Krylov Victor V
Department of Aeronautical and Automotive Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK.
Ultrasonics. 2016 Jul;69:279-84. doi: 10.1016/j.ultras.2016.01.011. Epub 2016 Jan 28.
In the present paper, directivity patterns of laser-generated sound in solids are investigated theoretically. Two main approaches to the calculation of directivity patterns of laser-generated sound are discussed for the most important case of thermo-optical regime of generation. The first approach, which is widely used in practice, is based on the simple modelling of the equivalent thermo-optical source as a mechanical dipole comprising two horizontal forces applied to the surface in opposite directions. The second approach is based on the rigorous theory that takes into account all acoustical, optical and thermal parameters of a solid material and all geometrical and physical parameters of a laser beam. Directivity patterns of laser-generated bulk longitudinal and shear elastic waves, as well as the amplitudes of generated Rayleigh surface waves, are calculated for different values of physical and geometrical parameters and compared with the directivity patterns calculated in case of dipole-source representation. It is demonstrated that the simple approach using a dipole-source representation of laser-generated sound is rather limited, especially for description of generated longitudinal acoustic waves. A practical criterion is established to define the conditions under which the dipole-source representation gives predictions with acceptable errors. It is shown that, for radiation in the normal direction to the surface, the amplitudes of longitudinal waves are especially sensitive to the values of thermal parameters and of the acoustic reflection coefficient from a free solid surface. A discussion is given on the possibility of using such a high sensitivity to the values of the reflection coefficient for investigation of surface properties of real solids.
在本文中，对固体中激光产生的声音的指向性图案进行了理论研究。针对最重要的热光产生机制情况，讨论了两种计算激光产生声音指向性图案的主要方法。第一种方法在实际中广泛使用，它基于将等效热光源简单建模为一个机械偶极子，该偶极子由两个沿相反方向作用于表面的水平力组成。第二种方法基于严格理论，该理论考虑了固体材料的所有声学、光学和热学参数以及激光束的所有几何和物理参数。针对不同的物理和几何参数值，计算了激光产生的体纵向和剪切弹性波的指向性图案以及产生的瑞利表面波的振幅，并与偶极子源表示情况下计算的指向性图案进行了比较。结果表明，使用激光产生声音的偶极子源表示的简单方法相当有限，特别是对于所产生的纵向声波的描述。建立了一个实用标准来定义偶极子源表示给出可接受误差预测的条件。结果表明，对于垂直于表面方向的辐射，纵向波的振幅对热学参数值和自由固体表面的声反射系数特别敏感。讨论了利用这种对反射系数值的高灵敏度来研究实际固体表面特性的可能性。
