中文摘要：

根据激光脉冲的时间分布、空间分布及材料物理参数与激光超声波形特征的关系，采用有限元方法（FEM）建立了激光超声的热弹有限元数值模型，得到了金属和非金属材料中的温度场分布，将温度场作为超声力源进而得到热弹激发的超声对心波形。根据对心波形，分析了前驱小波的成因和规律。结果表明，金属材料中由于热扩散效应在金属表面附近激发出一个位于表面的力源；非金属材料由于同时存在光学穿透效应，在材料表面层下形成一个有一定深度的体力源。激光辐照导致金属材料中加热区域急速膨胀进而产生垂直于表面方向的应力，而非金属材料该垂直表面方向的应力作用于材料表面下方一定深度处。因此金属材料中前驱小波为单极波形，而在非金属材料中表现为双极波形。前驱小波波形的特征反映了材料的物理性质和参数。该研究为利用前驱小波进行材料的定量检测和无损评价提供了依据。

英文摘要：

Based on the relationship between the features of laser ultrasonic waveform and the spatial distribution, temporal distribution of the laser pulse as well as the material physical properties, a model for the thermoelastic generation of ultrasonic wave in metals and non-metals is established by using finite element method （FEM）. The temperature distributions in metal and non-metallic materials excited by laser pulse are obtained, and the ultrasonic waveforms at the epicenter caused by thermoelastic expansion are calculated by utilizing the temperature distributions as ultrasonic force source. The generation reason and rules of precursor are analyzed according to the waveforms at the epicenter. The results show that, because of the thermal diffusion, a force source is excited on the metal surface, whereas a body force source with relatively large depth is caused due to the optical penetration in nonmetals. The stresses normal to surface are excited by rapid expansion of material heated by laser pulse, but the location that the stresses act on is below the surface in the case of non-metals, so the precursor is monopolor in metals and it becomes dipolar in non-metals. The precursor waveforms include the material physical properties and parameters, so this research is useful for nondestructive materials test and evaluation by utilizing the characteristic of precursor.