Research On Laser-induced Surface Acoustic Wave Technology For Various Media Testing

As the industrial technology is developing very fast and gets advanced very much in recent years,the required quality of media and materials gets higher.And the surface/subsurface integrity and completeness of the materials decide the performance and working life of the equipment and devices.Besides,accompanying the fast developed industrial technology,the air condition gets worse and water resources get polluted.As a result,some disease cases increase especially skin disease.Recently,the morbidity of skin disease in China is increasing year by year,and it becomes a trend that more young people get affected.Therefore the nondestructive detection for surface cracks and pathological changes on skin is very important and in great need.Laser-induced Surface Acoustic Waves technology as a nondestructive detection approach has shown promising results in the characterization of surface feature.The work done in this dissertation is following:1,the state-of-art of the Laser-induced Surface Acoustic Waves technology is explored.The shortness of the current researches and technologies are concluded,based on which,the research content and goal is finally determined.2,FEM simulations under thermal-stress analysis mode are conducted by the software of ANSYS to explorer the effects that pulse laser parameters,including pulse energy,rise time,beam radius,absorption and scattering coefficients,have on the generated Surface Acoustic Waves.And the range of parameters to generate Surface Acoustic Waves with big amplitude and high frequency components are given.Pulse laser with these parameters is applied on skin with melanoma.Through the dispersion curves of the detected signals,the relation between the mesh size of the skin model and the minimum detectable melanoma is found.3,In addition,Surface Acoustic Waves propagating on aluminum sample with surface cracks is simulated by FEM simulations under structural analysis mode.Surface waves propagate through signal cracks with different depth and through multiple cracks are simulated.And the dispersion curve of the surface wave signals detected on the surface of the multiple crack models are calculated,base on which the cracks' numbers can be roughly ascertained.3,the working principles of two optical experimental systems called classical Michelson interferometer system and polarized Michelson interferometer system,respectively,both with differential detection,are introduced.These two systems both include the Surface Acoustic Waves generation module,detection module and collection and processing module.In the polarized system,polarizer,?/4 and ?/2 wave plates and polarized beam splitters are added differently from the classical one,which makes He-Ne laser light polarized and signal noise less.The performances of these two systems are also tested.FIR low-pass digital filter is designed to filter the high frequency noise in the signal.4,surface wave signals detected in the experiments are processed to get the velocity of surface wave propagating in mental media.The location and width of single surface crack on aluminum are detected by the curve of peak to peak value of surface waves detected when the pulse laser and He-Ne laser scans over the crack.5,conclusions and work that needs to be done in the future are listed,including the summarization of the work done in this dissertation and the advice on the simulation and experiment work where can be improved.