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A DC-Biased Scanning Induction Thermographic System for Characterizing Surface Cracks in Ferromagnetic Components

Detecting surface cracks in ferromagnetic components during the manufacturing process is of critical importance to guarantee high-quality production. Scanning induction thermography (SIT) can be used to dynamically scan components and identify such cracks. However, in testing ferromagnetic components, the capability of SIT to characterize crack depths and orientations is limited under certain operating conditions, such as cracks parallel to the coil, quick detection via early heating. To address these problems, this article proposes the use of a DC-biased scanning induction thermographic system, which relies on the effective permeability variation in physics and a thermal data reconstruction procedure in image processing. Specifically, the thermal peak is used to characterize crack depths, and the average of thermal contrast and a morphology-based method are introduced to characterize crack orientations. Results show that the proposed system can effectively characterize crack depths up to 6.0 mm and orientations varying from 0° to 90°.