The near-field imaging method based on the spatio-temporal gradient analysis

Ultrasonic guided waves, because of their long range inspection ability, are now being used more and more as a very efficient and economical NDE method for large area inspection. The use of Lamb-waves has recently been interested in the nondestructive evaluations for thin-plate-like structures. The conventional time-of-flight technique is not reliable for locating the defects even in isotropic plates because of the dispersiveness in the Lamb-wave propagation. Avoiding difficulties caused by the dispersiveness, this paper proposes a novel spatio-temporal gradient imaging method which has an ability to localize cracks and to reconstruct the near-field images of cracks. The proposed method utilizes the determinant of the covariance matrix defined over the shear strain field over the surface. The determinant denotes the distributions of correlated intensity modulation (DCIM) of the incident and scattered wave fronts. The DCIM increases significantly in the near-field of the point-like defects. In this study, the computational process in wave field near-the defects is discussed and their physical meanings are investigated through FDTD-simulations.