A high-resolution imaging method of phased array ultrasonic Lamb wave based on DORT-SCF for multiple defects

Ultrasonic imaging system is limited by Rayleigh criterion, making it difficult to distinguish the features of multiple defects when the defect spacing is less than resolution limit. Therefore, this paper proposes a high-resolution imaging method of phased array ultrasonic Lamb wave based on DORT and SCF. Firstly, the scattered signals of Lamb wave with full matrix capture are utilized to construct the time reversal operator. Then singular value decomposition is performed to extract the singular vectors corresponding to significant eigenvalues. Immediately, TFM is performed after weighting the scattered signals by the constructed multi-channel SCF. Finally, the experiments about phased array ultrasonic Lamb wave imaging for multiple defects are carried out. The experimental results show that when the defect spacing is 28mm (Rayleigh limit is about 9.21mm), the signal-to-noise ratio of the DORT-SCF method is 44.5% higher than that of TFM. Furthermore, when the defect spacing is about 5mm (far less than Rayleigh limit), the DORT-SCF method can still identify the two defects, while the SCF-TFM is not, which means breaking through the Rayleigh limit and realizing super-resolution imaging. Meanwhile, the average pixel intensity of DORT-SCF method is reduced by 26.7% compared to that of SCF-TFM.