Study On Propagation Mechanism Of Laser Ultrasonic Surface Wave For Surface Damage Detection

The application of laser ultrasonic surface wave technology in surface defect detection of material processing is a new research direction in the field of industrial detection technology in recent years.Due to its non-contact,high-precision and non-damage detection characteristics,it has gradually become a non-destructive testing method of great concern.One.Based on the thermoelastic effect of the laser heat source on the surface of the material,the relationship between the temperature change of the thermoelastic material and the thermal stress and thermal strain under the action of the heat source is described.The excitation of the surface acoustic wave signal in the metal material is studied by the finite element method.With the propagation characteristics,the influence of surface layer material parameters on the propagation of surface acoustic wave signals is analyzed.Furthermore,laser surface acoustic wave excitation and detection experiments are carried out on different material processing surfaces.Aiming at the weak dispersion of experimental signals,a new method for obtaining the surface acoustic wave dispersion curve is proposed,combined with nanoindentation test and repeatability.Experiment to verify.The results of the research are as follows:1.The research status,numerical simulation and signal processing research progress of laser ultrasonic surface wave technology are discussed respectively.On this basis,the research objectives of the subject are put forward.Based on the basic theory of thermoelasticity,the laser energy input is used as the equivalent heat source,and the relationship between the temperature change of the elastomer and the thermal stress and thermal strain under the action of external heat source is studied.Starting from the finite element theory,the finite element method of transient heat conduction equation and thermoelastic motion equation is analyzed.2.Based on the above finite element solution principle,a simulation model of laser surface source excitation wave signal is established in the finite element analysis software ANSYS.The finite element model meshing method is optimized,and the equivalent thermal load function of the laser heat source is given,and the thermal stress sequence coupling is solved.By analyzing the simulation results of transient temperature field,stress field and displacement field,the mechanism of the generation and propagation of surface acoustic wave signals under heat source loading conditions is obtained.A layered material model is established to study the mechanism of the influence of the physical parameters of the surface layer on the propagation of surface acoustic wave signals.3.The experimental study on the propagation characteristics of laser ultrasonic surface wave signals on different materials and an algorithm for obtaining the dispersion curve is proposed.A DPSS(diode-pumped solid-state)laser was used as an excitation source to generate ultrasonic surface wave signals on the processed surfaces of single crystal silicon wafers,steel sheets,and aluminum alloy sheets,respectively.Aiming at the weak dispersion phenomenon of experimental signals,this paper proposes a time-frequency analysis of the experimental signals using wavelet transform method,and obtains the dispersion curve of the surface acoustic wave signals propagating on the surface of the test piece,and through the repeatability measurement method and the nanoindentation test.The validity of the analytical method is verified.The research content of this paper provides a basis for the study of the excitation and propagation mechanism of laser ultrasonic surface waves,and it has certain significance for further broadening the application range of laser ultrasonic surface wave non-destructive testing technology.