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Understanding and analysing the complex phenomena related to elastic wave propagation has been the subject of intense research for many years and has enabled application in numerous fields of technology, including structural health monitoring (SHM).

We study the propagation of in-plane elastic waves in a soft thin strip, a specific geometrical and mechanical hybrid framework which we expect to exhibit a Dirac-like cone. We separate the low frequencies guided modes (typically 100 Hz for a 1-cm-wide strip) and obtain experimentally the full dispersion diagram. Dirac cones are evidenced together with other remarkable wave phenomena such as negative wave velocity or pseudo-zero group velocity (ZGV). Our measurements are convincingly supported by a model (and numerical simulation) for both Neumann and Dirichlet boundary conditions. Finally, we perform one-way chiral selection by carefully setting the source position and polarization. Therefore, we show that soft materials support atypical wave-based phenomena, which is all of the more interesting as they make most of the biological tissues.

Elastic waves involving mechanical particle motions of solid media can couple volumetric and shear deformations, making their manipulation more difficult than electromagnetic waves. Thereby, circularly polarized waves in the elastic regime have been little explored, unlike their counterparts in the electromagnetic regime, where their practical usage has been evidenced in various applications. Here, we explore generating perfect circular polarization of elastic waves in an isotropic solid medium. We devise a novel strategy for converting a linearly polarized wave into a circularly polarized wave by employing an anisotropic medium, which induces a so-far-unexplored coupled resonance phenomenon; it describes the simultaneous occurrence of the Fabry-Pérot resonance in one diagonal plane and the quarter-wave resonance in another diagonal plane orthogonal to the former with an exact 90° out-of-phase relation. We establish a theory explaining the involved physics and validate it numerically and experimentally. As a potential application of elastic circular polarization, we present simulation results demonstrating that a circularly polarized elastic wave can detect an arbitrarily oriented crack undetectable by a linearly polarized elastic wave. The authors introduce a method for non-destructive testing based on circularly polarized ultrasound in solids, enabled by metamaterials, allows for detecting internal defects regardless of their orientation. This innovation will redefine ultrasonic non-destructive examination.

Due to the special ability of wave manipulation, elastic wave metamaterials have many interesting applications in mechanical engineering, vibration and sound isolation. This work proposes a movable metamaterial driven by nonlinear elastic waves with stop band and nonreciprocal crawling. Inspired by stop band properties, the metamaterial employs a periodic structure of nonlinear diatomic chain. A defect mass is introduced near the wave source to break the periodicity and symmetry, which leads to the nonreciprocal propagation of nonlinear elastic waves. With the asymmetric friction, the nonreciprocal and one-directional crawling of the metamaterial can be observed, which behaves as a movable mechanical diode. Furthermore, theoretical analysis is performed to derive the band structure and crawling velocity, which are supported by experiments. This research wishes to introduce nonlinear elastic wave metamaterials into new designs of robotics.

A new model for electro-elastic Bernoulli–Euler beams of centrosymmetric cubic materials is proposed, which incorporates microstructure and flexoelectric effects. The wave equations and boundary conditions are derived simultaneously through a variational approach based on Hamilton’s principle. The new beam model is then applied to predict elastic wave band gaps in a periodic electro-elastic composite beam structure. Bloch’s theorem and the transfer matrix method for periodic structures are used to solve the wave equations and determine band gaps. The current model reduces to its flexoelectric and classical elastic counterparts as special cases. To illustrate the new model, the effects of microstructure, flexoelectricity, beam thickness, unit cell length and volume fraction on band gaps are investigated through a parametric study. The numerical results show that the microstructure and flexoelectric effects lead to increased band gap frequencies, and these two effects are important when the beam thickness is at the submicron and micron scales. In addition, it is found that the unit cell length and volume fraction can significantly affect the band gap size at all length scales. These findings indicate that band gap frequencies and size can be tailored by adjusting the microstructural and material parameters.

The author introduces the technical principles of elastic wave CT method and seismic imaging method, taking the detection results of a dangerous road in southern Shaanxi as a research example. The application effect of comprehensive geophysical methods in solving engineering exploration problems was analyzed and studied, and the two methods were mutually verified, improving the accuracy of interpretation. The results show that the elastic wave CT method and seismic imaging have high recognition and resolution ability in the detection of dangerous roads in goaf collapse areas, and are effective technical methods to solve such geophysical exploration problems.

We report the first experiments, where simultaneous electrical resistivity and elastic wave velocity measurements are acquired during the triaxial deformation of granite under brine-saturated conditions. Both the resistivity and elastic wave velocity increase slightly during the early stage of deformation owing to crack closure, and then decrease systematically owing to crack development as the sample approaches failure. We observe a complex relationship among the resistivity, elastic wave velocity, and porosity during deformation that is likely attributed to their different sensitivities to crack orientation, tortuosity, and connectivity. The electrical resistivity changes tend to decline as the sample approaches failure owing to the nearly complete crack connectivity, whereas the elastic wave velocities continue to decrease. These characteristic changes in resistivity and velocity at the discrete stages of deformation may provide a clue to understanding structural changes in crystalline basements that are related to crack development and fluid infiltration.

For effective maintenance and failure prevention of mine tailing in geo-resource field, it is essential to assess porosity in both partially and fully-saturated tailing dams. This study aims to evaluate sand-hematite mixtures’ porosity, considering various hematite content (HC) and saturation levels, using electrical resistivity and elastic wave velocity as geophysical methods. Hematite powder is mixed with sand particles in a weight ratio of 0 to 30%. The experimental setup includes a specially designed cell equipped with a bender element (BE), piezo disk element (PDE), electrical resistivity probe (ERP), and time domain reflectometry (TDR) probe at the top and bottom. Additionally, a porous stone disk connected to a silicon tube regulates the water level in the cell. Test results reveal significant variations in dielectric constant and electrical resistivity within the water level range of 0.250 m to 0.200 m. Elastic waves show changes at the water level of 0.200 m due to hematite and capillary effects. For porosity evaluation, the electrical resistivity-based method proves more reliable than the elastic wave-based method, considering error norms influenced by various factors. This integrated experimental framework provides an effective tool for assessing porosity in tailing materials, contributing to enhanced geotechnical monitoring and sustainable resource management.

A wind-wave generation model over an ice-covered sea is proposed. The wind velocity over the ice upper surface is decomposed into the mean velocity profile of the boundary-layer flow and small perturbations, while the ice cover is modelled as a viscoelastic layer, with the water part modelled as an inviscid fluid. The present model is based on two-dimensional linear flow-instability theory, with no-slip boundary conditions at the air–ice interface, and both normal and shear stress boundary conditions matched on the air–ice interface. It is shown that the model converges to the field and experimental data for open-water cases. The ice elasticity is found to be the critical factor for generating wind waves, and the generation of flexural-gravity waves and elastic waves in ice is analyzed.

Elastic metamaterials with unusual elastic properties offer unprecedented ways to modulate the polarization and propagation of elastic waves. However, most of them rely on the resonant structural components, and thus are frequency-dependent and unchangeable. Here, we present a reconfigurable 2D mechanism-based metamaterial which possesses transformable and frequency-independent elastic properties. Based on the proposed mechanism-based metamaterial, interesting functionalities, such as ternary-coded elastic wave polarizer and programmable refraction, are demonstrated. Particularly, unique ternary-coded polarizers, with 1-trit polarization filtering and 2-trit polarization separating of longitudinal and transverse waves, are first achieved. Then, the strong anisotropy of the proposed metamaterial is harnessed to realize positive-negative bi-refraction, only-positive refraction, and only-negative refraction. Finally, the wave functions with detailed microstructures are numerically verified.