Anomalous Regulation Of Elastic Waves In Solids Based On Composed Plates/Beams

The realization and application of anomalous regulation of electromagnetic wave and optical wave bring about great breakthrough in science and technology.The corresponding anomalous manipulation has become a hot spot in other fields(such as acoustic field).However,due to the problems of modal transition in the interface of materials and structure,the application of traditional wave control methods in elastic wave manipulation is difficult.Nowadays,the research on singular control of elastic waves is still lacking.As a mature engineering element,thin plate has great potential in the design of elastic wave metasurface.Therefore,the design of elastic wave anomalous regulation method based on thin plate is easy to be realized,which is beneficial to the wide application such as filter,unidirectional transmission devices(elastic wave diodes),energy control,sensors and so on of elastic wave in solid.In this thesis,the metasurface design of elastic wave filtering,refraction,focusing and asymmetric transmission in solid are realized based on thin plate structure and it is verified by full numerical simulation.The details are as follows:1.For the elastic SV waves in metals,a high-quality narrow passband filter that consists of aligned parallel thin plates with small gaps is designed.In order to obtain a good performance,the thin plates should be constituted by materials with a smaller mass density and Young's modulus,such as polymethylmethacrylate(PMMA),compared to the embedded materials in which the elastic SV waves propagate.Both the theoretical model and the full numerical simulation show that the transmission spectrum of the designed filter demonstrates several peaks with flawless transmission within [0-20]KHz frequency range.The peaks can be readily tuned by manipulating the geometrical parameters of the plates.Therefore,the current design works well for both low and high frequencies with a controllable size.Even for low frequencies on the order of kilohertz,the size of this filter can be still limited to the order of centimeters,which significantly benefits the real applications.The investigation also finds that the same filter is valid when using different metals and the reason behind this is explained theoretically.Additionally,the effect of bonding conditions of interfaces between thin plates and the base material is investigated using a spring model.2.We report a novel approach to control the wave fronts of SV-waves in solids using metasurfaces constituted by a stacked array of composite plates,which are composed of two connecting parts made of different materials.The metasurfaces are connected at two ends to the half-space solids where the elastic SV-waves propagate.The incident SV-waves in the left half-space solid induce flexural waves in the composite plates,and subsequently are converted back to SV-waves in the right half-space solid.The time delay of flexural waves in each composite plate of the metasurfaces is tuned through the varying length of the two connecting components.To quantitatively evaluate the time delay in each composite plate,a theoretical model for analyzing the phase of the transmitted SV-waves is developed based on the Mindlin plate theory.To control the SV-waves at will,each composite plate in the metasurface is delicately designed according to the proposed theoretical model.For illustration purposes,two metasurfaces are designed and numerically validated.3.We report a novel approach to control flexural waves in thin plates using metasurfaces constituted of an array of parallel arranged composite beams with their neutral planes the same as that of the host plate.The composite beams are composed of two connecting parts made of different materials,and have a thickness identical to that of the host plate.To steer flexural waves in thin plates,a rectangular zone is subtracted from the thin plate and is then filled with the designed metasurface.The time delay of flexural waves in each composite beam of the metasurface is tuned through the varying length of the two connecting components,while keeping the total length fixed.To quantitatively evaluate the time delay in each composite beam,a theoretical model for analyzing the phase of the transmitted flexural waves is developed based on both Mindlin plate theory and Timoshenko beam theory.To control the flexural waves at will,each composite beam in the metasurface is delicately designed according to the proposed theoretical model.For illustrative purposes,the refracted and focusing metasurfaces are designed and numerically validated.4.In this thesis,by combining the metasurface above and phononic crystal,we propose an asymmetric transmission structure(ATS)for elastic shear vertical(SV)waves in solids,which has been relatively unexplored.The ATS is constituted by a metasurface and a phononic crystal(PC)possessing a directional band gap.While the metasurface aims to redirect the incident wave,the PC acts as a directional filter.The metasurface is composed of a stacked array of composite plates with two connecting parts made of different materials.To examine the performance of the designed ATS,full numerical simulations have been conducted.The numerical results indicate that the proposed ATS offered a relatively broad working frequency band and had a one order of magnitude difference in terms of transmission between the positive and negative incidences.Our study provides an alternative method to control elastic SV waves and could benefit applications in various fields,such as Micro-Electro-Mechanical System(MEMS),in which thin plates are frequently used components.