Numerical Simulation On Laser Ultrasoinc Buried Force Source And Ultrasound Waveforms

A numerical model dealing with laser-generated ultrasound in solid materials with different properties is presented by using the finite element method (FEM). 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.Taking into account of the finite width and duration of the laser source, the effects of thermal diffusion and optical penetration, as well as the temperature dependence of material properties, 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.The generation of buried force and propagation of consequent ultrasound in transparent coating/substrate system are studied using FEM. The relationship between the buried force source and the ultrasound waveforms is analyzed, and the influence of transparent coating thickness on force source and ultrasound waveforms is presented. The numerical results indicate that the increase of the coating thickness has different effects on temperature field and stress field in the transparent coating/substrate system. Due to the constraint of transparent coating, the axial stress increases with the increase of the coating thickness, however, when the transparent coating thickness reaches a certain value, the laser-generated ultrasonic buried force source does not change any longer.The generation of laser ultrasonic force source and propagation of laser-induced Lamb waves in thin unidirectional fiber-reinforced composite plate, transversely isotropic, is simulated. The mechanical generation process of Lamb wave is presented intuitively by analyzing the stress field propagation and the deformations of plate in detail. By the same method, the generation and propagation of laser-generated ultrasonic bulk waves in thick composite plate are also studied. The propagation features of the laser ultrasonic stress filed and ultrasound waves in principal planes are analyzed. The numerical results indicate that the features of laser-generated ultrasound waveforms have closely relation with the propagating direction in anisotropic specimen.The method in this paper provides insight into the generation and propagation of the laser-generated ultrasound wave in different property solid materials and establishes quantitatively relationship between the ultrasound waveform and the laser input as well as the specimen parameters. It presents theoretical basics to optimize ultrasonic signal generation in nondestructive measurement and evaluation by using laser-induced ultrasonic in metallic material, non-metallic material, coating/substrate system and composite materials.