Phase-Extraction-Based MFL Testing for Subsurface Defect in Ferromagnetic Steel Plate

Magnetic flux leakage (MFL) based on phase extraction for detecting the subsurface defects in ferromagnetic steel plate was investigated. The relationship between electromagnetic field phase and the subsurface defect was analyzed. Low-frequency alternating current (AC) excitation source and high-power magnetizer arrangement with Hall sensor were used to increase the skin depth of the MFL. Experiments results showed that 12 mm deep subsurface defect can be detected by using the phase extraction means, which is about two times higher than that by using the amplitude method.