![]() pointed out that the parasitic reflections always appeared at metasurfaces designed based on the GSL, and they proposed an equivalent impedance matrix model to eliminate the parasitic reflections and improve the power efficiency of the redirected waves. The latter are also called as the parasitic reflections in some studies. The EMs mentioned above are designed based on the generalized Snell's law (GSL), which can only predict the propagation of the fundamental diffraction mode but not the high order ones. These are just what we consider and discuss in this work. Besides, the existing TEMs rarely involve the boundary effect that must be considered in practical application scenarios, and in their design the force and moment interface impedances of flexural waves are not considered and clearly expounded up to now. However, the existing TEMs usually have complicated structures or poor stabilities caused by external coupling effects like the electromechanical coupling, ,, , or fluid-structure interaction, which brings challenges in engineering applications. were proposed, realizing the tunable transmission, wave propagation directivity, , and wave filtering. ![]() To gain the broadband and tunable functions, tunable EMs (TEMs) composed of fish-bone-like units, Terfenol-D pillars, piezoelectric patches shunted with circuits, ,, ,, sensing-and-actuating units, etc. Generally, only a fixed manipulation function can be realized by a passive EM, and the working frequency band is quite narrow. Based on the proposed EMs, various manipulation functions such as abnormal refraction or reflection, focusing, asymmetric transmission and source illusion were realized. With the deepening of studies, amplitude-shift, topology enhanced, ,, ,, and transmodal, , EMs were also developed. In recent years, various EMs consisting of subunits constructed by zigzags, ,, ,, , pillars, ,, ,, , resonators, ,, ,, , ], beams with gradient thicknesses, , and lengths, or composite materials, etc. The proposed IAS possesses a flexible and reliable layout for TEM design and has potential engineering applications in vibration control, health monitoring, etc.Īn elastic metasurface (EM) is a kind of metamaterial with subwavelength thicknesse, which can abnormally manipulates the propogations of elastic waves. Its accuracy and the modulation function adjustability of the TEM are demonstrated by the finite element (FE) simulations and experiments. ![]() By using the diffraction theory and coupled-mode theory, an analytical model is established for the metasurface subjected to a Gaussian flexural beam incidence, based on which, the total wavefield consisting of all propagating diffraction orders is solved. The boundary effect is considered, and the difference between the impedances of the flexural and acoustic waves is demonstrated for the subunits with free and clamped boundary conditions. Based on the impedance theory, the adjustable mechanism of the phase shift of the IAS is clarified according to the derivations for the relationship between the force/moment interface impedances and complex reflection coefficients. Different from the common tunable elastic metasurface (TEM), like piezoelectric active metasurface, in this paper, we propose a TEM composed of impedance-adjustable subunits (IASs) to realize tunable manipulations of flexural waves. ![]()
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