The Supernormal Manipulation Of Elastic Waves And Its Application Design By Artificial Compound Structures

In recent decades,artificial composite structures have received a great attention due to the novel acoustic characteristics beyond nature materials.The early study of artificial composite structures concentrated on Sonic Crystals(SCs),which can realize the supernormal regulation of sonic waves or elastic waves.It is convenient to analyze the acoustic characteristics of SCs by studying band structures.Hence,many novel structures with different functions have been made by modulating band structures.With the further research of SCs,Sonic Metamaterials(SMs)are developed by local resonant SCs become research focus rapidly.Compared with the traditional SCs,SMs can be able to control the waves with sub-wavelength.Nevertheless,the properties of SCs and SMs can't be modified easily once they are designed and manufactured that seriously hinder the development of corresponding design.The acoustic properties of piezoelectric composite structures can be modified by the external shunt circuit.Therefore,the acoustic properties of piezoelectric composite structures have a great tunability,and corresponding works have gradually emerged in recent years.In this paper,one functional material and three application structures/devices are designed to realize the supernormal regulation or measurement for elastic waves.Including triple-negative SMs,transmission structure for unidirectional elastic waves based on mode converting of SCs waveguides,elastic waves polarization-sensitive device based on unidirectional transmission structures and tunable sonic impedance matching layers based on piezoelectric shunt circuit.In the second chapter,we designed a triple-negative SMs that possesses multiple local resonances.At the same frequency range,the SMs possess simultaneously negative effective mass density,bulk modulus and shear modulus induced by dipolar,monopolar and quadrupolar resonances,respectively.Therefore,two negative bands arise,one is longitudinal wave,another is transverse wave.Subsequently,the negative refraction simulation test was performed to demonstrate the negative bands.The results show that both transverse and longitudinal waves have a negative refraction at the same frequency.That is to say,at the same time,SMs have the ability to control two types of elastic waves.We also note that when the longitudinal wave is incident,the outgoing wave only has a transverse wave.So the SMs can also be used to design a device for converting elastic waves between transverse and longitudinal waves.In the third chapter,we proposed an unidirectional elastic waves transmission structure.Based on this structure,a polarization-sensitive device of elastic waves was designed.The unidirectional transmission structure is composed of two mutually perpendicular SCs waveguides connected by a resonant cavity.We demonstrate that when symmetrical waves incident into the structure from input waveguide,the output waves in output waveguide are converted into asymmetrical modes on the basis of the mode matching mechanism between waveguides and the cavity.However,when symmetrical waves incident into the structure from output waveguide,no output waves are transmitted into input waveguide due to the mismatch of modes in waveguides and the cavity.Based on unidirectional transmission structure,we designed a polarization-sensitive structure to measure the polarizing angle of hybrid elastic plane waves.This polarization-sensitive structure may have a potential application in elastic wave detection.The unidirectional transmission and mode conversion mechanism may also be applied to design an elastic wave diode for acoustic integration devices and a convertor of the elastic wave waveguide modes.In the fourth chapter,we designed a tunable gradient sonic impedance matching layers,which consists of ten layers of piezoelectric ceramic plates.Each of plates is connected with different capacitor to adjust their impedance.Firstly,we theoretically confirmed that the longitudinal vibration of piezoelectric ceramic could be affected by the shunt circuit.And then the dispersion relationship of periodic piezoelectric ceramic plates was deduced.Results indicated that,in the long wavelength limit,the capacitance can affect the effective velocity of longitudinal wave.Besides,we find that negative capacitance can affect the effective velocity in a greater extent.Then we achieved negative capacitance theoretically and experimentally.Finally,a simulation experiment was performed.The gradient sonic impedance matching layers located between steel rod and aluminum rod,and longitudinal wave incidents into steel and outputs from aluminum.Results show that the flow transmittance is only about 77% when matching layers was absent;When the sonic impedance of matching layers is distributed by “gradient 1”,a stable and high transmission rate(close to 100%)can be obtained between 50 kHz and 90 kHz;When the sonic impedance is distributed according to the “ gradient 3”,the ideal transmission rate(more than 95%)has been achieved between 40 kHz and 150 kHz.This tunable sonic impedance matching layers has potential applications in transducer.