用有限元方法数值模拟了脉冲线源激光作用于厚铝管时产生的温升以及由此温升而产生的表面声波的情况,得到了逆时针向探测点和波源之间角度从φ=5°到φ=180°范围内一系列表面法向位移的时域波形,并对相同厚度不同外径的铝管的表面波进行对比。数值结果表明表面声波中显著的波为掠面纵波、头波和瑞利波,第一个瑞利波脉冲的极性在传播过程中发生了倒转现象,逆时针向看探测点离波源的角度φ,当φ较小时瑞利波脉冲是单极性的(负的),随着角度的增大,瑞利波脉冲逐渐变为双极性的,且双极性中正向极性逐渐增大,到φ=90°时变为完全双极性(正向和负向对称)。随着角度的进一步增加,双极性中负向极性逐渐减小,到φ=180°时瑞利波脉冲又变为单极性(正的)。不考虑衍射效应时,圆管中第一个瑞利波脉冲的极性和试样的尺寸无关,仅和探测点离波源的角度相关。

The transient temperature fields and the surface acoustic waves generated by a pulsed line-source laser in thick hollow cylinders are analyzed by using a finite element method. A series of the time domain waveforms of the surface normal displacements are shown for different angles between the source and the detection point from =5° to =180° in an anti-clockwise direction. And the surface acoustic waves generated in hollow cylinders with the same thickness and the different outer radii are compared. The numerical results denoted that the main features of the surface waves are three kinds of waves—the surface skimming longitudinal wave, head wave and the Rayleigh wave. And the first Rayleigh pulse′s polarity changes gradually with the propagating angle increment. The Rayleigh pulse is monopolar (negative) near the source, becomes bipolar completely at the point of =90° and monopolar (positive) at =180°. If neglected the diffraction effects, the polarity of the first Rayleigh pulse is only determined by the angle between the source and the detection point, and not related to the size of the specimen.