A high-resolution magnetisation method for enhancing the maximum MFL signal of inner-wall defects in thick-walled ferromagnetic pipes

The magnetic flux leakage (MFL) field generated by inner-wall defects in thick-walled ferromagnetic pipes is weak, posing a major challenge for MFL testing. Although increasing the magnetisation strength can enhance the MFL field, the leakage field amplitude of inner-wall defects exhibits a non-monotonic dependence on magnetisation strength, resulting in the presence of a maximum value in the MFL response of internal defects. To break this limitation, this study proposes a high-resolution magnetisation method. On the basis of a conventional primary magnetiser, a high-resolution local magnetising device composed of a permanent magnet and a magnetic yoke is introduced. Its small-gap configuration concentrates magnetic energy at the surface layer of the sample, weakening the magnetic shielding effect, ultimately enhancing the leakage field of inner-wall defects. Numerical simulations and experimental studies were conducted to compare the effects of different structural parameters of the high-resolution local magnetising device and to reveal the underlying physical mechanism. Results demonstrate that, compared with conventional MFL testing, the proposed method increases the leakage field amplitude of 1-mm-deep inner-wall defects in 20 mm thick pipes by more than 30%, significantly improving the detectability of internal defects in thick-walled components.