激光远场激发表面波在开口缺陷处的散射回波

Scattered echo of surface defect in the far field of Rayleigh wave generated by laser

为了实现激光超声技术对表面缺陷的定量检测,采用有限方法模拟了激光激发的表面波与表面开口矩形缺陷作用的复杂过程,比较了近场和远场两种不同激发方式下相同接收点的位移信号,发现远场激发下各种模式转换波和反射表面波能充分分离,有利于分析表面波与缺陷作用的复杂过程。在远场激发方式下分别探讨了表面波与缺陷前沿和后沿的作用过程,提取缺陷产生的散射回波特征,并用惠更斯原理分析其产生原因。最后研究了缺陷深度和宽度变化对散射回波特征的影响。结果表明,散射回波特征点到达时间差与缺陷深度和宽度有线性关系。该研究为反问题估算缺陷尺寸提供了理论依据。

In order to detect the surface defect quantitatively with laser ultrasonic technique,the interaction between the laser-generated surface wave and the open rectangular defect was simulated by means of the finite element method. After comparing the displacement signals at the same receiver generated in the near-field and far-field laser irradiation,it was found that reflected waves could be separated the from the mode-converted waves sufficiently when the laser was located in the far field of the defect,which may be helpful to analyze the complicated process of the interaction between the incident wave and the defect. Then,the wave interaction with either the front or the back of the defect was studied in detail,and Huygens principle was further employed to interpret the possible interaction features among the reflected signals. Finally,the influences of depth and width of the defect on the scattered echo were also studied. The results show that the arrival time difference of the scattered echo is linearly related to the depth and width of the defect. This research may provide the theoretical basis to quantify the defect.