Subsurface crack detection by spation-temporal gradient method

This paper presents a defect detection method by spatio-temporal gradient analysis over the lamb wave field. The proposed method has an ability to classify the zero order anti-symmetric mode lamb-wave field through the rank of the covariance matrix which is defined by the four-dimensional vector with following components: a vertical displacement, its vertical velocity, and a pair of out-of-plane shearing strains of the surface. The covariance matrix provides the information about defects. Its determinant, therefore, is proposed as the index of inhomogeneity of the object surface. In this study, the computational process in the lamb wave field near the defects is discussed and their physical meanings are investigated through FDTD-simulations and acoustic experiments.