Electromagnetic image recognition of a defect profile on a metal surface with a protective layer based on magnetic disturbance

In order to obtain defect information quickly and effectively and improve the accuracy and evaluation ability of traditional electromagnetic non-destructive testing (NDT), an electromagnetic image recognition method for the defect profile based on magnetic field disturbance is proposed in this paper. The excitation coil structure is designed, the excitation mode of the signal source is optimised and a three-dimensional electromagnetic transient analysis model is established for defect profile identification of a metal surface with an anti-corrosion protective layer. The research shows that the disturbed magnetic field Bz has the characteristics of high-resolution imaging and symmetry. The orientation of the defect on the surface has different effects on the clarity of image recognition. The larger the angle between the defect boundary and the induced current, the more complete and clear the image formed by the disturbed magnetic field Bz . A rectangular square wave is the best excitation signal for defect recognition. Its Bz image at t = 0 can present complete shape and position information about the defect. In addition, the excitation coil structure based on the principle of the disturbed magnetic field must provide a uniform induced current to produce a pronounced disturbed magnetic field. It is concluded that electromagnetic imaging technology based on the disturbed magnetic field Bz can better detect and characterise the shape of metal surface defects without damaging the metal protective layer and has good application potential for NDT and safety evaluation of in-service equipment.