Study On Asymmetric Acoustic Transmissions In Acoustic Metamaterials

The dissertation is mainly devoted to study on asymmetric acoustic transmissions in acoustic metamaterials. The main contents of the dissertation are divided into four parts:(1). The basic theory of asymmetric acoustic transmissions induced by asymmetric plate-like structures;(2). Study on asymmetric acoustic transmissions in a single layer of asymmetric plate-like structures;(3). Study on enhancement of asymmetric acoustic transmissions in multi-layered asymmetric composite structures;(4). Study on control of asymmetric acoustic transmissions. The main contents are described briefly as follows:1. The basic theory of asymmetric acoustic transmissions induced by asymmetric plate-like structuresIn the second chapter, we mainly introduce the basic theory of asymmetric acoustic transmission (AAT) in an asymmetric plate-like structure immersed in a fluid, which is composed of a one-dimensional periodical grating structure and solid plates. First, according to the basic equations of the acoustic grating diffraction, the diffractive characteristics of acoustic waves through a one-dimensional periodical grating immersed in the fluid are calculated. In addition, based on the wave equations of the elastic wave, the general form and the potential function form of the fluid-solid coupling equations and boundary conditions are obtained, and the potential function form of the fluid-solid coupling equations and boundary conditions are transformed by using integral transform method. Therefore, the dispersive equations of the Lamb wave in the solid plate immersed in the fluid are derived, and the dispersive curve and leaky angle curve of the Lamb wave are calculated. The studies provide the theoretical bases for studying the physical mechanisms of the AAT in the asymmetric plate-like structures. 2. Study on asymmetric acoustic transmissions in a single layer of an asymmetric plate-like structureIn the third chapter, the AAT in a single layer of an asymmetric plate-like structure immersed in water, and the physical mechanisms of the AAT are studied. The asymmetric plate-like structure is composed of two different surfaces, in which one surface is a periodical rectangular grating, and the other side is a smooth surface. As the acoustic wave is incident from the side of grating surface, the acoustic wave is severely diffracted when it is reaching the grating surface, and the diffracted waves induce stronger anti-symmetric Lamb modes. However, as the acoustic wave is incident from the side of the smooth surface, most of the acoustic wave is reflected by the smooth surface. The anti-symmetric Lamb mode is also excited in the second case, but the excitation is very weak. Therefore, the AAT originates from the enormously asymmetric excitation of the anti-symmetric Lamb mode in the asymmetric plate-like structure. The numerical simulations show that in the frequency bands where the AAT appeared, the relative transmittance can be up to0.9, which indicates that the asymmetric plate-like structure has good performance. Meanwhile, we also experimentally study the AAT of the asymmetric plate structure, and the results of the experimental measurements agree well with the numerical simulations.In addition, we investigate the influence of different structure parameters on the AAT. It shows that as all of the structure parameters are changed with the same ratio simultaneously, the frequency bands of the AAT are translated, but the transmittance, bandwidth, and transmission spectrum shapes keep constant basically. It is indicated that the frequency ranges of the AAT can be systematically controlled. The structure has the advantages of broader bandwidth, tunable frequency range, and simple structure as well as being easy to be achieved.3. Study on enhancement of asymmetric acoustic transmissions in multi-layered asymmetric composite structuresIn the fourth chapter, we study an enhanced AAT through a multi-layered asymmetric composite structure consisted of a periodical grating made of cylinders and accompanied by multi-layered plates immersed in water, and also investigate the physical mechanisms of the enhanced AAT in detail. Compared the AAT with other composite structures, the acoustic rectifying ratio and transmitted acoustic energy flux in this structure are enhanced obviously, and the transmitted acoustic energy flux is only divided into two branches. The results show that the transmittance and rectifying ratio of the composite structure can be as high as0.7and103respectively. Even higher rectifying ratio can be obtained with more plates, but the number of the plates has less influence on the transmitted acoustic energy flux. The transmitted acoustic energy flux arises from the leaky A0mode in the multi-layered plates immersed in the water which is excited by both the0-and±1-orders diffracted waves. The enhanced AAT originates from the asymmetric excitation of the leaky A0mode in the composite structure.In addition, we introduce two inclined plates as mirrors placed at the left side of the grating to optimize the composite structure. Both reflected diffraction energy beams are reflected again by both inclined plates and transmit through the multi-layered plates. The AAT are further enhanced in the optimized composite structure, and the transmittance and rectifying ratio of the optimized structure are up to0.97and104respectively. Meanwhile, we also experimentally measured the transmittance spectra in both structures, and the experimental measurements agree well with the numerical results. The enhanced AAT in the composite structure has also the advantages of broader bandwidth, more high efficiency, and low energy loss.4. Study on control of asymmetric acoustic transmissionsIn the fifth chapter, we investigate the control of the AAT in the asymmetric composite structure consisted of a periodical grating accompanied by two layers of identical plates immersed in water. Based on the method of the geometrical adjustment, the influences of the structural parameters on the AAT are studied by changing the structural parameters of the composite structure successively and separately. The results show that the frequency band of the AAT is related to the lattice constant (a) of the grating and plate thickness (h). The frequency band of the AAT greatly shifts to the low frequency range with the increase of a. However, the frequency band of the AAT slowly moves to the high frequency range with the increase of h.Based on these results, we study the control of the frequency band, transmittance, and the direction of the transmitted acoustic energy flux. The variation of α have a great influence on the frequency band range of the AAT, however, the transmittance is reduced obviously if a is great changed from its optimized value. Thus, we investigate the frequency band range and transmittance simultaneously by adjusting the combination of a and another parameter. The results show that the suitable change of the combination of a and h may increase the transmittance in the band moved to the high or low frequency range, especially to the high frequency range. The change of the combination of a and the diameter of the cylinder (d) increases the transmittance in the band moved to the low frequency range. The variation of the combination of a and the distance between the grating and the plate (b) enhances the transmittance in the band moved to the high frequency range. The frequency band the AAT is translated with the variation of all of the structure parameters, but the frequency band and transmittance remain unchanged basically.The propagation angle of the transmitted acoustic energy flux is the same as that of the leaky angle of the Ao mode in the frequency band of the AAT. However, the frequency band of the AAT is related to the parameters a and h. and the angle curves of the leaky Ao mode and the±1-orders diffracted waves are shifted with the variations of a and h, which cause the change of the propagation angle of the transmitted acoustic energy flux. Therefore, we can control the propagation angle of the transmitted acoustic energy flux by modifying the parameters a and/or h. In this chapter, we study the control mechanisms of the frequency range, transmittance, and the direction of the transmitted acoustic energy flux of the AAT. The results provide the theoretical basis for designing tunable unidirectional acoustic devices, which has important scientific significance and applied potential.Finally, an appendix is attached to describe the studies on laser-generated ultrasonic waves in viscoelastic materials and also its applications to angled crack detections. We mainly study the laser-generated ultrasonic waves in a single thin plate, a single thick plate, and a coating/substrate structure made of the viscoelastic materials. Moreover, the detection mechanisms of angled cracks with laser-generated Rayleigh waves are studied, and the position, length, and orientation angle of the angled crack is detected. Furthermore, the influences of the crack width and the material viscoelasticity on the Rayleigh wave propagations and crack detections are also obtained. The studies provide the theoretical basis for evaluating mechanical properties and nondestructive detecting cracks in different types of viscoelastic structures.