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A Generalized Time-Reversal-Based Imaging for Ultrasonic Nondestructive Evaluation

In this paper, a generalized time-reversal-based imaging algorithm is presented and introduced into the area of ultrasonic Nondestructive evaluation (NDE) for imaging defect in solids. It can be expressed in different forms, including central frequency time-reversal with multiple signal classification (CF-TR-MUSIC), as well as its multi-frequency forms over a given frequency bandwidth, namely multi-frequency TR-MUSIC (MF-TR-MUSIC) and a related phase-coherent form (PC-MUSIC). These imaging algorithms are tested with experimental ultrasonic array data, and their performance is evaluated in terms of lateral resolution and sensitive to noise, two particularly important indicators for industrial NDE. It is shown that all three methods are capable of resolving lateral targets spaced closer than the Rayleigh limit, achieving super-resolution imaging. Both CF-TR-MUSIC and MF-TR-MUSIC can locate the positions of targets correctly, whereas the results from PC-MUSIC are less clear because of the presence of multiple peaks in the vicinity of target. However, an advantage of PC-MUSIC is that it can overcome the elongated point spread function that appears in TR-MUSIC images, and hence provide enhanced axial resolution. For high noise levels, MF-TR-MUSIC and PC-MUSIC are shown to provide stable image and suppress the presence of artifacts seen in the CF-TR-MUSIC image.