Accurate Identification for 3-D Position of Hybrid Defects in Ferromagnetic Pipe Using External Remote Field Eddy Current Testing

Remote field eddy current (RFEC) testing is extensively used for the defect evaluation of ferromagnetic pipes, which depends on accurate measurement and complete information. In practice, the misalignment between the probe and defect can affect the accuracy of response signals. In addition to defect size, the 3-D position of a defect is critical information for pipe defect evaluation as well. However, existing RFEC methods find it difficult to identify the radial position of defects, especially the hybrid defect. In this article, an external RFEC (ERFEC) method under sine and pulse excitation is proposed to address these issues. By studying the influences of the axial offset distances and circumferential deflection angles between the probe and defect against RFEC signals, the strategy for position correction of the probe placement is provided to reduce signal error. A pulse signal with optimal excitation parameters is utilized to drive the probe. The time-domain characteristics of the pulsed RFEC signal in shape and magnitude are used for the radial position identification of pipe defects. The results indicate that the probe misalignment effects can be removed using RFEC signal features. Besides, the axial and circumferential positions of hybrid defects can be identified using the probe scanning method, while the radial position identification of the hybrid defect needs to use the valley feature of pulsed RFEC signals combined with the RFEC signal phase. In general, this work makes it possible for depth quantification and accurate profile inversion of the hybrid defect.