Fatigue Crack Initiation And Propagation In Rails Under Rolling Contact Loading

The wheel-rail contact loading is a time-dependent multi-axis non-proportional loading that causes cracks on the surface or subsurface of the rail.Most of the damage of the rail is related to the crack caused by inclusion and the crack propagation on the surface of the rail.Therefore,it is necessary to study the initiation and propagation of the fatigue crack of the rail under the rolling contact load.This is helpful to understand the damage mechanism of rail spalling,squat,―Kidney-shaped‖crack,rail fracture and proposes a rational suggestion for reducing rail damage.The main work of this thesis is as follows:（1）The two-dimensional rail model with inclusion was simulated by the finite element method,and the crack initiation risk coefficient around the inclusion was calculated by the use of the Dang Van multi-axis fatigue crack initiation criterion.The simulation results show that the crack initiation zone around the inclusion is―butterfly‖,the crack initiation zone of the hard inclusion is larger than that of the soft inclusion,and the cracks are more likely to initiate around hard inclusions.（2）The variation of the amplitude of circumferential strain at the crack tip of biaxial fatigue tensile specimen with angle was analyzed.The propagation direction of fatigue crack under multi-axial non-proportional load was predicted by the criterion of maximum circumferential strain amplitude.The forking propagation of fatigue crack was explained.The predicted crack propagation direction is in good agreement with the experimental results.（3）The fatigue crack on the rail surface was analyzed under the framework of linear elastic fracture mechanics,and the crack propagation direction was predicted by the maximum circumferential strain amplitude criterion.The results show that the small angle oblique crack on the rail surface easily deflects upward and propagates towards the rail surface,which leads to the spalling of the rail surface.But the large-angle crack（30°,40°and 45°）is easy to fork and propagate.The friction coefficient of the crack surface has little effect on K_Ⅰ.The larger the friction coefficient of the crack surface is,the smaller the?K_Ⅱ is.The friction coefficient of the crack surface has little effect on the crack propagation direction.When the friction coefficient of the crack surface is greater than 0.3,the crack propagation direction is almost unchanged.（4）The plastic zone at the crack tip of the rail surface was simulated by finite element method with the number of loading cycles.When the friction between wheel and rail is small（μ=0.1）,the plastic zone size of the crack tip satisfies the "small range" yield condition of the linear elastic fracture mechanics.When the friction between wheel and rail is large,the cyclic plasticity of the crack tip increases remarkably,and the cyclic plastic zone size of the large-angle crack tip（θ=30°）is significantly larger than that of the small-angle crack tip（θ=15°）.