Depth quantification of surface-breaking cracks in ferromagnetic materials using DC-biased magnetization based induction thermography

摘要

A common challenge in maintenance of critical structures is how to fast localize and quantify geometries of surface-breaking cracks. Induction thermography (IT) is a potential technique to detect this type of defects. However, for testing ferromagnetic structures, if a deep defect exists in the wall structure, only its shallower part within the skin-depth layer affects the early thermal response of IT, whereas its deeper part hardly has any influence on the early response. To extend the detectable depth range of traditional IT for ferromagnetic materials, a DC-biased magnetization based induction thermography (DCMIT) technique is introduced in this paper. Based on the permeability distortion in the skin-depth layer, DCMIT can build a monotonic relation between the thermal feature and the defect depth. Results show that when the coil direction is parallel to the notch's length direction, DCMIT can quantify the notch with a depth up to 6.0 mm whereas traditional IT can hardly achieve this depth detection.