Time-reversal imaging for sub-surface defect detection over the lamb-wave field

Combining the time reversal mirror (TRM) with the spatio-temporal gradient analysis has an ability to localize a scatterer such as a crack in a thin plate. The TRM provides the means to narrowly converge wave front in time and space. If the focal point is a point-like scatterer, the near-field of the converging point provides linear independency among the four signals: (1)a vertical (z-directional) displacement, (2)its vertical particle velocity, (3)x-directional and (4)y-directional out-of-plane strains. On the other hand, the complete independency is not induced elsewhere in the far-field. In this paper, we explore analytically and numerically the defect imaging by time reversal method based on the spatio-temporal gradient analysis. Time-reversed signals propagate backwards through the time-independent medium and go through all the multiple scattering, reflections, and refraction that they underwent in the forward direction, which is why refocusing occurs.