Abstract: On the basis of the further derivation and analysis of magnetic refraction, a new principle of magnetic flux leakage(MFL) based on magnetic vacuum leakage is proposed. Then, a new MFL testing method based on magnetic vacuum leakage is put forward, in which the strong background magnetic field is eliminated, so that the magnetic flux in the ferromagnetic materials to be tested may thoroughly leak at the place of defect into the created magnetic vacuum region without any background magnetic pressure, thus forming a maximum leakage field and being induced by the magneto sensors placed in the region. The feasibility of the proposed principle and method is verified by simulation and experiment. Finally, their advantages are also presented. The proposed principle and method based on near-zero background magnetic field enhance the spatial dimension and intensity of the magnetic leakage field of defects, resulting in an increase of lift-off distance of magneto sensors and a real long-distance and non-contact inspection. Additionally, the information distortion of defects, magnetic noise and the phenomenon of failure of magneto sensors beyond saturated status are reduced or eliminated. Summarily, the proposed principle and method promote the further sufficient application of MFL method. Meanwhile, they also further richen and perfect the theory system of MFL.

Key words: Magnetic flux leakage(MFL), Magnetic refraction, Magnetic shielding, Magnetic vacuum leakage, Nondestructive testing(NDT)