Research on defect detection in non-ferromagnetic materials based on an induced magnetic field testing method

This study introduces a novel defect detection method based on induced magnetic field measurements. The technique employs a millimeter-scale permanent magnet as the excitation source and utilizes a high-precision tunnel magnetoresistance sensor to detect perturbations of the induced magnetic field generated by eddy currents, thereby identifying defects. In contrast to conventional eddy current testing, the proposed method eliminates the need for excitation coils, significantly reducing the size and weight of the detection apparatus while offering increased detection accuracy. Vector analysis is conducted through both experiments and numerical simulations, and the signal characteristics are further interpreted using an equivalent parallel circuit model of eddy current flow. An experimental setup is developed to detect microdefects, successfully identifying flaws as small as 10 μm, thus demonstrating the feasibility of the proposed method. Additionally, the influences of four key parameters, namely, the defect diameter, the rotational tangential speed of the copper plate, the skin depth, and the lift-off distance, on the detection performance are systematically studied via both experiments and simulations. The results indicate that smaller defects substantially increase the difficulty of detection, higher motion velocity increases the detection sensitivity, but at the expense of reducing detection depth, and a lift-off distance of 0.2 mm is identified as better. Owing to its compact structure and high precision, the proposed sensor system shows great potential for practical applications in non-destructive testing engineering, such as fatigue crack detection of non-ferromagnetic high-manganese steel commonly used in high-speed railway tracks.