Rapid Quantitative Non-destructive Detection Approach For RCF Cracks In High-speed Rails Based On Motion Induced Eddy Current

At present,China's railway transportation is developing rapidly,and the construction mileage of high-speed railway is planned to reach about 38,000 kilometers by 2025.The long-term cyclic loading between wheel and rail will lead to rolling contact fatigue crack(RCF)on the top surface of the rail,and the continuous propagation of the RCF crack will cause the fracture of rail,which seriously endangers the safety of high-speed railway.With the increasing speed and traffic density of high-speed rail,the time available for rail maintenance and crack detection is shortened accordingly.Therefore,to prevent rail fracture caused by RCF cracks,the rapid quantitative non-destructive testing(NDT)approach for RCF cracks in high-speed rails based on motion induced eddy current is investigated in this dissertation,and the experimental verification is carried out to realize rapid and accurate characterization of RCF cracks.Specifically,the main research contents are as follows:The main research contents and innovations of this dissertation are as follows:(1)The generation mechanism,influence mechanism and the distribution of motion induced eddy current in high-speed moving electromagnetic NDT are analyzed.The governing and diffusion equations of motion induced eddy currents in metals are derived.Followed that,the composition of magnetic field and the eddy current distribution in electromagnetic NDT system are investigated.Furthermore,the basic principle of the electromagnetic NDT of rail crack is expounded when the excitation magnetic field is alternating magnetic field and constant magnetic field respectively,which provides a theoretical basis for the study of the rapid quantitative NDT approach of RCF cracks.(2)To obtain a fast and quantitative approach for the detection of RCF in the rail,the PEC technique under velocity effect is investigated.A 3D transient finite element model for high-speed PEC technique of rail with cracks is established.Furthermore,the distribution of eddy current in the rail at different speeds,as well as the relationship between detection speed and detection signal are analyzed.Finally,a high-speed electromagnetic NDT platform is designed to verify the proposed quantitative characterization approach of crack in high-speed motion experimentally.The results show that with the increase of speed,the stronger the motion induced eddy current is and the more obvious the drag effect is,which has an effect on the detection signal.It is feasible to characterize the location of the crack.(3)In response to the current difficulties in achieving rapid quantitative characterization of RCF cracks,a fast and quantitative electromagnetic NDT approach for RCF straight crack detection based on motion induced eddy current is proposed.The influence of detection speed on eddy current distribution under direct current(DC)excitation is analyzed by finite element simulation.The quantitative relationships between crack depth,width and detection signal are investigated.The experimental platform of high-speed DC electromagnetic NDT is built and the quantitative characterization approach of cracks is verified.The results show that the DC electromagnetic NDT approach can quantitatively characterize RCF straight cracks of different depths and widths at high-speed,even at detection speed up to 20 m/s.(4)In the high-speed motion state,the drag effect will cause the change of motion induced eddy current and magnetic field distribution inside the rail,which can affect the crack detection accuracy.To deal with this issue,the optimal probe structure for high-speed electromagnetic NDT is designed with the goal of obtaining high detection signal strength and sensitivity.The multi-convolution method is applied to evaluate the measurement uncertainty of the experimental results.Furthermore,the change of magnetic field distribution inside the rail due to the drag effect is revealed by experiments.The accuracy of quantitative electromagnetic NDT of rail RCF crack under high-speed motion is improved effectively.(5)RCF cracks sprouting in the rails are usually inclined at different directions on the surface as well as inside the rails.To solve this problem,the effect of crack inclination angle and depth on the detection signal is investigated by finite element method and experiments,the quantitative relationship between crack extension angle,depth and detection signal is obtained.A quantitative characterization method for the angle and depth of RCF oblique cracks is proposed using the peak and peak-to-valley spacing of the detection signal as the characteristic signal.The angle and depth of RCF oblique crack in rail are inverted by decoupling algorithm.The proposed approach is of great significance for predicting and evaluating the RCF crack propagation direction.