Enhancement of the Spatial Resolution Using a Novel Rayleigh Wave EMAT Based on Temporal–Spatial Pulse Compression

Rayleigh wave electromagnetic acoustic transducer (RW-EMAT) has been widely used in nondestructive testing (NDT). It is suitable for metal surface crack detection due to its energy concentration within a small depth. However, the electromagnetic acoustic transducer (EMAT) has low energy conversion efficiency and poor signal quality. The spatial resolution is degraded after applying a coil with several turns and an excitation signal with a few cycles. To enhance the spatial resolution of RW-EMAT, a new technique based on temporal–spatial pulse compression (TSPC) is proposed. The transmitter with a linear wavelength modulated coil acts as a spatial matched filter. It spatially compresses the temporal excitation signal with corresponding linear frequency modulation (LFM). Then, a novel ultrashort ultrasonic field for Rayleigh waves is generated. The finite element (FE) model and experimental results indicate that the TSPC technique compresses the ultrasonic field into a spike distribution. Compared with the conventional approach, the spatial signal width of RW-EMAT is reduced by about 98%. For the adjacent defects which cannot be identified by the conventional approach, the TSPC method can identify them clearly, and the calculation error of defect spacing is less than 2%. This work significantly enhances the spatial resolution of RW-EMAT.