Fast frequency-domain symbolic coherence total focusing method for array electromagnetic acoustic transducer

Array-based non-destructive testing (NDT) has become a key advancement in modern NDT, offering large-area coverage, rapid imaging, and real-time data acquisition, significantly improving inspection efficiency and underpinning automated and intelligent inspection systems. Among array-based methods, electromagnetic acoustic transducers (EMATs) stand out as a non-contact method that eliminates the need for coupling agents, making them particularly well-suited for inspecting coated components and performing high-temperature in-line monitoring. Moreover, Array EMAT provide more intuitive, higher-resolution imaging, enhancing defect characterization and interpretation. Based on the full matrix capture of array EMAT, this study develops a fast frequency-domain signed coherence total focusing method. Derived from the rigorous solution of the wave equation, the frequency-domain algorithm offers clearer images compared to the time-domain delay-and-sum method. The introduction of the double-square-root vertical wavenumber significantly reduces the computational complexity of wavefield extrapolation. Meanwhile, the combination of frequency-domain zero-padding and time-domain interpolation effectively balances computational efficiency and spatial resolution. Gaussian filtering and the signed coherence factor are incorporated to suppress incoherent noise and mitigate the blind zones caused by electrical pulse crosstalk between array elements. Experimental results demonstrate that the array EMAT integrated with the advanced frequency-domain algorithm achieves an effective detection depth of 75 mm, with a single image computation time of only approximately 0.59 s. Moreover, it provides high-contrast and high-spatial-resolution detection results for multiple defects and adjacent defects. This research demonstrates the potential of array EMAT, combined with the proposed frequency-domain imaging algorithm, for future on-line non-destructive testing applications.