Experiments on cloaking in optics, thermodynamics and mechanics.

In this study, we design periodic grille structures on a single homogenous thin plate to achieve anisotropic acoustic metamaterials that can control flexural waves. The metamaterials can achieve the bending control of flexural waves in a thin plate at will by designing only one dimension in the thickness direction, which makes it easier to use this metamaterial to design transformation acoustic devices. The numerical simulation results show that the metamaterials can accurately control the bending waves over a wide frequency range. The experimental results verify the validity of the theoretical analysis. This research provides a more practical theoretical method of controlling flexural waves in thin-plate structures.

In this paper, the linear transformation method (LTM) to control flexural waves propagating in thin plates is presented. Unlike earlier studies, only a small number of homogeneous materials with no requirement of in-plane forces or pre-stress are needed, which tremendously simplifies the implementation of devices for flexural waves. An invisibility cloak with homogeneous materials is studied to confirm the validity of the present approach, and to show its imperfection due to impedance mismatch at interfaces. Required materials can be further simplified as layered isotropic materials using the effective medium theory. Finally, the LTM can be extended to the case of flexural waves propagating in anisotropic thin plates. The present method opens a promising avenue toward the realization of advanced structured shields and other devices.

Ribbed elastic metasurface with lateral scalability for flexural wave manipulation

Achromatic ribbed elastic meta-structure for ultra-broadband flexural wave manipulation

Compact functional elastic waveguides based on confined mode

We designed and analyzed a piezo-lens to focus flexural waves in thin plates. The piezo-lens comprises a host plate and piezoelectric patches bonded on the surfaces of the plate. The piezoelectric patches are shunted with negative capacitance circuits. The effective refractive indexes inside the piezo-lens are designed to fit a hyperbolic secant distribution by tuning the negative capacitance values. A homogenized model of piezo-mechanical system is adopted in the designing process of the piezo-lens. The wave focusing effect is studied by the finite element method. Numerical results show that the piezo-lens can focus flexural waves by bending their trajectories, and is effective in a large frequency band. The piezo-lens has the ability to focus flexural waves at different locations by tuning the shunting negative capacitance values. Potential applications of the piezo-lens include energy harvesting, SHM, etc.

A new wave confinement effect is predicted in graded crystals with a concave slownesssurface under conditions of growth of the phase velocity with decreasing distance from thewaveguide axis. This finding overturns the common notion about the guiding and‘antiguiding’ profiles of wave velocity in inhomogeneous media. The waveguide effect foundis elucidated by means of ray analysis and particular exact wave solutions. The exactsolution obtained for localized flexural waves in thin plates of graded cubic and tetragonalcrystals confirms the predicted effect. Since this solution is substantially different withrespect to the existence conditions from all others yet reported, and it cannot be deducedfrom the previously known results, the predicted waves can be classified as a new type ofwaveguide mode in graded anisotropic media. Although the concrete calculations aregiven in the article for acoustic waves, its general predictions are expected to bevalid for waves of various natures, including spin, plasma, and optical waves.

Mixing of ultrasonic Lamb waves in thin plates with quadratic nonlinearity

In investigating small displacements and the resulting state of stress in thin plates or shells obeying a linear stress-strain law, the three-dimensional problem is reduced to a two-dimensional one by means of a hypothesis concerning the distribution of the relevant geometric and mechanical quantities along the thickness of the shell or plate. The simplest hypothesis in bending is the Kirchhoff hypothesis, while for the motion “in the xy plane” it is usually assumed that a plane state of stress prevails. However, in the case of high frequencies and short wave lengths, particularly in the case of anelastic materials the equations of which contain higher time derivatives, the above assumptions are incorrect and it is necessary to establish a system of equations taking into account additional phenomena, such as rotary inertia, shear displacement, etc. This is done by changing the hypothesis or employing a method of successive approximations.

Quantum revival is described as the time-periodic reconstruction of a wave packet initially localized in space and time. This effect is expected in finite-size systems which exhibit commensurable discrete spectrum such as the infinite quantum well. Here, we report on the experimental observation of full and fractional quantum revival for classical waves in a two dimensional cavity. We consider flexural waves propagating in thin plates, as their quadratic dispersion at low frequencies mimics the dispersion relation of quantum systems governed by Schrödinger equation. Time-dependent excitation and measurement are performed at ultrasonic frequencies and reveal a periodic reconstruction of the initial elastic wave packet.

This paper is concerned with the influence of a viscous liquid on the propagation of acoustic waves in thin plates of lithium niobate. The characteristics of the three lowest order wave modes that such a plate can support (zeroth order antisymmetric Lamb wave A/sub 0/, symmetric Lamb wave S/sub 6/, and quasi-shear-horizontal wave QSH/sub 6/) are investigated. It is assumed that the liquid is isotropic such that its viscous properties are described by two independent components of viscosity tensor /spl eta//sub 11/ and /spl eta//sub 44/. It is found that the attenuation of the waves depends primarily on the shear component of viscosity /spl eta//sub 44/. The influence of expansion component /spl eta//sub 11/ is negligible. The attenuation per unit length is proportional to (/spl eta//sub 44/)/sup 1/2 / and f/sup 1,3/, where f is the frequency of the wave. Under identical conditions, the lowest value of attenuation is for the S/sub 0/ mode, while the highest is for the A/sub 0/ mode. For example, for h//spl lambda/=0.025 (h=plate thickness, /spl lambda/=acoustic wavelength) and /spl eta//sub 44/=0.003 Ns/m/sup 2/, the attenuation in dB/cm at a frequency of 1 MHz for A/sub 0/, S/sub 0/ and QSH/sub 0/ modes is 13.5, 0.024, and 0.073, respectively. The above results indicate that the S/sub 0/ wave is most suitable for use in devices operating in contact with a viscous liquid. The results also show that by using the three different wave modes, one can develop a viscosity meter with a very wide measurable range of viscosity.

A feasible approach is proposed in this work to realize an acoustic omnidirectional invisible lens for flexural waves in thin elastic plates. These devices require extreme values of the refractive index close to its center, and in this work this drawback is overcome by reducing gradually the thickness of the elastic plate, which has the same effect as increasing proportionally the refractive index. Although ideally the thickness of the plate should be zero at the central point, it is shown that a finite but small minimum thickness is enough to provide a good invisibility effect. Numerical simulations are performed by the finite element method, which supports the theoretical predictions, showing that an invisible lens is feasible for flexural waves.

A pulsed laser has been used to generate ultrasonic transients in samples of metal and fiber-reinforced polymer composite material. These have been detected using an air-coupled piezoelectric transducer. It is demonstrated that such a transduction system can be used for longitudinal waves in bulk material, Rayleigh waves at solid surfaces and Lamb waves in thin plates.

We have developed a method for detecting acoustic fields in solids irradiated with dense electron beams. The method is based on laser Michelson interferometry. The electron source is a high-current DZhIN electron accelerator. The detection system features a short baseline Michelson interferometer located inside the experimental chamber with the sample, a stabilized initial beam pathlength difference within the interferometer, high temporal resolution, an analog-to-digital converter with output to a personal computer, and a program for reducing the interferometer data. We can measure both long pulses with minimum displacements of 10−10 m and durations of 10−8 sec, and flexure waves with large-amplitude displacements of 10−5 m and oscillation periods of 10−3 sec. We present results from studies of flexure waves in thin plates and rods of copper, silicon, alkali-halide crystals, quartz glasses, and D16T aluminum alloy irradiated by nanosecond high-density electron beams.

As a new kind of elastic materials, elastic wave metasurface has great research significance in the field of elastic wave regulation. However, most of the researches on elastic wave metasurface are guided by traditional Generalized Snell’s Law (GSL), and the effect of higher order diffraction waves caused by structural periodicity is not considered. Under the action of higher order diffraction wave, the incident wave will produce more complex transmission phenomenon when passing through the metasurface, and the angle of transmission does not conform to GSL in some cases. In order to verify whether the modified GSL theory considering the higher-order diffraction term is still applicable to the regulation of solid elastic waves, this paper designs a helical metasurface based on the elastic wave theory of plate-beam structure, which is composed of helical lines of different lengths, and uses this structure to explain the complex transmission phenomenon of elastic wave metasurface. Finally, the asymmetric transmission, modal separation and waveguide of Lamb waves in thin plates are realized by combining the theory with structural design, which proves that the structure has great application potential in ultrasonic detection and other fields.