Earthquakes represent one of the most catastrophic natural events affecting mankind. At present, a universally accepted risk mitigation strategy for seismic events remains to be proposed. Most approaches are based on vibration isolation of structures rather than on the remote shielding of incoming waves. In this work, we propose a novel approach to the problem and discuss the feasibility of a passive isolation strategy for seismic waves based on large-scale mechanical metamaterials, including for the first time numerical analysis of both surface and guided waves, soil dissipation effects, and adopting a full 3D simulations. The study focuses on realistic structures that can be effective in frequency ranges of interest for seismic waves, and optimal design criteria are provided, exploring different metamaterial configurations, combining phononic crystals and locally resonant structures and different ranges of mechanical properties. Dispersion analysis and full-scale 3D transient wave transmission simulations are carried out on finite size systems to assess the seismic wave amplitude attenuation in realistic conditions. Results reveal that both surface and bulk seismic waves can be considerably attenuated, making this strategy viable for the protection of civil structures against seismic risk. The proposed remote shielding approach could open up new perspectives in the field of seismology and in related areas of low-frequency vibration damping or blast protection.
Topologically protected waves in classical media provide unique opportunities for one-way wave transport and immunity to defects. Contrary to acoustics and electromagnetics, their observation in elastic solids has so far been elusive because of the presence of multiple modes and their tendency to hybridize at interfaces. Here, we report on the experimental investigation of topologically protected helical edge modes in elastic plates patterned with an array of triangular holes, along with circular holes that produce an accidental degeneracy of two Dirac cones. Such a degeneracy is subsequently lifted by careful breaking of the symmetry along the thickness direction, which emulates the spin orbital coupling in the quantum spin Hall effect. The joining of two plates that are mirror-symmetric copies of each other about the plate midthickness introduces a nontrivial interface that supports helical edge waves. The experimental observation of these topologically protected wave modes in elastic continuous plates opens avenues for the practical realization of structural components with topologically nontrivial waveguiding properties and their application to elastic waveguiding and confinement.
The appearance of nonlinear effects in elastic wave propagation is one of the most reliable and sensitive indicators of the onset of material damage. However, these effects are usually very small and can be detected only using cumbersome digital signal processing techniques. Here, we propose and experimentally validate an alternative approach, using the filtering and focusing properties of phononic crystals to naturally select and reflect the higher harmonics generated by nonlinear effects, enabling the realization of time-reversal procedures for nonlinear elastic source detection. The proposed device demonstrates its potential as an efficient, compact, portable, passive apparatus for nonlinear elastic wave sensing and damage detection. DOI: 10.1103/PhysRevLett.118.214301 In recent years, phononic crystals (PCs) have attracted great attention due to their unconventional dynamic behavior, with effects such as negative refraction [1], frequency band gaps [2,3], wave filtering or focusing [4][5][6], acoustic cloaking [7][8][9], subwavelength sensing [10,11], etc. Their periodic structure, rather than single material constituents, is responsible for their behavior, which exploits Bragg scattering [12,13]. Their attractive properties to act as stopband filters [12] or to concentrate energy in selected frequency ranges [14] makes them potentially interesting for nonlinear elastic source detection and to reveal the presence of defects, e.g. cracks, in a sample. This is because, in general, a nonlinear response is generated at the defect location and several possible features may appear, including the generation of higher order harmonics [15][16][17] or subharmonics [18,19], the nonlinear dependence of the elastic modulus and of attenuation coefficients on strain [20][21][22], and, as a consequence, the shift of the resonance frequency with increasing excitation amplitude [23,24] and the failure of the superposition principle [25,26]. All of these possible signatures can be used to detect and monitor the presence and evolution of damage, exploiting the greater sensitivity of nonlinear detection techniques compared to conventional linear ones [27].In the past years, nonlinear imaging techniques such as b scan, c scan, and tomography [28] have attracted much interest. A particularly robust and efficient approach is the combination of time reversal (TR) and nonlinear elastic wave spectroscopy (NEWS). This technique (TR-NEWS) exploits space-time focusing of the wave field achieved in TR [29] and applies it to a defect acting as a source of nonlinear elastic waves [30][31][32][33]. The scattered signal is recorded, the frequency generated by the primary source is filtered out using a bandpass filter, and the resulting signal is time reversed and reinjected by the receiver: due to the (t → −t) symmetry, the wave field back propagates to its original (nonlinear) source, focusing energy at the defect location at a specific time. Many studies have proved the efficiency and robustness of TR-NEWS in various configurations, for ...
a b s t r a c tVarious strategies have been proposed in recent years in the field of mechanical metamaterials to widen band gaps emerging due to either Bragg scattering or to local resonance effects. One of these is to exploit coupled Bragg and local resonance band gaps. This effect has been theoretically studied and experimentally demonstrated in the past for two-and three-phase mechanical metamaterials, which are usually complicated in structure and suffer from the drawback of difficult practical implementation. To avoid this problem, we theoretically analyze for the first time a single-phase solid metamaterial with socalled quasi-resonant Bragg band gaps. We show evidence that the latter are achieved by obtaining an overlap of the Bragg band gap with local resonance modes of the matrix material, instead of the inclusion. This strategy appears to provide wide and stable band gaps with almost unchanged width and frequencies for varying inclusion dimensions. The conditions of existence of these band gaps are characterized in detail using metamaterial models. Wave attenuation mechanisms are also studied and transmission analysis confirms efficient wave filtering performance. Mechanical metamaterials with quasi-resonant Bragg band gaps may thus be used to guide the design of practically oriented metamaterials for a wide range of applications.
Attenuating low-frequency sound remains a challenge, despite many advances in this direction.Recently developed acoustic metamaterials enable efficient subwavelength wave manipulation and attenuation due to exotic effects such as unusually high reflectivity, negative refraction or cloaking. In particular, labyrinthine acoustic metamaterials can provide broadband sound reduction and exhibit extremely high effective refractive index values due to their characteristic topological architecture. In this paper, we design a novel labyrinthine metamaterial with hybrid characteristics compared to previously proposed structures, by exploiting a spider web-inspired configuration. The developed metamaterial structure is characterized by additional tunability of the frequencies at which band gaps or negative group velocity modes occur, thus enabling versatility in the functionalities of the resulting structures. Time transient simulations demonstrate the effectiveness of the proposed metamaterials in manipulating wave fields in terms of transmission/reflection coefficients, amplitude attenuation and time delay properties in broadband 2 frequency ranges. Results could find applications in the development of practical lightweight acoustic shielding structures with enhanced broadband wave-reflecting performance.
In this paper the relaxed micromorphic continuum model with weighted free and gradient micro-inertia is used to describe the dynamical behavior of a real two-dimensional phononic crystal for a wide range of wavelengths. In particular, a periodic structure with specific micro-structural topology and mechanical properties, capable of opening a phononic band-gap, is chosen with the criterion of showing a low degree of anisotropy (the band-gap is almost independent of the direction of propagation of the traveling wave). A Bloch wave analysis is performed to obtain the dispersion curves and the corresponding vibrational modes of the periodic structure. A linear-elastic, isotropic, relaxed micromorphic model including both a free micro-inertia (related to free vibrations of the microstructures) and a gradient micro-inertia (related to the motions of the microstructure which are coupled to the macro-deformation of the unit cell) is introduced and particularized to the case of plane wave propagation. The parameters of the relaxed model, which are independent of frequency, are then calibrated on the dispersion curves of the phononic crystal showing an excellent agreement in terms of both dispersion curves and vibrational modes. Almost all the homogenized elastic parameters of the relaxed micromorphic model result to be determined. This opens the way to the design of morphologically complex meta-structures which make use of the chosen phononic structure as the basic building block and which preserve its ability of "stopping" elastic wave propagation at the scale of the structure.
Spider silk is a remarkable example of bio-material with superior mechanical characteristics. Its multilevel structural organization of dragline and viscid silk leads to unusual and tunable properties, extensively studied from a quasi-static point of view. In this study, inspired by the Nephila spider orb web architecture, we propose a design for mechanical metamaterials based on its periodic repetition. We demonstrate that spider-web metamaterial structure plays an important role in the dynamic response and wave attenuation mechanisms. The capability of the resulting structure to inhibit elastic wave propagation in sub-wavelength frequency ranges is assessed, and parametric studies are performed to derive optimal configurations and constituent mechanical properties. The results show promise for the design of innovative lightweight structures for tunable vibration damping and impact protection, or the protection of large scale infrastructure such as suspended bridges. Published by AIP Publishing. [http://dx
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