A new NDT method based on permanent magnetic field perturbation

On the basis of permanent magnet, perpendicular oriented to the surface of the component under inspection and surrounded by a pick-up coil, the so-called PMFP-effect (permanent magnetic field perturbation-effect) can be observed when the magnet is moved in this perpendicular position with a well defined lift-off along the surface and a surface-breaking or near-surface – but hidden – defect is coming into the influence range of the magnetic field. A new NDT (nondestructive testing) technique by using PMFP so can be proposed for the first time, and its detection mechanism is presented and analyzed by simulations. Afterwards, its testing characteristics for defects are given and its feasibility is further confirmed by experiments. Meantime and particularly, its inspection depth and the effect of the position and attitude of PMFP sensors on inspection signals are analyzed. Accordingly, a multi-differential method used for improving signal-to-noise rate is presented and some testing apparatus using PMFP method is developed. Finally, the proposed PMFP method is discussed specially compared to MFL in detail.