Numerical Simulation Of Laser-induced Ultrasonic Waves In Hollow Cylindrical Metallic Materials Using Finite Element Method

In this paper, a theoretical model of thermo-elastic generation of laser-ultrasound in hollow cylindrical metallic materials has been studied and established. By using the finite element method, the transient temperature fields and the thermal stress fields generated by a pulsed line-source laser in hollow cylinders are analyzed. And using the same method, the generation of laser ultrasonic wave and its propagation, waveform characteristics are also studied by theoretical analysis and numerical simulation.In accordance with the classic thermal conduction equation, this paper analyzed the changing of material's optical and thermal physical properties under laser irradiation. The transient temperature fields are obtained. The numerical results denoted: the transient temperature field inside the material is being distributed in a very small-localized region, where great temperature gradient exists quite unevenly.In accordance with the linear elastic dynamics equation, a method of linear elastic finite element numerical simulation laser thermal stress in a quasi-static process is used. The stress fields are obtained based on the thermal analysis above. The distribution of Von Mises stress is similar to the temperature distribution, there appears circumferential compressive resistance near the laser irradiation range and circumferential tensile stress far away the laser irradiation range. The thermal stress is the origin of the laser-ultrasound.Since the temperature field induces the stress and displacement fields and the effect of the stress and displacement fields on the temperature field is negligibly small, the sequential field coupling is used in the present modeling. An optimized finite element is developed to simulate the laser thermo-elastic generation and propagation of surface acoustic waves in hollow cylinders based on a well understanding of the influences of element size and integration time step on the solution stability. A series of the time domain waveforms of the surface normal displacements are shown according to the angle between the source and the detection point from 5° to 180°. The numerical results denoted: in a cylindrical shell, the typical ultrasonic guided wave is Lamb wave like in a thin plate and the So mode is the fastest transmitting and less dispersion of guided waves; the main features of the surface waves in a thick hollow cylinder are three kinds of waves-- the surface skimming longitudinal wave , head wave and the Rayleigh wave, the first Rayleigh pulse's polarity changes gradually with the propagating angle increment, and reverses one...