Local Defect Detection of Ferromagnetic Metal Casing Based on Pulsed Eddy Current Testing

Remote-field eddy current testing (RFECT)-based methods have been widely used in conductive pipe evaluation. However, the response signals of RFECT are affected both by the casing thicknesses around the driver coil and in the vicinity of the pickup coil, making it difficult to invert and separate the thicknesses around the coils. Moreover, the relative permeability difference between the near-field and remote-field zones causes a further deviation in the thickness inversion. To avoid these problems, a near-field pulsed eddy current testing (PECT) method is proposed for defect evaluation. Time-domain transient features, which can be applied for defect detection, are analyzed based on the theoretical data of PECT. A prototype of the testing apparatus and a defective ferromagnetic metal casing are designed to verify the feature performance for defect evaluation. The experimental results indicate that the time-related features are sensitive to circumferential defects and insensitive to the axial defects, and the magnitude-related feature is sensitive to both the circumferential and axial defects. Therefore, the magnitude-based method can be used to detect the local defects of ferromagnetic metal casings.