Department of Mechanical Engineering, University of Bristol, Bristol, UK.
IEEE Trans Ultrason Ferroelectr Freq Control. 2011 Oct;58(10):2171-80. doi: 10.1109/TUFFC.2011.2066.
The roughness of crack-like defects affects ultrasonic wave scattering and this, in turn, affects defect detection and characterization. The first part of this paper is concerned with the efficient numerical modeling of scattering from rough cracks, i.e., a finite element local scattering (FELS) model. The scattered field is presented in the form of a scattering matrix, which describes the far-field scattering coefficient for all possible combinations of incident and scattering directions. The scattering matrices for many different realizations of rough cracks are simulated using both a FELS model and a model based on the Kirchhoff approximation. It is shown that the difference between scattering matrices extracted from the Kirchhoff model and the FELS model is less than 8%, for rough cracks with a standard deviation less than 0.3 wavelengths and a correlation length longer than 0.5 wavelengths, at incident and scattering angles ranging from -80° to 80° relative to the normal direction of the mean surface. Because the Kirchhoff model is significantly more efficient than the FELS model, it is used for subsequent simulations in which many realizations of rough cracks are studied to gain insight into the statistical nature of the scattering process. In line with previous work, a distinction is made between the coherent and diffuse contributions to the overall scattered field, in which the former represents the ensemble average over multiple surface realizations. The coherent and diffuse contributions of scattered field from various types of rough cracks are simulated. It is shown that surface roughness directly affects the coherent contribution to scattering behavior, whereas the diffuse contribution is affected by both surface roughness and correlation length, especially for rougher cracks.
裂纹状缺陷的粗糙度会影响超声波散射，从而影响缺陷检测和定性。本文的第一部分关注的是高效数值模拟粗糙裂纹的散射，即有限元局部散射（FELS）模型。散射场以散射矩阵的形式呈现，它描述了所有入射和散射方向组合的远场散射系数。使用 FELS 模型和基于 Kirchhoff 近似的模型模拟了许多不同的粗糙裂纹的散射矩阵。结果表明，对于标准偏差小于 0.3 波长且相关长度大于 0.5 波长的粗糙裂纹，在相对于平均表面法线的入射和散射角度从-80°到 80°的范围内，从 Kirchhoff 模型提取的散射矩阵与 FELS 模型之间的差异小于 8％。由于 Kirchhoff 模型比 FELS 模型效率高得多，因此在后续模拟中使用了许多粗糙裂纹的实现，以深入了解散射过程的统计性质。与之前的工作一致，将整体散射场的相干和漫散射贡献区分开来，其中前者代表多个表面实现的总体平均值。模拟了来自各种类型粗糙裂纹的散射场的相干和漫散射贡献。结果表明，表面粗糙度直接影响散射行为的相干贡献，而漫散射贡献受表面粗糙度和相关长度的影响，对于更粗糙的裂纹尤其如此。
