Damage Evaluation of Structural Components by Ultrasonic Attenuation

主要研究结果如下：1.针对三维弹性固体中裂纹的散射问题，建立了确定裂纹张开位移的边界积分方程。对裂纹尖端引入四分之一点单元，并对边界积分方程组的超奇异核进行正则化。2.从三维弹性动力格林函数出发，得到了纵向和横向散射幅度的远场表达式。然后由远场散射幅值推导出弹性动力散射截面的表达式。3.数值计算了微分散射截面的角分布，研究了弹性动力散射截面的频率特性。研究发现，随着频率的增加，微分散射截面的纵向部分局部化到入射波的传播方向。当入射波的波长变为裂纹直径的数量级时，总散射截面出现一个峰值。4.假定结构构件中裂纹的稀疏分布，得到了弹性动力散射截面、衰减与裂纹密度的关系。提出了通过测量损伤构件中的衰减来估计裂纹密度。

The results of the study are summarized as follows.1.The boundary integral equation to determine crack opening displacements was formulated for scattering problems of a crack in three dimensional elastic solids. The quarter point elements were introduced for the crack tip and the regularization was performed for the hyper singular kernel of the boundary integral equation.2.Far-field expressions of longitudinal and transverse scattering amplitudes were obtained from the three-dimensional elastodynamic Green's function. Then the expression of elastodynamic scattering cross-sections was formulated from the far-field scattering amplitudes.3.The angular distributions of differential scattering cross-sections were numerically calculated and the frequency characteristics of the elastodynamic scattering cross-section were investigated. It was found that the longitudinal part of the differential scattering cross-section is localized to the propagation direction of the incident wave when the frequency increases. The total scattering cross section has a peak when the wavelength of the incident wave becomes the order of crack diameter.4.The relation of elastodynamic scattering cross-section, attenuation and crack density was obtained from the assumption of the dilute distribution of cracks in structural components. It was proposed to estimate the crack density by measuring the attenuation in damaged components.