| In recent years,the rapid development of my country’s railway industry has also increased the quantity and quality requirements for steel rails.As a new type of rail maintenance technology compared with traditional grinding wheel grinding,abrasive belt grinding has great advantages: elastic contact,cold grinding,etc.The distribution of the residual stress and gradient plays a vital role in fatigue life of the rail.The thesis aims to reveal the relationship between the residual stress gradient and the fatigue life of the rail,and proposes an abrasive belt grinding mode that reduces the stress gradient,thereby improving the fatigue life of the rail,which is of great significance for enriching the rail maintenance operation mode.Firstly,the thermal-mechanical coupling finite element model and several measurement indicators of residual stress and gradient are established.The distribution law of the residual stress and gradient of the rail along the profile direction and the depth direction under different grinding process parameters is revealed.The three-factor three-level regression orthogonal experiment of stress gradient was accomplished to analyze the influence of different grinding parameters on the stress gradient.Thereby based on the fitted stress gradient function model,the best grinding parameters are obtained.Afterwards,based on Kalker theory,the wheel-rail contact area is divided into slip zone and creeping zone,and the wheel-rail contact stress model is established to analyze the influence of contact stress and residual stress on rail fatigue cracks.Based on the method of initial stress setting,the residual stress of the grinding parameters optimized is imported into the wheel-rail contact finite element model.Thereby the evolution law of residual stress and gradient under different axle loads and driving speeds is analyzed.Then based on the designed two-factor five-level full factorial experiment,the relationship between axle load、driving speed and minimum fatigue life is analyzed.The model of the relationship between stress gradient and wheel-rail contact parameters,and the model of the relationship between minimum fatigue life and residual stress gradient are established.Then,according to the analysis of the relationship between the grinding mode parameters(grinding speed,times,distance)and the stress gradient,the best parameters are got.Then the number of grinding heads and deflection angles are arranged for the purpose of reducing the stress gradient,which is a new grinding mode.The minimum fatigue life of the rail is increased by 37.26% and 43.5% respectively after 2～3 times of grinding in this grinding mode compared with before grinding.And a grinding mode including the entire grinding range of the rail is given.Finally,based on the rail belt grinding experiment,the distribution area of tensile and compressive stress in abrasive belt grinding is analyzed.The finite element model of abrasive belt grinding and the theoretical model of stress gradient are verified.And the change law of the residual stress gradient of the rail under the different degree of staggered grinding belt is studied.The arrangement of the grinding heads proposed in the thesis can reduce the residual stress gradient of the rail,which is confirmed by experiments that stress gradient in X and Y directions are reduced by 53.85% and45.83% respectively. |
