The Effect of Motion-Induced Eddy Current on Circumferential Magnetization in MFL Testing for a Steel Pipe

Eddy current effect essentially occurs when a relative movement happens between steel pipes and the circumferential magnetizer in high-speed magnetic flux leakage (MFL) testing. To improve the MFL testing for longitudinal defects, the effect of motion-induced eddy current (MIEC) on circumferential magnetization is analyzed. First, in terms of Maxwell equations, the eddy current effect and the MIEC distribution in the steel pipe are analyzed. Then, the finite-element simulations are conducted to investigate the MIEC distribution, the magnetization status change, and the sensitivity difference. With the rotational speed increasing, the MIEC and circumferential magnetic flux density gradually become asymmetrical and non-uniform. The MFL testing signal amplitudes of external and internal defects in the arriving and leaving parts change in different variation trends. Finally, the relevant high-speed MFL testing experiments for steel pipes are performed to confirm the effect of MIEC. The experimental results match well with the theoretical analysis and numerical simulation results.