Numerical Simulation Of Surface Defect For Metal Materials Based On The Laser Ultrasonic Waves

A theoretical model based on the thermoelastic theory is developed to analyze the laser thermoelastic generation and propagation of ultrasonic guided waves in solid materials with different properties. The laser-induced temperature field and acoustic field are numerical simulated by means of Finite Element Method, while the quantitative relationship between the laser source distribution and the physical characteristics of the corresponding ultrasonic waveform is also established. By means of this model, the features of laser buried force and consequent ultrasound generated by pulsed laser, as well as the relationship between the force source and the ultrasonic waveforms are studied numerically. Moreover, good agreement is obtained in a comparative study which uses integral transform analytical methods. The research provides a theoretical basis for the Laser-induced thermoelastic ultrasonic waves applied to nondestructive evaluation. The specific research contents include the following aspects:The transient temperature field in the solid materials with different properties caused by the laser pulse line source is analyzed based on the classical heat conduction equation. As a result, the temperature gradient field is identified as the body excitation source of ultrasonic, while the spatial and temporal distribution of which is also discussed. Based on the mechanism of Laser-induced thermoelastic ultrasonic, the coupling solution of thermoelastic governing Equations and heat conduction equation of elastic medium are derived by using integral transform method. Under the circumstance of solid half space, the analytical solution of mechanical components in the transform domain and the characteristic equation of ultrasonic field are obtained. Thus the solution of surface acoustic waves (SAWs) in time domain, the dispersion curves are also obtained.Taking into account of the finite width and duration of the laser source, the effects of thermal diffusion and optical penetration, the generation of laser ultrasonic force source and propagation of laser-generated ultrasound in metallic material and non-metallic material are simulated, respectively, and waveform characteristics of ultrasound waveforms in metallic materials and non-metallic materials are analyzed. The relationship between the different force models and ultrasound waveforms are presented based on epicenter waveforms.Based on the classical theory of laser-generated ultrasound thermoelasticly, a finite element model of pulsed laser line source acting on the edge of defects is built. The temperature field and the ultrasonic field induced are simulated. The influence of the defect depth on the waveform of the SAWs is also discussed. So the phenomena, that the amplitude of received SAWs increases when the laser line source arrives at the edge of the edge, can be explained. The finite element models are used to simulate the procedure of the pitch-catch method and the near field detection method. The defect information obtained.The results of this study not only provide a solid theoretical basis for the Laser-induced thermoelastic guided waves applied to nondestructive evaluation in solid materials, but also serve as a complement to the theory system of laser ultrasonic.