表面垂直裂痕诱发瑞利波散射的数值分析

Numerical simulation of scattering of Rayleigh wave by a surface-breaking crack

激光激发的声表面波为材料表面缺陷的检测提供了有力的工具。针对含缺陷材料在模型边界上的复杂性,建立了基于平面应变的有限元模型并选取了相同厚度但含有不同深度的表面裂痕的单层铝板进行了对比计算,得到了声表面波经过不同深度的表面裂痕时产生的反射及透射信号波形的时域特征。进而引入了基于Wigner-Ville分布理论的时-频分析方法计算裂痕前、后散射的瞬态表面波的能量在时间-频率平面内分布的情形。结果显示:声表面波接近中心频率的某一频率成分在经过深度小于其中心波长的表面缺陷时,随着裂痕深度的增加,对应于该频率的反射系数呈现单调递增的趋势;而透射系数呈现递减的特征,这一结果可以为激光超声检测表面缺陷提供一种定量的表征手段。

Surface-breaking cracks can be detected by observing their interaction with surface acoustic waves generated by the pulsed laser. Taking into consideration the complicated boundary of the material with a surface crack, an optimized finite element model based on the plane strain is developed to simulate the scattered surface acoustic waves propagated on an A1 plate with varied depth surface-breaking crack. From this we attained the characteristics of reflected and transmitted Rayleigh wave from the crack. Furthermore, we introduced Wigner-Ville theory to analysis the spectrogram of the two signals recorded on both sides of the surface crack. The results show that for the certain frequency component, the reflection coefficient becomes larger as the depth of the surfacebreaking crack increased; while the transmission coefficient appears to be smaller. This establishes a quantitative basis for the application of laser ultrasonics in nondestructive evaluation of the inaterial properties.