激光在近表面弹性性质梯度变化的材料中激发超声波的数值分析

Numerical simulation of laser-generated ultrasonic waves in steel with gradient changes of near-surface elastic property

激光激发超声波为评价材料近表面弹性性质提供了有效的手段.考虑到由于冲击硬化、表面热处理、表面氧化等引起的金属材料近表面层弹性性质的变化,建立了一种激光在基底上的梯度材料中激发超声波的理论模型.用有限元方法模拟了热弹条件下脉冲激光作用于材料上表面激发出的超声波及其传播过程,研究了近表面层离散的层数对超声波的产生和传播的影响,并分别讨论了表面层"变硬"和"变软"两种情况下声表面波的模式变化及用二维傅里叶变换得到各模式的色散曲线.为进一步研究近表面层的弹性性质建立合理的计算模型及材料性质的反演提供了理论依据.

Laser-generated ultrasonic wave is effective for evaluating near-surface elastic property. Taking account of the gradient changes of the near-surface elastic property caused by surface heat treatment or laser shock peening, a finite element method is developed to simulate the laser thermo-elastic generation and the propagation of acoustic waves in the system of multiple gradient layers of the substrate. The number of discretized layers has an effect on the generalized wave modes. Two systems with the surface layer softened and the surface layer hardened substrates are respectively considered. And the two-dimensional fast Fourier transform method has been used to obtained the phase velocity dispersion of different modes. The conclusion could be useful to the further study of the near-surface elastic property.