DC-Biased Magnetization Based Eddy Current Thermography for Subsurface Defect Detection

Eddy current thermography (ECT) as one of the emerging nondestructive testing and evaluation techniques has been used for defects detection in critical components, e.g., fatigue cracks in turbine blades, bond wire lift-off in IGBT modules, lack of fusion in welded parts, etc. However, in fast inspection using the early thermal response, the thin eddy current penetration depth (skin depth) of ferromagnetic materials limits ECTs capability of detecting subsurface defects. In order to increase the detectable depth range, this paper proposes a dc-biased magnetization based ECT (DCMECT) technique. Based on the nonlinear magnetic permeability in ferromagnetic material, DCMECT can increase the thermal contrast between the defective and sound areas by the enhanced permeability distortion in the skin-depth layer. Specifically, the influences of dc-biased magnetization direction and intensity on the thermal responses (of the defective and sound areas) and their thermal contrast are investigated. Results show that the dc-biased magnetization direction has the strongest influence on the thermal response when it is parallel to the ac magnetization direction generated by the coil. Both the thermal responses of defective and sound areas decrease with the magnetization intensity increasing. Whereas, the thermal contrast between two areas increases with the magnetization intensity, which presents the enhanced defect detectability of DCMECT. The proposed technique can detect the subsurface defect with a buried depth up to 6 mm.