Magnetic flux leakage (MFL) testing is a widely used nondestructive testing (NDT) method for the inspection of ferromagnetic materials. This review paper presents the basic principles of MFL testing and summarizes the recent advances in MFL. An analytical expression for the leakage magnetic field based on the 3D magnetic dipole model is provided. Based on the model, the effects of defect size, defect orientation, and liftoff distance have been analyzed. Other influencing factors, such as magnetization strength, testing speed, surface roughness, and stress, have also been introduced. As the most important steps of MFL, the excitation method (a permanent magnet, DC, AC, pulsed) and sensing methods (Hall element, GMR, TMR, etc.), have been introduced in detail. Finally, the algorithms for the quantification of defects and the applications of MFL have been introduced.

There has not been an electric-magnetic flux leakage (MFL) detector enabling nondestructively testing the in-service mine hoist wire ropes fixed at both ends mainly due to the tubular configuration of solenoids as electric magnetizers. Hence, an opening electric magnetizer is proposed based on the magnetic control for a C-like electric loop-coil, featured by an opening structure capable of directly encompassing and centering the endless object in it. Subsequently, the magnetization effect similar to the conventional solenoids is confirmed by simulations and experiments with the designed 3-D models and prototypes. Finally, an opening electric-MFL detector chiefly consisting of the opening electric magnetizer is presented, capable of directly encircling the endless ropes, and meanwhile two application cases of the relevant apparatuses for the in-service mine hoist wire ropes are given with a good detection effect. The emergence of the opening electric magnetizer and its detector can expand the application ranges beyond of the traditional tubular solenoids and their corresponding tubular sensors, including other electromagnetic excitation or nondestructive testing for the endless objects, such as railway and coiled tubing in service.

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.