An improved magnetiser for magnetic flux leakage inspection of 20 mm-thick steel plate

Large storage tanks often use 20 mm-thick steel plates as pressure-bearing parts, which need periodic non-destructive inspection to ensure their structural safety and operational reliability. It is difficult to saturate 20 mm-thick steel plate using conventional magnetisers due to their limited penetration capability, resulting in weak back-side defect signals, low signal-to-noise ratio and insufficient inspection sensitivity. To address this problem, this study proposes an improved magnetiser structure based on the Halbach array design. A finite element model was developed to simulate and analyse the axial component of the magnetic flux leakage (MFL) field of a spherical hole defect with a depth of 4 mm on the back side of a 20 mm-thick steel plate, thus verifying that the improved magnetiser enhances the detectability of the MFL field signals from the back-side defect, with a 170% increase in MFL field amplitude. An experimental platform was constructed and the results show that the improved magnetiser can detect the spherical hole defect with a depth of 4 mm on the back side of a 20 mm-thick steel plate at 15 mm lift-off of the improved magnetiser and 3 mm lift-off of the sensor.