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In this paper, the wave characteristics of true surface waves (TSWs) and pseudo surface waves (PSWs) in two-dimensional (2D) seismic metamaterials are explored. The 2D seismic metamaterial is formed by placing periodic concrete pillars on the surface of the soil substrate. The complex dispersion curves of the seismic metamaterial are numerically calculated by the ω–kapproach. The Kelvin–Voigt model is adopted to describe the material viscoelasticity. The results show that not only the TSW modes but also the PSW modes should be taken into account to evaluate the mitigation capacity of the seismic metamaterials. Due to the introduction of material viscoelasticity, the TSWs and PSWs convert from the propagative mode to the evanescent mode. When the PSW modes propagate along the substrate surface, their energy gradually leaks into the bulk of the substrate, leading to their transmission coefficient smaller than that of the TSW modes. Compared to the concrete pillars on the substrate surface, the viscoelasticity of the soil substrate plays a more important role in surface wave reduction. Moreover, the wave propagating length in the seismic metamaterials can be predicted by using complex dispersion relations. [ABSTRACT FROM AUTHOR]

In this work, we numerically and experimentally investigate topological rainbow trapping and energy amplification of acoustic waves in a gradient phononic crystal (PC) structure. Thanks to the acoustic valley Hall effect, topological interface states (TISs) are generated along the interface between two PCs with different topological phases. To achieve rainbow trapping, we introduce the gradient into a 3D-printed PC structure by varying the geometrical parameter of scatterers along the interface. The incident acoustic waves at different frequencies split, stop, and, hence, are significantly amplified at different positions. Notably, the rainbow trapping of TISs is immune to random structural disorders. The topological rainbow trapping is promising for the design of broadband energy harvesters with excellent robustness. [ABSTRACT FROM AUTHOR]

The topology concept in the condensed physics and acoustics is introduced into the elastic wave metamaterial plate, which can show the topological property of the flexural wave. The elastic wave metamaterial plate consists of the hexagonal array which is connected by the piezoelectric shunting circuits. The Dirac point is found by adjusting the size of the unit cell and numerical simulations are illustrated to show the topological immunity. Then the closing and breaking of the Dirac point can be generated by the negative capacitance circuits. These investigations denote that the topological immunity can be achieved for flexural wave in mechanical metamaterial plate. The experiments with the active control action are finally carried out to support the numerical design. [ABSTRACT FROM AUTHOR]

A phoxonic crystal is a periodically patterned material that can simultaneously localize optical and acoustic modes. The acousto-optical coupling in two-dimensional air-slot phoxonic crystal cavities is investigated numerically. The photons can be well confined in the slot owing to the large electric field discontinuity at the air/dielectric interfaces. Besides, the surface acoustic modes lead to the localization of the phonons near the air-slot. The high overlap of the photonic and phononic cavity modes near the slot results in a significant enhancement of the moving interface effect, and thus strengthens the total acousto-optical interaction. The results of two cavities with different slot widths show that the coupling strength is dependent on the slot width. It is expected to achieve a strong acousto-optical/optomechanical coupling in air-slot phoxonic crystal structures by utilizing surface acoustic modes. [ABSTRACT FROM AUTHOR]

In this paper, a novel design concept for active auto-adaptive metamaterial (AAAMM) plates is proposed based on an active auto-adaptive (AAA) control strategy guided by the particle swarm optimization (PSO) technique. The AAAMM plates consist of an elastic base plate and two periodic arrays of piezoelectric patches. The periodic piezoelectric patches placed on the bottom plate surface act as sensors, while the other ones attached on the top plate surface act as actuators. A two-dimensional (2D) simplified metamaterial (MM) plate model is established by the Hamilton principle. The finite element method (FEM) with quadratic shape functions is adopted for numerically solving the 2D simplified MM plate model, which is validated by two more accurate but also much more complicated FEM models using three-dimensional (3D) piezoelectric and 3D elastic solid elements, and by its 2D counterpart model solved by the plane wave expansion (PWE) method. The conventional displacement, acceleration and velocity feedback control methods are introduced and analyzed. Then, a novel AAA control strategy based on combining the displacement and acceleration feedback control methods and guided by the PSO technique is developed. The main objective of the present paper aims at the vibration or flexural wave suppression and isolation of elastic plate structures based on an auto-adaptive control strategy, which has wide-range engineering applications. Numerical results will be presented and discussed to show that the proposed AAAMM plates can automatically and intelligently evolve different feedback control schemes to adapt to different stimulations on demand. Compared to the conventional MM plates, the proposed AAAMM plates exhibit improved and enhanced band-gap characteristics and suppression performance for flexural waves at frequencies inside and outside the band-gaps. [ABSTRACT FROM AUTHOR]

In this study, we propose the three-dimensional (3D) phononic crystal-based coupled resonator waveguides (PnCCRWs) for the guidance of acoustic waves along complex routes. The proposed 3D PnC exhibits a complete acoustic band-gap. The point defect mode with a frequency in the band-gap is achieved via the introduction of a point defect into the perfect PnC. The 3D PnCCRWs are constructed by appropriately setting the PnC point defects in the designed paths. These PnC point defects interact with the neighboring ones, which leads to the propagation of the acoustic wave energy along the designated PnCCRWs. Notably, the interaction between the neighboring PnC point defects can be well described by the tight-binding model. The waveguiding ability is demonstrated numerically and experimentally for three different PnCCRWs integrated in the same PnC structure. In addition, the viscothermal effect on the transmission properties of the PnCCRWs is also investigated. The proposed 3D PnCCRWs exhibit their flexibility, which may stimulate additional design rules for complex 3D phononic circuits. • The design of phononic crystal-based coupled resonator waveguides is proposed. • The waveguiding along 3D complex routes is verified numerically and experimentally. • The tight-binding model describes the interaction between phononic crystal defects. • The viscothermal effect on the transmission property is investigated. [ABSTRACT FROM AUTHOR]

The phoxonic crystal sensor is theoretically investigated in this paper. Based on the phoxonic crystal with dual surface mode bandgaps, both optical and acoustic waves can be confined near the surface. Thus dual cavities, one optical cavity and one acoustic cavity, are designed and integrated in the same phoxonic device. The sensor aims at determining the refractive index and sound velocity of different liquids by utilizing surface optical and acoustic waves. The sensitivities can be estimated by the variations of the cavity resonant frequencies in the response spectra. The relation between the frequency and the measuring physical parameters, i.e., the refractive index or the sound velocity, can be approximated by a linear function. By optimizing each cavity the photonic and phononic sensitivities can reach 199 nm/RIU and 2.79 MHz/ms −1 , respectively. This work can serve as a reference for the design of novel phoxonic sensors. [ABSTRACT FROM AUTHOR]

The propagation of elastic waves in a piezoelectric slab covered with periodically structured coatings or the so-called stubbed phononic crystal slab is investigated. Four different models are selected and the effects of distribution forms and geometrical parameters of the structured coatings on complete bandgaps are discussed. The phononic crystal slab with symmetric coatings can generate wider complete bandgaps while that with asymmetric coatings is favorable for the generation of multi-bandgaps. The complete bandgaps, which are induced by locally resonant effects, change significantly as the geometry of the coatings changes. Moreover, the piezoelectric effects benefit the opening of the complete bandgaps. [ABSTRACT FROM AUTHOR]

In this paper, a phononic crystal (PnC) Luneburg lens is designed for harvesting the mechanical energy of Rayleigh surface waves. The square-latticed PnC is formed by drilling holes periodically in an aluminum matrix. The PnC Luneburg lens for Rayleigh surface waves is constructed by spatially modulating the hole radius of each PnC unit-cell according to the refractive index profile of a conventional Luneburg lens. To convert mechanical energy into electricity, a piezoelectric disk is attached at the focal point of the PnC Luneburg lens, where the Rayleigh surface waves are focused. Compared with the reference harvester on a homogeneous aluminum matrix, the electric power outputs are significantly enhanced by the PnC Luneburg lens harvester for two typical directions, namely the Γ X and Γ M directions. For either the Γ X or Γ M direction, the amplification ratio of the PnC Luneburg lens harvester is over 14. [Display omitted] • The phononic crystal Luneburg lens is proposed for focusing Rayleigh surface waves. • The piezoelectric energy harvester based on the phononic crystal Luneburg lens possesses high amplification ratios. • The phononic crystal Luneburg lens exhibits high rotational isotropy in the piezoelectric energy harvesting. • The phononic crystal Luneburg lens can also be used as a collimator for Rayleigh surface waves. [ABSTRACT FROM AUTHOR]

Abstract: Phoxonic crystal is a promising artificial periodic material for optomechanical systems and acousto-optical devices. In this paper, two-dimensional phoxonic crystals with veins in square, triangle and honeycomb lattices are investigated. Detailed discussion is presented on the variation of the photonic and phononic bandgaps with the geometry and lattice form of the structure. The results show that simultaneous large complete photonic and phononic bandgaps can exist over a wide range of geometry parameters for the square and honeycomb lattices. These large regions offer good tolerance for the technological realization and flexibility in designing this phoxonic crystal structure. [Copyright &y& Elsevier]

In this paper, we investigate the topological characteristics of the phoxonic crystal (PxC) arrays. For the PxC system, the electromagnetic transverse electric mode and the elastic in‐plane mode are considered. The PxC possesses the photonic and phononic bandgaps simultaneously. Based on the bandgaps, we design the PxC cavity which can confine the electromagnetic and elastic waves. The interaction strength between the neighboring PxC cavities can be tuned by changing the distance between these cavities. The PxC cavity arrays with the topologically non‐trivial and trivial phases are obtained. We combine the PxC arrays with the topologically non‐trivial and trivial phases together. The electromagnetic and elastic topological interface states can be simultaneously observed at the interface of the PxC cavity arrays. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the elastic band structures of two-dimensional solid phononic crystals (PCs) with both negative and positive Poisson''s ratios are investigated based on the finite difference domain method. Systems with different combinations of mass density ratio and shear modulus ratio, filling fractions and lattices are considered. The numerical results show that for the PCs with both large mass density ratio and shear modulus ratio, the first bandgap becomes narrower with its upper edge becoming lower as Poisson''s ratio of the scatterers decreases from −0.1 to −0.9. Generally, introducing the material with a negative Poisson''s ratio for scatterers will make this bandgap lower and narrower. For the PCs with large mass density ratio and small shear modulus ratio, the first bandgap becomes wider with Poisson''s ratio of the scatterers decreasing and that of the host increasing. It is easy to obtain a wide low-frequency bandgap by embedding scatterers with a negative Poisson''s ratio into the host with a positive Poisson''s ratio. The PCs with large filling fractions are more sensitive to the variations of Poisson''s ratios. Use of negative Poisson''s ratio provides us a way of tuning bandgaps. [Copyright &y& Elsevier]

• A control method based on the non-simplified equations of motion of the lattice sandwich structures with considering the effects of the sensors and actuators is established. • The effects of active feedback control on the band-gaps of the pyramidal core lattice sandwich structures based on the non-simplified equations of motion are investigated. • The results show that the proposed simple control strategy is sufficiently accurate, highly efficient and can significantly expand the band-gaps. In this paper, we propose a simple active adaptive feedback (AAF) control strategy to enhance the vibration mitigation and isolation properties of the conventional pyramid-core lattice sandwich structures (PSSs) and the modified chessboard-designed pyramid-core lattice sandwich structures (CPSSs) via the lead zirconate titanate (PZT) ceramic patches. The proposed simple AAF control strategy is based on the active feedback (AF) control by relating the AF control parameters to the frequency. Firstly, the effects of the AF control parameters on the band structures are analyzed. Subsequently, the mathematical expressions of the frequency-dependent AAF control parameters are derived by the curve fitting technique. The results show that the proposed simple AAF control strategy is sufficiently accurate, highly efficient and can significantly expand the band-gaps compared to the conventional AF control, hence leading to improved vibration mitigation and isolation properties of the PSSs and CPSSs. The proposed simple AAF control strategy provides a simplified and generalized method to enhance the vibration mitigation and isolation properties of the lightweight lattice sandwich structures (LSSs) while maintaining their high stiffness-to-density ratio, which may offer various potential applications in engineering sciences. [ABSTRACT FROM AUTHOR]

Pyramid-core lattice sandwich structures (PCLSSs) have been widely used in various engineering fields due to their high stiffness-to-density ratio. Although the advantages of such structures are obvious, the rather poor vibration isolation property restricts their applications. In the present work, we develop the chessboard-designed and gradient PCLSSs in order to improve their out-of-plane vibration isolation performance. The numerical results are obtained by the finite element method (FEM). Our results show that the chessboard-designed PCLSS has a lower and wider band-gap for the out-of-plane vibration than the classical PCLSS. The effects of the geometry and material parameters on the band-gap are analyzed. A gradient PCLSS with the core radius varying linearly is proposed. Compared to the classical PCLSS, the gradient PCLSS can expand the frequency range of the band-gaps significantly while maintains the high stiffness-to-density ratio. Moreover, the spatial gradient of the pyramid-core is utilized to realize the rainbow trapping and the nonreciprocal transmission of the elastic flexural waves. The novel meta-designed PCLSSs show potential applications where both a good vibration isolation and a high stiffness-to-density ratio are needed.0 [ABSTRACT FROM AUTHOR]

A tunable and multi-functional one-dimensional metasurface, which is formed by engraving periodic semi-ellipse grooves on the surface of an aluminum half-space, is proposed in this paper. One characteristic of the metasurface is the manipulation of multi-physical fields, i.e. it could be utilized to manipulate surface elastic and acoustic waves simultaneously. The dispersion curves of the elastic and acoustic waves can be effectively tuned by adding liquids into the grooves. Based on the tunability different applications can be realized by adding different volumes of different liquids into the grooves. As an example, simultaneous rainbow trapping of the surface elastic and acoustic waves is demonstrated in the metasurface. Moreover, a resonant cavity where the elastic and acoustic waves are highly confined is reported. The proposed metasurface paves the way to the design of multi-functional devices for simultaneous control of elastic and acoustic waves. [ABSTRACT FROM AUTHOR]

Non-Fourier heat conduction models assume wave-like behavior does exist in the heat conduction process. Based on this wave-like behavior, thermal conduction controlled in a one-dimensional periodical structure, named thermal wave crystal, has been demonstrated through both theoretical analysis and numerical simulation based on the Cattaneo-Vernotte (CV) heat-conduction model. The transfer matrix method and Bloch theorem have been applied to calculate the complex dispersion curves of thermal wave propagating in thermal wave crystals. And the temperature responses are obtained by using the FDTD method. The results show that the band gaps with pronounced heat reduction do exist in non-Fourier thermal transfer process because of the Bragg scattering. The mid-gap frequency is well predicted analytically based on the Bragg scattering mechanism. Finally, the key parameters determining the band gaps are presented and discussed. This study shows the potential applications of these materials in heat isolation and reduction. [ABSTRACT FROM AUTHOR]

Phoxonic crystals (PXCs) which exhibit simultaneous phononic and photonic bandgaps are promising artificial materials for optomechanical and acousto-optical devices. In this paper, we theoretically investigate the phononic and photonic guided modes in the three-dimensional topology-type PXCs with a line defect. By varying the geometrical parameters, simultaneous guidance of the slow elastic and light (electromagnetic) waves can be realized. Both elastic and optical energies can be highly confined in and near the defect region. Small elastic and optical group velocities with small group velocity dispersions can be achieved. The group velocities are about 10 and 20 times smaller than the transverse velocity of the elastic waves in silicon and the speed of light in vacuum, respectively. [ABSTRACT FROM AUTHOR]

Sandwich structures (SSs) are widely deployed in engineering applications, such as for the low-frequency vibration or acoustic mitigation and isolation. However, it is challenging to enhance their low-frequency vibration mitigation and isolation performance while maintaining their multi-functionalities, such as lightweight, high bending stiffness, and high impact absorption ability, etc. To achieve this objective, local resonators are introduced into a hollow-tube core lattice sandwich structure (HLSS) in this paper, which is referred to as the metamaterial sandwich structure (MMSS-1). The proposed MMSS-1 is composed of periodically distributed thin-walled hollow tubes with attached resonators as the core, and two thin cover-plates or face-sheets. The band structures and the corresponding eigenmodes of the unit-cell of the MMSS show that both the Bragg scattering and the local resonance band-gaps coexist in the proposed MMSS-1. The influences of the key geometrical and material parameters of the MMSS-1 on the band-gaps are analyzed in details. Then, the piezoelectric patches are attached to the top and bottom face-plate surfaces of the MMSS-1 and resonators to form the MMSS-2, and they are shunted via an active feedback control to build the MMSS-3. The effects of the displacement feedback control gains on the band-gaps of the MMSS-3 are studied. Based on the active displacement feedback control and differential evolution (DE) optimization algorithm, a self-sensing and self-actuating strategy is developed to lend the self-adaptive capability to the MMSS-3, which is referred to as the self-adaptive MMSS. The vibration or flexural wave propagation in the finite-sized self-adaptive MMSS is numerically simulated to demonstrate its outstanding ability for low-frequency vibration mitigation and isolation. It shows that the proposed self-adaptive MMSS can evolve different control parameters automatically to widen and merge Bragg scattering band-gaps and local resonance band-gaps without human intervention. In addition, the vibration attenuation inside the band-gaps is also enhanced. The proposed self-sensing and self-actuating strategy can improve the vibration mitigation and isolation performance of the HLSSs significantly in the low-frequency range. The proposed self-adaptive MMSS may pave a novel and feasible way for the design and applications of the HLSSs for the low-frequency vibration mitigation and isolation in civil engineering, mechanical engineering, and aerospace engineering etc. Moreover, the proposed self-adaptive strategy via self-sensing and self-actuating can be also applied to other types of the SSs for improving their vibration mitigation and isolation performance, and for the NVH (noise, vibration and harshness) of vehicles, high-end equipment and high-precision instruments. [ABSTRACT FROM AUTHOR]