Laser Shock Hardening Material Surface Roughness On Laser Saw Dispersion Effects

In this paper,by using theoretical analysis and experiment,the effect of the surface roughness of the laser shock peened metals on the dispersion of laser-generated surface acoustic waves is studied.A brief introduction of the theory of laser-generated surface acoustic waves is given.A theory of surface acoustic waves which are generated by pulsed laser in thermo-elastic mechanism and the property of surface acoustic waves are discussed.Methods of excitation and detection of surface acoustic waves are also discussed.From the methods of the detection,it's better to use PVDF transducer to detect surface acoustic waves on the rough surface.All these works are preparations for the experiment.The theoretical model of the propagation of laser-generated surface acoustic waves on the weak roughness surface is built.When the transverse correlation length of the rough surface is much shorter than the wave length of laser-generated surface acoustic waves,the explicit expressions for the dispersion of laser-generated surface acoustic waves on the rough surface are obtained by using green's function.The dispersion of laser-generated surface acoustic waves is related to not only the roughness parameters(rmsδ,correlation lengthα),but also the poisson's ratio,young's modulus of elasticity,density of the medium. The surface roughness of laser shock peened metals has linear effect on the dispersion of laser-generated surface acoustic waves.Laser-generated surface acoustic waves are excited by Nd:YAG and the signals are detected by PVDF transducer.The paper discusses laser-generated surface acoustic waves in the smooth and non-laser shock peened medium by using the experimental system.It can conclude that the experimental system's precision is so high enough.The signals of laser-generated surface acoustic waves in aluminum alloy under different impact times by laser shock penning are detected.The signals spectrum and phase velocity dispersive curve are obtained.Then dispersive curve of the phase velocity which is affected by the surface roughness of the laser shock peened medium is also obtained.The surface roughness of the laser shock peened medium has effect on the high frequency of laser-generated surface acoustic waves.As the impact times of the laser shock peening increase,the surface roughness has little effect on the dispersion of laser-generated surface acoustic waves. From the theoretical curve and experiment curve,it can conclude that the expression of the dispersion of laser-generated surface acoustic waves on the rough surface is true.And it can used to analyze the effect of surface roughness on the propagation of laser-generated surface acoustic waves.The result in this paper can provide a foundation for studying the effect of residual stress and cold work(texture,dislocation,density increase)on the dispersion of laser-generated surface acoustic waves.It can also provide insight into near-surface properties of laser shock peened metals.