Quantitative evaluation of surface crack depth based on wideband surface waves electromagnetic acoustic transducer

Accurate quantitative evaluation of surface crack depth is essential for the safety of metallic structures but remains challenging over a wide depth range. In this research, a Halbach magnet-based variable distance meander-line coil electromagnetic acoustic transducer (HBVD-EMAT) is used to generate wideband pulse compression surface waves for non-contact crack depth evaluation. The interaction mechanisms between wideband surface waves and surface cracks are investigated through theoretical analysis and experimental validation, focusing on frequency-dependent reflection, transmission, and diffraction-induced time-delay effects. Experimental results indicate that reflection and transmission coefficients are highly sensitive to shallow crack depths, enabling effective amplitude-based evaluation. For deeper cracks, severe attenuation of transmitted waves limits the reliability of amplitude characteristics. To overcome this limitation, time-of-flight differences between multiple transmitted wave packets are introduced, showing a strong linear relationship with crack depth and improving deep-crack evaluation accuracy. For small cracks, wideband signals are decomposed into multiple frequency components, and a neural network model is employed to capture the nonlinear relationship between crack depth and multi-frequency characteristics. The proposed multi-scale, multi-characteristic framework enables crack depth evaluation over a wide range, providing a practical EMAT-based solution for surface crack detection.