Inspection of subsurface defects in the composite materials by divergence-based analysis over the phase velocity vector field

In recent decades, CFRP materials have been extensively used in engineering process and become an important material in many industries. Ultrasonic guided waves, Lamb waves, are useful for evaluating the integrity of CFRP structures. However, the variation of the Lamb wave velocity causes difficulties for interpretation of observed signals. It is important, therefore, to establish the crack detection criterion independent of wave numbers. The aim of this work is to retrieve divergence of the phase velocity vector field caused by the scattered waves and to reconstruct the image of sub-surface defects. In this study, the problem of reconstructing a super-resolved image of defects in a shell structures over Lamb wave field is considered. This study discusses the image reconstruction process based on the divergence of the phase velocity vector field of the scattered wave fronts.