The evaluation of opposite-side defects in thick ferromagnetic objects (for example, steel plates) is essential for safety maintenance. Eddy-current testing (ECT) can detect defects in metallic specimens; however, owing to the skin effect, its use is limited to the evaluation of surface or subsurface defects. Therefore, in this paper, we propose a non-contacting inspection system that consists of two excitation coils and a detection coil to detect slit defects on the backside of a steel plate using low-frequency ECT (LF-ECT). Using the proposed system, we examine whether LF-ECT mainly detects the leakage or the flux induced by the eddy current, and also examine which direction of the excitation currents in the two excitation coils is suitable for detecting slit defects. The results suggest that the method to drive the current in the same direction and to detect the imaginary part of the magnetic flux density, which corresponds to the flux induced by the eddy current, is suitable for detecting slit defects. Subsequently, we examine the detection limit of the height of the slit defect when the thickness of the steel plate is 10 mm and the width and length of the slit defect are constant, at 50 and 1 mm, respectively. The results demonstrate that the slit defect can be detected by using the proposed system when the height is greater than or equal to 4 mm. Therefore, the proposed system is a promising tool to detect critical defects on the backside of steel plates.
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