Sensitivity difference caused by eddy-current magnetic field in Hi-speed MFL testing and its elimination method

Abstract

Eddy current effect essentially occurs when relative movement happens between steel pipe and axial magnetization coil in Hi-speed MFL (magnetic flux leakage) testing. Based on the forming mechanism of eddy-current, the eddy-current magnetic field distributions in different parts were obtained, and it was found that the eddy-current magnetic field and the original magnetizing field in pipe tail have the same direction, resulting in the increase of the magnetization intensity; however, the direction of the eddy-current magnetic field is opposite to the one of the original magnetizing field in pipe head, which brings about the reduction of magnetization intensity; and that the eddy-current magnetic field in pipe body nearly has no influence on the magnetization. Due to the different magnetization intensity in different parts of the steel pipe, sensitivity difference is formed, lowering the accuracy of MFL testing. Further, the elimination method for sensitivity difference by building extending magnetic pole was proposed. Lastly, the relevant experiments for steel pipe were conducted to verify the influence of the eddy-current magnetic field and the effectiveness of elimination method.