Analysis of the eddy-current effect in the Hi-speed axial MFL testing for steel pipe

Abstract

According to the Lenz's Law, eddy-current essentially exists in MFL(magnetic flux leakage) testing due to the relative movement between steel pipe and excitation coil, apparently affecting the MFL signals with the testing speed rising. Hence, the constitutive equation of eddy-current in the steel pipe was established by using Lenz's Law and it was found that: eddy-current in the arriving zone has the different direction from excitation current, resulting in the reduction of magnetization intensity; but they have the same direction in the leaving zone, where the magnetization intensity is enhanced; and there is few eddy-current formed in the center; besides, the eddy-current density has direct proportion to the testing speed. Further, the effect on the MFL signal was investigated and it was shown that not only the magnitude but also the features are altered. Finally, a confirmation for the aforementioned analysis was conducted by numerical simulations.