利用理论推导及实验验证的方法研究了激光倾斜入射支持燃烧波时纵波声场的指向性。根据激光超声的产生机理,在喷溅物质垂直作用于工件表面的前提条件下,推导了椭圆形声源作用时远场质点的法向位移。获得了纵波声场的指向性函数,分析了影响纵波声场指向性的因素,并进行了实验验证。利用功率密度低于爆轰波点燃阈值的激光烧蚀工件,使用峰值频率为5 MHz压电探头接收纵波,进而获得纵波声场的指向性实验数据,结果表明,实验数据与理论数据能够较好地进行吻合。在光斑短轴长度恒定时,与激光垂直入射相比,各倾斜入射角度下纵波声场的声束轴线指向均保持不变,并且指向性图形随倾斜角度的增加而变得细窄,纵波声场的能量也更加集中。

Acoustic field directivity patterns of longitudinal wave are theoretically and experimentally analyzed,when the oblique laser supported combustion wave exists. Based on laser ultrasonic generation mechanism, the normal displacement of particle in the far field is deduced under the condition that splash materially vertically impacts on the surface of workpiece. The directivity function is obtained, and the relevant factors are analyzed. A measurement system for directivity patterns is established to verify the function. Longitudinal wave is received by piezoelectric transducer whose peak frequency is 5 MHz in a state of laser supported combustion. The experimental data and theoretical data agree well with each other. For the longitudinal wave under the same size of spot, the directivity patterns become narrower with the increase of the inclined angle, which means that the energy of the longitudinal wave field is also more concentrated, but the angle of the acoustic beam axis remains unchanged.