Magnetic Flux Leakage Technique For Rail Cracks Online High Speed Detection

Wheel-rail contact stress can cause various types of fatigue crack damage on the railway track.Rail crack defects, as the typical degradation, easily lead to fatal traffic accident. Efficient andaccurate rail failure detection methods and equipments are urgently needed. ElectromagneticNon-Destructive Testing (ENDT) is suitable for the characterization of changes in the ferromagneticmaterials’ structure, because the magnetization processes are closely related to their microstructure. Inthis paper, Magnetic flux leakage (MFL) technique is proposed for the non-contact, online high speedrail cracks detection. Velocity effect, produced in the rapid relative motion between the detectiondevice and measured rail track, is analyzed. The detection methods and devices, suitable for highspeed rail cracks detection, are proposed and verified with the experiments.On the analysis of the MFL detection in Finite Element Method (FEM) simulation, defect MFLfield divergent distribute in3-D space with the "bubble" shape, cracks with different width, depth andangle of inclination corresponding to different MFL field intensity and distribution. Multiplecontinuous cracks with small spacing will cause MFL field stack distribution. Sensor lift-off andsaturation magnetization of the measured material are important for cracks detection. It have obviousMFL field near the surface of measured material with the0.1mm width and1mm depth cracks insimulation, MFL method has the feasibility to achieve tiny cracks detection. The experiment results ofhigh speed rail cracks MFL detection show that it can achieve fast rail cracks detection within thescope of200km/h, which the minimum crack width and depth are0.2mm and2mm, respectively.Magnetic hysteresis effect will enhance MFL field, and the opposite, magnetic field induced by eddycurrent will weaken MFL field and increase the background magnetic field. Lateral combine withlongitudinal magnetization along the rail will be helpful to the comprehensive crack damage detection.With the detection speed increasing, amplitude of the defect MFL signal collected by hall and coilsensors increases obviously. But the hall signals will be drift and fluctuation with different detectionspeed. More cracks information can be achieved, if the3D hall, coil sensors and the detection coils,winding in the arm of magnetic yoke, are integrated together. It is helpful to the quantitative analysisand damage assessment of various types of rail crack defects under different detection speeds.