Laser-generated Lamb Wave Propagation Characteristics In Thin Plate With Thickness Variation

In this thesis,based on the characteristics of laser-generated Lamb waves which combined frequency dispersion,noncontact excitation,broadband of laser generation,and application in complex structures,the propagation of Lamb waves of MHz in uniform thin plate structure(plate structure with linear thickness variation and acoustic black hole structure with parabolic profile)is studied by theoretical analyse,numerical calculation and experiment.On the basis of wave equation and thermoelastic theory,a three dimensional finite element model is established to simulate Lamb waves generated by laser in uniform thin plate.The time domain waveforms of laser-generated Lamb waves are recorded.Experimentally,the shearing speckle setup based on the Michelson interferometer was built up.The setup includes an auto time-delay system to control pulsed laser which can realize the full field measurement of acoustic wave's dynamic propagation.With this setup,the dynamic propagation process of laser-generated Lamb waves are achieved.The propagation of AO mode Lamb waves in thin plate with linear thickness variation is studied.The propagation path of Lamb waves in the plate is calculated on the basis of Geometrical Acoustic Approximation theory.Through theoretical and numerical research,it is found that the Lamb waves with incidence angle ?s propagates along stationary orbit in the area of conical hole,the radius of stationary orbit decreases with growing frequency of Lamb waves;for the wave with smaller incident angle than ?s,the wave is directed towards the center.Efficient conversion of the Lamb waves into the wave propagating along the edge takes place if the tangential components of their wave vectors match;for the Lamb waves with incident angle bigger than ?s,the waves propagate away from the cone in a form of circulating emission.The built shearing speckle setup based on the Michelson interferometer is used to detect the propagation of Lamb waves in area with linear thickness variation which shows wave propagation intuitively and validates the theoretical results.The propagation of AO mode Lamb waves in thin plate with two dimensional acoustic black hole with parabolic profile is studied.By studying the varying characteristics of the Lamb waves' beam width and amplitudes,it is found that with Lamb waves propagating towards the center,the beam width becomes smaller and the amplitude becomes larger,the increasing speed of amplitudes is inversely proportional to radius;the decreasing of velocity is also studied.The propagation of Lamb waves is also researched by an all-optics generation and detection system based on Laser Doppler Vibrometer experimentally.The experimental results show the wave focusing function of structure and the effect of decreasing velocity.The waveforms are analysed using two-dimensional fast Fourier transform(2D-FFT)to achieve dispersion curves.By comparing with the dispersion curves of wave generated in uniform thin plate,the charactersic of mode selection is founded.The three dimensional model of Lamb wave propagation in acoustic black hole is built,the wave trapping function of acoustic black hole is validated in view of energy.The research results could provide theoretical and experimental basis for the generation and detection of slow wave and application in damping.