Study On Dynamic Expansion Of Wheel-rail Rolling Contact Fatigue Crack

With the increase of train speed and the increase of axle load,the rolling contact fatigue between wheel and rail has gradually become one of the main forms of rail damage at home and abroad.If the treatment is not timely,fatigue crack will continue to expand and eventually lead to break.It is very important to study the initiation and propagation of fatigue crack on the surface of rail.With the development of computer technology,some scholars has studied the expansion of surface crack behavior based on Abaqus and Ansys and other finite element software in the past few decades,and some other scholars studied the fatigue life of rails based on Sympack and other dynamic software,but most of them tried to establish a static or quasi-static model to study two-dimensional crack problems.Because of the shortcomings of previous scholars' research,this paper uses the virtual crack closure technique to study the transient propagation behavior of 3D crack,which can support the mechanism of rolling contact fatigue and optimize its control measures.In order to study the transient propagation behavior of three-dimensional crack,this paper establishes a three-dimensional wheel-rail transient contact model considering the fatigue crack on the rail surface by using ANSYS-LS/DYNA,which is based on a certain high-speed transmission line in China.The geometric profile of the wheel and rail can be taken into account,and the contact between the wheel and the track and the crack surface is defined by the "face-to-face"contact algorithm,and the tangential contact satisfies the Coulomb's friction law.In this paper,the modle can be used to solve the law between the wheel-rail and the crack surface by setting the"face-to-surface" contact.It can also be used to get the transient rolling contact response and the wheel-rail contact spot "sticky-slip" distribution.And the impact behavior of the wheel and the crack tip stress field is analyzed emphatically.In this paper,the crack tip stress field strength factor is calculated by using the virtual crack closure technique,which does not consider the gap between the crack surfaces,and the discrete nodes on the two crack surfaces are completely coincident.In this paper,the calculation process is mainly divided into two steps.First,the strain energy release rate is calculated based on the crack tip force and the displacement difference between the joints near the crack tip.Then,the K??K?and Km can be calculated based on the relationship between the strain release rate and the stress field strength factor.Since the three-dimensional crack is established in the model,the crack tip is composed of some discrete nodes,so the distribution of the crack tip stress factor along the crack tip can be studied.In order to verify the necessity of the model,a static model with three-dimensional crack is established.The static model is created by moving the wheels to different positions on the surface of the rail.By this way we can obtain the contact between the wheel and the track and the SIF.At the same time,the dynamic model is used to obtain the results under the same crack size.The results of static calculation are compared with the dynamic.Finally,it is found that the results obtained by using the static model are lower than the dynamic,mainly due to the neglect of the transient impact of the wheel when it passes through the crack in the static crack model.After the analysis of the model,this paper considers a simple model,which is the three-dimensional wheel-rail transient contact model with straight crack on the surface of the rail.After considering the friction coefficient of the crack in the traction state,the adhesion coefficient between the wheel and the rail,the influence of crack depth and train speed,we can get the following conclusions:The straight crack is closed and the K? value is 0 in the wheel rolling condition.When the friction coefficient of the crack surface changes from 0 to 0.5,|K?|max is reduced by 20.66%,and |K?|max,is reduced by 18.53%.When the wheel-rail adhesion coefficient varies between 0 and 0.5,|K?|max,changes 3.98%,and |K?|max,decreases by 20.8%.When the crack depth increases from 0.5mm to 3mm,|K?|max,increases by 30.88%,and |K?|max,decreases by 20.06%.When the train speed is in the range of 250-3 50km/h,the contact force between the wheel and rail and the crack surface and the strength factor of the crack tip stress do not change.In the actual field investigation,there are few cases of straight crack,and most of them are at different angles with the rail surface.Therefore,in order to fit the actual situation,this paper establishes a three-dimensional transient wheel-rail rolling contact model with oblique cracks on the rail surface.The influence of oblique crack angle,oblique crack length and traction coefficient on the transient propagation behavior of the oblique crack on the rail surface is considered in the traction state when the wheel and the track are linear elastic materials.Based on the obtained results,the following conclusions can be drawn:under different inclination,the oblique crack is in the closed state,and the K? value is close to O.In the studied four angles,after considering the equivalent stress field strength factor Keq as the evaluation criteria,the value of Keq will reduce first and then increase,and it reached the maximum at ?=30°,and it reached the minimum at?=75°,the difference between the two is 25.18%.When the traction coefficient changes from 0 to 0.4,the crack will open,the amplitudes of the intensity factors |K?|max,and |K?|max,are increased by 48.81%and 25.76%,respectively.Finally,after considering the transient propagation behavior of straight crack and oblique crack,this paper summarizes the contents of the research and prospects the future research based on the existing results.