Research On Ferromagnetic Materials Surface Defects Detection Using Magneto-optic Imaging Method

Non-destructive testing and evaluation(NDT&E) plays an important role in ensuring the safety and integrity of the ferromagnetic materials, which are widely used in various engineering systems. The current techniques for NDT&E of ferromagnetic materials are: ultrasonic testing, eddy current testing, magnetic particle testing and magnetic field leakage(MFL). Test results of these techniques are not intuitive, while defect visualization is a trend of NDT&E. Magneto-Optic Imaging(MOI) is a new emerging NDT&E technique, which combines eddy current and faraday magneto-optic effect. The major advantage of MOI over other techniques is the potential in rapid real-time visual inspection of large areas. However, MOI is seldom applied for ferromagnetic materials defect detection.Because of the drawback of current techniques used for ferromagnetic materials defect detection, a new MOI method based on MFL is proposed, taking into account the fact that there has been large numbers of magnetization devices for ferromagnetic materials detection. The feasibility of the new method is verified by comparing images light intensity based on an experiment platform.Firstly, the domestic and overseas development situation of MOI is expounded deeply, as well as the basic principle. The possibility of MOI based on MFL is analyzed, which combines the faraday magneto-optic effect and leakage magnetic field near defects when magnetizes the measured ferromagnetic materials.Secondly, the MOI experiment platform based on MFL is designed, after comparing two typical MOI structures from aspects of the way magnetic field generated, light path, and magneto-optic sensor. The selection of important parts such as magnet, magneto-optic sensor, laser source, beam expander, polarizer, and image sensor is illustrated explicitly, especially focused on qualitative analysis of light propagation and imaging quality caused by coated magneto-optic glass with anti-reflective film, reflective film and protective film.Finally, the MOI experiment platform is set up, a surface defect with silicon steel sheet is fabricated and experimented. The experiment results have proved the feasibility of the new method used for ferromagnetic materials surface defect detection, and established foundation of subsurface defect visualization and quantitative analysis.