism, termed a two-stage failure mode, was discovered. Moreover, two dominant factors were identified as influencing the strength of the bonded joints. The optimized nonflat-interface specimens exhibited improvements in the ultimate load and average shear stress of up to 45% and 52%, respectively, compared with typical flat-interface specimens. Applying the proposed nonflat joints together with artificial defects has little influence on the ultimate load of specimens and further reduces total weight, and joints can be fabricated without using an additional mold. Therefore, these joints are significantly advantageous for promoting this technique in the industry.

This paper presents the use of elastic metamaterials for impact attenuation and blast-wave mitigation. Metamaterials represent a novel and emerging research area where materials exhibit exceptional properties not commonly found in natural materials. These unique properties are enacted by specifically designed microstructures. In this study, a single-resonator model and a dual-resonator microstructural design are proposed to exhibit negative effective mass density. The effect of negative effective mass density is explicitly confirmed by analysis of wave propagation using numerical simulations. Results evidently show that impact stress wave attenuation occurs over a wider frequency spectrum for the dual-resonator model as compared to the narrow band gap of a single-resonator design. Parametric studies of blast-wave simulation reveal that the mass and number of internal resonators have significant influence over the frequency range of blast-wave attenuation. The effectiveness and performance of the single-resonator and dual-resonator models on blast-wave mitigation are examined and discussed. Finally, practical ways to design and manufacture elastic metamaterials with negative effective mass density are presented and explored.

Two continuum methods were investigated for modeling the dynamic behavior of an acoustic metamaterial in the form of a composite material with internal resonators. First, an effective homogeneous classical continuum model was proposed. The effective elastic constants for this continuum are obtained by taking the static equivalence between the continuum and the composite, while the effective mass density adopts the form of a second order tensor. The second model is also a continuum model that is described by two displacement variables. In addition to the usual displacement vector, a displacement vector for the motion of the resonator mass is included, thus making this multi-displacement model quite different from the classical model for elastic solids. It was shown that the dispersion relations predicted by the proposed two approaches were practically the same. The accuracy of the dispersion curves was verified by finite element analyses. Simplicity is the main advantage of the first approach. However, it has to adopt an unusual frequency-dependent effective mass density which may become negative in certain frequency range. On the other hand, the multi-displacement model can be constructed based on the actual material properties of the composite and, in general, is more versatile for further extensions to complex microstructures.

A mechanical model with local resonators is proposed as an acoustic metamaterial that exhibits an unusual frequency-dependent effective stiffness. If treated as an equivalent elastic solid, its effective Young's modulus can become unbounded or vanishingly small at two respective frequencies. Moreover, in a certain frequency range, the effective Young's modulus would become negative, resulting in a band gap that coincides with this frequency range. The wave attenuation behavior and mechanism are studied through numerical simulations on the acoustic metamaterial model. The capability of the metamaterial to selectively block or filter unwanted waves is demonstrated by a numerical simulation example.

We propose a bidirectionally tunable and multiconfiguration acoustic diode. The acoustic characteristics of the proposed prototype acoustic diode (PAD) were investigated experimentally and numerically. The results indicated that the PAD could achieve a favorable tunability index (rate of total changes in contrast ratio) of up to 124.3%. The PAD can not only turn on or off unidirectional transmission but realize the “bidirectionally tunable effect” to tune unidirectional transmission in the forward or backward direction within multibands. In particular, the PAD employs a unique control system for switching the types of tunability from unidirectional to bidirectional. Moreover, we propose a plate-type acoustic diode (PTAD). The PTAD was observed to demonstrate favorable tunability within multibands, and the rate of total variation in its contrast ratio approached 108.0%.

This study numerically and experimentally investigated the wave propagation and vibrational behavior of a metamaterial beam with lateral local resonators. A two-dimensional simplified analytical model was proposed for feasibly and accurately capturing the in-plane dispersion behavior, which can be used for the initial design. The out-of-plane wave motions, however, required advanced three-dimensional (3D) modeling. Through experimental validations, 3D finite element simulations were demonstrated to be suitable for advanced design and analysis. This study provided a basis for designing metabeams for transverse wave mitigation. The proposed concept can be further extended to 3D metamaterial plates for wave and vibrational mitigation applications.

A microstructure design of anisotropic resonant inclusions is investigated for the elastic metamaterial plate with the aid of the numerically based effective medium model. Experimental validation is then conducted in the anisotropic metamaterial plate through both harmonic and transient wave testing, from which the anisotropic effective dynamic mass density, group, and phase velocities are determined as functions of frequency. The strongly anisotropic mass density along two principal orientations is observed experimentally and the prediction from the experimental measurements agrees well with that from the numerical simulation. Finally, based on the numerically obtained effective dynamic properties, a continuum theory is developed to simulate different guided wave modes in the elastic metamaterial plate. Particularly, high-order guided wave coupling and repulsion as well as the preferential energy flow in the anisotropic elastic metamaterial plate are discussed.

A two-dimensional (2D) metamaterial possessing an effective anisotropic mass is investigated. This metamaterial is a composite material in the form of an internal mass connected in two directions to the host medium. A 2D mass-in-mass lattice model is used to characterize the dynamic behavior of the metamaterial. If modeled as an effective spring-mass lattice system, the metamaterial may possess a frequency-dependent effective mass. Moreover, if an equivalent homogenous elastic continuum is used to represent the metamaterial, an anisotropic mass density may result and may assume negative values for wave frequencies that are near the local resonance frequency of the internal mass. In fact, it was found that negative mass density occurs in the band-gap of the metamaterial. Unusual wave motion arises from the anisotropic band gap structure. In the present study, wave propagation in the representative continuum model for the metamaterial is studied in order to understand the unusual features of the dynamic behavior of the metamaterial.Copyright © 2009 by ASME

Metamaterials are materials with manmade microstructures. Recently, researchers have looked at a class of metamaterials whose microstructures contain internal degrees of freedom that are different from those of the macro-medium. These metamaterials exhibit unusual dynamic behavior and if modeled as homogeneous solids then their effective mass densities would become negative in certain frequency range. Specifically, a new stop band in the vicinity of the local resonance frequency of the internal mass in the microstructure would result. In this paper, a one dimensional metamaterial is employed to investigate the meaning of the negative mass density in the material and the energy flow in and out of the microstructure. In addition, numerical solutions are used to illustrate the phenomenon.Copyright © 2008 by ASME