Research On Excitation Mechanism And Propagation Manipulation In Periodic Structures Of Elastic Guided Waves

Elastic wave is a kind of wave that propagates in an elastic medium.The energy coupling and mode conversion caused by the propagation of elastic guided waves between different media have important meanings and effects in the fields of geoacoustic communication,earthquake early warning and metal nondestructive testing.As the carrier of geoacoustic signals,Scholte waves can carry various information of ocean and seabed media in the elastic seabed medium when propagating on the offshore seabed.However,in the practical application,the excitation and long-distance propagation of the elastic guided waves in complex media still face many theoretical and practical problems.By designing and constructing artificial periodic structures to intervene the coupling and propagation of guided wave modes,it is helpful for people to regulate the spectral band structure of elastic waves,so as to obtain and analyze marine information in offshore environment.Aimed at the excitation of the elastic guided waves in a solid plate from the underwater source,the model of the aluminum plate immersed in water was established,and it was studied how to use the elastic guided waves to transmit information.Periodic structures can affect the propagation properties of the elastic guided waves producing the band gap,defect state,and other physical phenomena,as to realize the information communication.This thesis has researched the elastic wave modes utilizing various research tools including experimental platforms such as laser ultrasonic system and underwater ultrasonic system,finite element method and relevant guided wave propagation theory.Furthermore,various types of wave manipulation structures have been designed to extract and control the guided waves purposefully,including surface acoustic waveguide,elastic waveguide,underwater phononic crystal structure,and so on.Firstly,in this thesis,the excitation mechanism of the elastic guided wave modes from the underwater sound source under the condition of liquid-solid plate-liquid media was studied.The underwater ultrasonic system and laser vibrometer system were utilized to carry out the experimental research on the elastic guided wave excitation of the water-immersed plates.The finite element method was used to establish an acoustic-structure coupling simulation model to calibrate the Lamb wave modes excited in the elastic plate under different incident angles.It was showed that the underwater sound source couldn’t excite the guided wave mode propagating along the aluminum plate when it is vertically incident on the aluminum plate,according to the experimental and simulated results.But when the incident angle is greater than the critical angle of the longitudinal wave,the stable A1 mode and S0 mode could be excited.The above findings provided theoretical support and experimental verification for the excitation and extraction of the elastic guided wave modes under the condition of fluid-solid layer-fluid media.On the basis that the elastic guided waves can be excited and propagate in the layer structure,the influencing factors of the band strucure of the periodically corrugated elastic waveguides were theoretically analyzed.The shape facter has been proposed to quantify the relationship between the forbidden band width and the undulation geometry.A conlusion was obtained theoretically that the band width is proportional to the shape factor and relative corrugation parameter.Furthermore,a single defect basing on the connection phase mismatching has been introduced between two waveguides.The simulation results revealed that the single-defect waveguide has special spectral characteristic and spacial localization.It occurred two kinds of modes: symmetric modes and antisymmetric modes.And then,we focused on the excitation and propagation manipulation of the surface acoustic waves on the metal surface.Combining the finite unit method simulations and laser ultrasonic experiment system,the phenomenon of the Bragg band and non-Bragg band of the surface acoustic wave caused by the periodically corrugated waveguide profile has been studied.It was shown that the waveguide with the symmetrical upper and lower surface periodic structure leads to the Bragg band,while the antisymmetric structure leads to the non-Bragg band.The manipulation effect of the waveguide structure proposed above on the elastic guided waves is beneficial to the design and realization of the functional wave control devices.Finally,the propagation and manipulation of the guided waves in the underwater phononic crystal structure with defect was researched.A basic hexagonal two-dimensional array of phnonic crystal structure was constructed based on the phononic crystal theory.Then,the periodic cavity array has been introduced between two phononic crystals with opposite topological properties.The results showed that a forbidden band appeared at some frequency,and a defect mode corresponding to the peak near the lower edge of the forbidden band was generated.The spatial energy localization phenomenon of the defect mode has been observed.The frequency tuning of the defect mode was realized by the effect of temperature feild on the physical parameters of the water.The frequency shifting range can reach 10.77%,which can contribute to the frequency selecting of the acoustic waves under water.In summary,it combines theoretical analysis,numerical simulation and experimental method to conduct in-depth research on the excitation mechanism of the elastic guided waves under various medium conditions in this thesis.A variety of wave control structures have been designed to realize the manipulation of the elastic guided wave modes.It provides a solid theoretical basis and experimental verification for the design of wave control devices.