Micro crack detection based on the spatio-temporal gradient method

第一个结果提出了一种基于Lamb波场时空梯度分析的缺陷表征方法。沿层向传播的表面超声波是无损评价（NDE）方法的潜在候选者。所提出的方法能够通过定义在四维矢量空间上的协方差矩阵的秩来对表面声波场进行分类，该向量空间由以下组件组成：垂直位移，垂直速度和表面的一对剪切应变。协方差矩阵提供了关于缺陷的信息。因此，提出协方差矩阵的行列式作为物体表面均匀性的指标。本文讨论了缺陷附近波场的计算过程，并通过fdtd模拟和声学实验研究了缺陷附近波场的物理意义。第二个结果如下所示。本文从理论上推导了基于时空梯度分析的局部相速度法。所提出的方法能够通过4维矢量之间的线性关系来表征兰姆波传播的相速度，该矢量由以下分量组成：(1)垂直（z方向）位移，(2)垂直粒子速度，(3)x方向和(4)y方向的面外应变。本文讨论了局部测速的计算过程，并通过数值和声学实验探讨了局部测速的物理意义。

1:The first results presents a defect characterization method by spatio-temporal gradient analysis over the Lamb wave field. The surface ultrasound waves that propagate in the direction of the layer are potential candidates of non-destructive evaluation(NDE) methodology. The proposed method has an ability to classify the surface acoustic wave field through the rank of the covariance matrix defined over the four-dimensional vector space which is spanned by following components : a vertical displacement, its vertical velocity, and a pair of shear strains of the surface. The covariance matrix provides the information about defects. The determinant of the covariance matrix, therefore, is proposed as the index of homogeneity of the object surface. In this study, the computational process in wave field near the defects is discussed and their physical meanings are investigated through FDTD-simulations and acoustic experiments.2.The second results are shown as follows.In this study, the local phase velocimetry based on the spatio-temporal gradient analysis is derived theoretically. The proposed method has an ability to characterize the phase velocity of the Lamb-wave propagation through the linearity among the 4-dimensional vector which is composed by following components : (1)a vertical (z-directional) displacement, (2)its vertical particle velocity, (3)x-directional and (4)y-directional out-of-plane strains. In this study, the computational process of the local velocimetry is discussed and their physical meanings are investigated through numerical and acoustical experiments.