Near-field acoustical imaging of cracks over the A0-mode Lamb-wave field

The use of Lamb waves has recently been interested in the non-destructive 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 imaging method utilizes the determinant of covariance matrix of dynamic shear strains which are defined over the object surface. The determinant denotes the distribution of correlated intensity modulation of the incident wave fronts with scattered ones. The determinant increases significantly in the near-field of the point-like defects. In this study, the computational process in the wave field near the defects is discussed and their physical meanings are investigated through FDTD-simulations and acoustic experiments.