Study On The Mechanical Behavior Of Rail Surface Cracks

Due to the increase of passenger,freight volume and the train speed,the material damage caused by rolling contact of wheel-rail was becoming more and more serious.However,the material surface failure mechanism was very complicated,so it was necessary to carry out corresponding theoretical research to explore its mechanical mechanism.It can provide a theoretical basis for practical engineering application.The micro-cracks will be formed on the surface or sub-surface of the rail by the contact force.And the crack surface will be subjected to extrusion or friction,which causes the crack surface to open or slip.Under cyclic loading,the crack surface was in a mixed state of local opening or closing for a long time.It was also accompanied by a misaligned displacement.On the other hand,when the rail material with surface cracks was continuously rolled by the wheels,the stress in the local area of the crack tip reached the yield limit and local plastic deformation occurred.The dislocations emitted by the crack tip caused plastic deformation.Considering this deformation mechanism,a theoretical model was established to characterize the plastic effect of the crack tip using continuously distributed dislocations.Most of the mechanical research of the crack on rail surface under rolling contact of wheel-rail was based on finite element simulation or experimental methods,and there was relatively little theoretical research work.Therefore,this paper used an effective method to solve the problem of complex crack,called the distributed dislocation method,which was used to study various crack models.The main idea of the distributed dislocation method was to decompose the main problem into two sub-problems by using the superposition principle.The first sub-problem was: in any plane,there were no defects such as dislocations and cracks,but only withstand external forces,and the sub-problem "1" can be solved based on the method in elasticity.The second sub-problem was: in any plane,there was no applied load,and only contained several columns of continuously distributed edge-type dislocations(There were several cracks will be virtual set up several lines of edge dislocations,their distribution position were consistent with the position of the cracks),and the sub-problem "2" can be solved based on the distributed dislocation method.Further,by superimposing these two sub-problems,we can get a complete solution to the main problem.Finally,the relevant mechanical parameters were obtained,and these parameters were discussed and analyzed in depth.Based on this method,this paper has carried out the following aspects of work:(1)For the problem that the stress on the crack surface cannot be directly integrated under the complex applied load,this paper presented an effective method.By using the Taylor expansion method for Hertz pressure or other complex loads,the stress expression of the crack surface can be deduced by Flamant's solution in elastic mechanics.Further,the polynomial function of the wheel-rail contact force under steady-state rolling was obtained by function fitting,and the stress expression of the crack surface for different wheel weight was deduced.(2)The theoretical model was established by simplifying the rail to a two-dimensional semi-infinite plane.The singular integral equations of several types of crack problems were established by the distributed dislocation method.By " Mathematica9.0 " programming,the numerical solutions of the singular integral equations were obtained.Then,the critical friction coefficient of the local adhesion about the closed crack surface,the stress intensity factor of the crack tip,and the opening crack length of the crack surface under partial tensile stress can be obtained.Further,the effects of load at different positions,the magnitude of the wheel-rail contact force,and different wheel-rail contact mode on the stress strength factor and the length of crack opening were analyzed.Moreover,the effects of crack length,crack embedding depth on the stress intensity factor at the crack tip and the critical friction coefficient of the crack surface were also analyzed.(3)The crack tip has a large stress concentration,so in the crack tip area,the material will undergo significant plastic deformation.In this paper,the continuously distributed dislocations were used to characterize the plastic deformation of the crack tip.A theoretical model containing an edge crack in a semi-infinite plane under uniaxial tension was established.The established model can accurately characterize the relationship between the plastic deformation of the crack tip and the distribution of dislocations in the plastic zone.Further,the variation of plastic zone size and dislocation-free zone size at crack tip was discussed.