Study On The Transmission Of Surface Acoustic Wave In Acoustic Metamaterials

In recent years, since the development of natural materials tends towards to a bottleneck, the research on metamaterials has attracted much attention. Generally speaking, due to their special micro-structures, metamaterials exhibit properties not found in naturally occurring materials or compounds, which open a new avenues of the modern material science. As a part, acoustic metamaterial is a composite or structured material that consists of two or more components with much different elastic properties. Its acoustic transmission characteristics mainly depend on the properties of the special micro-structures rather than physical properties of materials, which introduces many novel phenomena, such as acoustic band gaps, negative refraction, etc. These lead to a great advance in controlling the propagation of the elastic wave and have great value for academic study and application, for instance, high quality acoustic filters, vibration isolators in the high precise measurement systems, etc.Elastic properties of materials imply much important information such as mass densities, homogeneities, micro-structures, etc. Being general surface acoustic waves, Rayleigh wave and Lamb wave have strong potential applications on noncontact and nondestructive evaluation due to its superiority in speed, accuracy and efficiency, especially in the detection of alloy blocks and plates. In addition, the propagation of surface acoustic waves evolves sensitively along to the changes of interface, e.g. tiny stress, temperature, surface load. There has been a tremendous development of surface acoustic waves for microsensor and micromanipulation. These microsensors with being small, high accuracy, wide suitability have a good outlook in the field of real time measurement and high resolution remote sensing.This dissertation focuses on the study of elastic waves propagating in acoustic metameterials, which mainly contains five parts:the influence of artificial structures on the transmission properties, the physical mechanism of couple effect on periodic lattices in one-dimensional super phononic crystals, the transmit power spectra of sandwich-structured phononic crystal plates and the band gaps in two-dimensional phononic crystal plates. The dissertation is described briefly as follows:In chapter one, the related theoretical and experimental background are reviewed briefly, including contents, methods and progress of the research.In chapter two, we introduce the thermoelastic mechanism and solution procedure of the FEM in one-layered isotropic medium. On the basis of the theory, the travelling of Rayleigh waves on the aluminum block with designed periodical structures is simulated numerically. It is found that the cutoff frequency is controlled by the depth of surface structures. We design three kinds of the periodical structures as low-pass, high-pass and band-pass elastic wave filters and discuss the effect of structural parameters of artificial structures on filter performance.In chapter three, the travelling properties of Rayleigh waves propagating on the samples with surface defects are studied in use of laser ultrasonic nondestructive detection system. The results in the time and frequency domain indicate that the surface defects work as the frequency-division devices in the potent range. The bandwidth of reflecting signal varies directly with the depth of the surface defects? and the width of the surface defects alter the energy distribution of the reflecting signal, which conforms the conclusion of numerical results in chapter two.In chapter four, we study theoretically the transmission properties of Lamb waves in super lattice of one-dimensional sandwich-structured phononic crystal which comprised by periodic structure layers bilaterally deposited on both sides of a core layer. Through the calculations of band structures from supercell eigenvalue problems, the propagating Lamb waves are strongly modified by the core layer. The influence of the core layer embodies by two aspects:1) the core layer destroys the periodicity of the super lattice and weaken the Bragg scattering, which brings the narrowing of the band gaps; 2) the core layer leads to Lamb modes combination and reconfiguration supported by the sandwich-structured super lattice.In chapter five, the thermoelastic mechanism and the equations of the numerical calculation by the FEM in multi-layered anisotropic medium are introduced. With the finite element simulations, we investigate the propagation of Lamb waves in a sandwich-structured periodic plate. The transmit power spectra shows that the material and thickness of the sandwiched layer have great influence on the location and the bandwidth of the absolute forbidden bandgaps, which agree well with the theoretical results in chapter four.In chapter six, we make use of the laser ultrasonic nondestructive detection system to detect the propagating Lamb waves in two-dimension phononic crystal plates. The band structures are theoretically calculated in the same time. The transmit power spectra detected in the experiments and dispersion curves present band gaps in two-dimension phononic crystal plates, and the periodical arrangement of scatterers has much influence on the location of band gaps.The last chapter presents the conclusions of this dissertation, and prospects for future work.In summary, we study on the transmission properties of some acoustic metamaterials by the theoretical, numerical and experimental methods. The micro-structures are capable of controlling the propagation of elastic waves, which has significant value in theory and practice.