A theoretical approach is developed to describe the acoustic behavior of humid air-saturated porous media. Humid air is here considered as a binary mixture of dry air and water vapor, in which water vapor is likely to precondense (i.e., to be adsorbed) on a solid wall. The formulation of the acoustic problem includes the Navier-Stokes, mass conservation, heat diffusion, and mutual diffusion of water vapor in air equations. The boundary conditions account for the velocity continuity and the mass and heat flux conservation at the gas-liquid-solid interfaces. Indeed, precondensed water vapor forms a thin liquid film on the walls that undergoes thickness variations, resulting in non-zero vibration velocity at the liquid-gas interface. This film also acts as a mass and heat source. Its overall behavior is determined by the expression of its thermodynamic equilibrium with water vapor. The problem is analytically solved for straight pores having a constant cross section shape. The results obtained highlight the specific contributions of precondensation to the dynamic behavior of simple porous structures. These contributions, which are significant under extreme environmental conditions, essentially result in increasing the low-frequency limit of the normalized dynamic compressibility. Based on this analysis, a simple generalization to porous media is also proposed.

This study explores the possibilities of temporal control of one-dimensional phononic crystal consisting of a homogeneous piezoelectric plate on which metallic electrodes are structured. Previous works [1] have shown that Lamb-type guided modes in these periodically structured piezoelectric ceramic plates exhibit a tunable frequency dispersion that can be controlled by external electric circuits connected to the electrodes. In this study we develop a device enabling changes in the electric boundary conditions (EBC) from one condition to another by making use of switches driven by a microcontroller. This device agility is exploited to perform dynamic control of the crystal properties. To our knowledge, this is the first study devoted to the dynamic modulation of Lamb-type guided waves. Here, we do not simply change the EBCs as a function of time, but we create a "wave" of modulation of properties. Broadband translation of Lamb-wave dispersion curves in the frequency-wavenumber space (f, k) are observed, depending on the modulation speed. Experimentally, the translation of the S0 mode is detected. The relationship between the position of static bands and the position of bands after temporal modulation obeys a very simple vector rule in the (f, k) space. [1] Phys. Rev. B 99, 094302 (2019).

Shear wave excitation of an equilateral triangular solid bar shows the existence of horizontally polarized (SH) and vertically polarized (SV) shear waves. The SH wave modes are Lamé solutions of the Neumann problem for displacement and Dirichlet problem for stress in the equilateral triangle. It is shown that Lamé's solutions are obtained from superposition of three equal plane waves and their reflections. The Neumann SH modes have cutoff wavenumber 4mπ/(3a), where m=1,2,3,…, a being the side length of triangular cross section. The first Dirichlet SH mode has a higher cutoff wavenumber 4π7/(3a). The SV modes are related to the 30°-60°-90° and 30°-120°-30° sub triangles. Their cutoff wavenumbers lie between those of the first (m = 1) and second (m = 2) Neumann SH modes. The standing wave condition in each sub triangle is related to three unequal plane waves which decompose via symmetrical component analysis into two different sets of equal plane waves. A nonlinear product solution based on the new plane wave set leads to modal SV wave solutions. Dispersion curves calculated using COMSOL confirm the correct cutoff wavenumbers of both SH and SV waves. Normal displacement mode shapes are calculated analytically and verified experimentally using laser vibrometer with good agreement for all modes.

By using a structured tungsten-polyurethane composite that is impedance matched to water while simultaneously having a much slower longitudinal sound speed, we have theoretically designed and experimentally realized an underwater acoustic absorber exhibiting high absorption from 4 to 20 kHz, measured in a 5.6 m by 3.6 m water pool with the time-domain approach. The broadband functionality is achieved by optimally engineering the distribution of the Fabry-Perot resonances, based on an integration scheme, to attain impedance matching over a broad frequency range. The average thickness of the integrated absorber, 8.9 mm, is in the deep subwavelength regime (~λ/42 at 4 kHz) and close to the causal minimum thickness of 8.2 mm that is evaluated from the simulated absorption spectrum. The structured composite represents a new type of acoustic metamaterials that has high acoustic energy density and promises broad underwater applications.

A variety of multidispersive, localized, or extended in frequency, bands, induced by inductance-based external electric circuits in piezoelectric phononic plates, is studied both theoretically and experimentally in this work. Their origin, tightly related to an equivalent LC-circuit behavior, is analyzed in detail and their interaction with the Lamb-like guided modes of the plate is also discussed. These bands, easily tuned by the choice of the parameters of the external electric circuitry, lead to a non-destructive, real-time control of the dispersion characteristics of these structures. Our device and analysis can find application in the improvement of surface acoustic wave components by offering additional degrees of freedom.

Acoustic filters and metamaterials have become essential components for elastic wave control in applications ranging from ultrasonics to noise abatement. Other devices have been designed in this field, emulating their electromagnetic counterparts. One such case is an acoustic diode or rectifier, which enables one-way wave transmission by breaking the wave equation-related reciprocity. Its achievement, however, has proved to be rather problematic, and current realizations display a number of shortcomings in terms of simplicity and versatility. Here, we present the design, fabrication and characterization of a device able to work as an acoustic diode, a switch and a transistor-like apparatus, exploiting symmetry-breaking nonlinear effects like harmonic generation and wave mixing, and the filtering capabilities of metamaterials. This device presents several advantages compared with previous acoustic diode realizations, including versatility, time invariance, frequency preserving characteristics and switchability. We numerically evaluate its efficiency and demonstrate its feasibility in a preliminary experimental realization. This work may provide new opportunities for the practical realization of structural components with one-way wave propagation properties.

Phononic crystals made of piezoceramic materials allow for frequency band structure tunability due to the ease of electric command use. In this paper, we develop a full elastodynamic model, taking into account piezoelectric coupling effects for the band-structure calculation of Lamb eigenmodes of a phononic crystal consisting of a piezoceramic plate with a one-dimensional periodic array of electrodes set on both sides. The dispersion-relation characteristics of these eigenmodes are nondestructively tuned via external circuit impedance loads coupled to the phononic plate, e.g., through inductance loads inducing tunable resonant flat bands that hybridize with the Lamb modes of the elastic plate, thus opening up hybridization gaps. It is shown that additional control of the shape of these resonant bands can be achieved by incorporating an impedance interconnecting two adjacent electrodes of the same side, the whole structure forming an electric quadripole. This behavior can be easily predicted with the help of our formalism combined with a simplified electric line model of the phononic crystal plate.

We propose an experimental technique based on all-electric measurements to retrieve the frequency response of a one-dimensional piezoelectric phononic crystal plate, structured periodically with millimeter-scaled metallic strips on its two surfaces. The metallic electrodes, used for the excitation of Lamb-like guided modes in the plate, ensure at the same time control of their dispersion by means of externally loaded electric circuits that offer non-destructive tunability in the frequency response of these structures. Our results, in very good agreement with finite-element numerical predictions, reveal interesting symmetry aspects that are employed to analyze the frequency band structure of such crystals. More importantly, Lamb-like guided modes interact with electric-resonant bands induced by inductance loads on the plate, whose form and symmetry are discussed and analyzed in depth, showing unprecedented dispersion characteristics.

We demonstrate numerically and experimentally the opening of a locally resonant bandgap in an active phononic crystal (PC) made of a homogeneous piezoelectric plate covered by a 1D periodic array of thin electrodes connected to inductive shunts. The application of periodic electrical boundary conditions (EBCs) enables an at will tailoring of the dispersion properties of the PC plate, thus leading to a control of the dispersion of the propagating guided elastic waves in the plate. Depending on the nature of the EBCs, several bandgaps open up, the most important being a Hybridization Bandgap (HBG) in the subwavelength regime. The PC behaves as a locally resonant metamaterial. The HBG originates from the interaction of propagating elastic waves (Lamb modes) with an electrical resonant mode whose dispersion can be effectively described through an equivalent transmission line model.

The quest for large and low-frequency band gaps is one of the principal objectives pursued in a number of engineering applications, ranging from noise absorption to vibration control, and to seismic wave abatement. For this purpose, a plethora of complex architectures (including multiphase materials) and multiphysics approaches have been proposed in the past, often involving difficulties in their practical realization. To address the issue of proposing a material design that enables large band gaps using a simple configuration, in this study we propose an easy-to-manufacture design able to open large, low-frequency complete Lamb band gaps exploiting a suitable arrangement of masses and stiffnesses produced by cavities in a monolithic material. The performance of the designed structure is evaluated by numerical simulations and confirmed by scanning laser Doppler vibrometer (SLDV) measurements on an isotropic polyvinyl chloride plate in which a square ring region of cross-like cavities is fabricated. The full wave field reconstruction clearly confirms the ability of even a limited number of unit cell rows of the proposed design to efficiently attenuate Lamb waves. In addition, numerical simulations show that the structure allows to shift the central frequency of the BG through geometrical modifications. The design may be of interest for applications in which large BGs at low frequencies are required.

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 thicknessdirection,whichemulatesthespinorbitalcouplinginthequantumspinHalleffect.Thejoiningoftwo plates that are mirror-symmetric copies of each other about the plate midthickness introduces a nontrivial interface that supports helical edgewaves. 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.

EDITORIAL article Front. Mater., 19 September 2018Sec. Mechanics of Materials Volume 5 - 2018 | https://doi.org/10.3389/fmats.2018.00056

In this work, we numerically and experimentally investigate the influence of bioinspired hierarchical organization and material viscoelasticity on the wave dispersion diagram in metamaterials with self-similar structures at various spatial scales. The study reveals that the hierarchical architecture combined with viscoelastic material properties provides advantages for the dynamic performance with respect to conventional metamaterials.

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.

Propagation of guided Lamb waves in a piezoelectric phononic plate, on which metallic electrodes are laid on periodically, is studied. Electric boundary conditions (EBCs) applied on the electrodes induce some changes in the effective elastic properties of the plate. More particularly, the case of inductive impedance charges connected on the PC is investigated. This configuration enables the creation of low-frequency gap in the sub-wavelength regime.

The quest for large and low frequency band gaps is one of the principal objectives pursued in a number of engineering applications, ranging from noise absorption to vibration control, to seismic wave abatement. For this purpose, a plethora of complex architectures (including multi-phase materials) and multi-physics approaches have been proposed in the past, often involving difficulties in their practical realization. To address this issue, in this work we propose an easy-to-manufacture design able to open large, low frequency complete Lamb band gaps exploiting a suitable arrangement of masses and stiffnesses produced by cavities in a monolithic material. The performance of the designed structure is evaluated by numerical simulations and confirmed by Scanning Laser Doppler Vibrometer (SLDV) measurements on an isotropic polyvinyl chloride plate in which a square ring region of cross-like cavities is fabricated. The full wave field reconstruction clearly confirms the ability of even a limited number of unit cell rows of the proposed design to efficiently attenuate Lamb waves. In addition, numerical simulations show that the structure allows to shift of the central frequency of the BG through geometrical modifications. The design may be of interest for applications in which large BGs at low frequencies are required.

Using an extension of the full elastodynamic layer-multiple-scattering method to structures of fluid-saturated poroelastic spherical bodies, a comprehensive theoretical study of the acoustic response of double-porosity submerged liquid-saturated granular polymeric materials of specific morphology consisting of touching porous polymer spheres arranged in a fcc lattice, beyond the long-wavelength effective-medium description, is presented. Calculated transmission and absorption spectra of finite slabs of these materials are analyzed by reference to the acoustic modes of the constituent porous spherical grains as well as to relevant dispersion diagrams of corresponding infinite crystals, and a consistent interpretation of the results is provided. In particular, it is shown that resonant modes with very long lifetime, localized in the spheres in the form of slow longitudinal waves, which are peculiar to poroelastic materials, are formed when the viscous length is much shorter than the radius of the inner pores of the spheres. These modes, which can be easily tuned in frequency by adjusting the intrinsic porosity of the spheres, induce some remarkable features in the acoustic behavior of these double-porosity materials, such as narrow dispersionless absorption bands and directional transmission gaps.

The evaluation of the mesh resistance to opening of fishing nets is an important issue in assessing the selectivity of trawls by numerical methods. Sala et al. (2007) proposed a method using a relatively expensive experimental device ensuring the 2D deformation of net sample. De la Prada and Gonzales (2015) proposed a simple uni-axial experimental set-up, which stretches a sample in the transverse direction of the meshes while leaving free its deformation in the normal direction. Both authors (Sala and De la Prada) assumed that the deformation is uniform in the sample. The present study aims at developing a finite element model taking into account the mesh resistance to opening of nets, allowing the simulation of non-uniform deformation. Mechanical experimental tests were performed on a range of fishing nets commonly used in trawl codends, with varied dimensions of the sample and loading. The proposed model is in good agreement with these experimental results, and it captures the heterogeneous deformation of the netting samples. Consequently, a procedure for the assessment of the mesh resistance to opening using this model and a simple non-expensive experimental setup are proposed.