The multi-mode reverse time migration for defect characterization using ultrasonic array

An ultrasonic wavefield-based imaging technique, termed Multi-Mode Reverse-Time Migration (MMRTM) is implemented for characterizing complex-shaped surface-breaking cracks. This method enables the imaging and sizing such cracks by combining longitudinal (L) and/or shear (S) wave-mode images. The MMRTM builds upon the Reverse-Time Migration (RTM) by separating the wavefield into multiple modes prior to imaging. A 2D imaging approach for isotropic elastic materials is developed, involving the decomposition of source and receiver wavefields into L-and S-wave vectors using decoupled elastodynamic extrapolation. This method preserves the original stress and particle velocity components, ensuring accurate amplitude and phase information in the separated wavefields. Inner-product imaging condition is then applied to generate LL, LS, and SS images of surface-breaking cracks. Both simulations and experimental validations are conducted to showcase the efficacy of MMRTM in characterizing defects. Furthermore, the MMRTM is compared with the Total Focusing Method and conventional RTM, where it shows significant improvement in image reconstruction compared to the other two imaging methods. The results demonstrate its potential utility in evaluating various complex-shaped defects, including fatigue cracks and stress corrosion cracks.