Study Of Fatigue Crack Initiation And Propagation Of Railroad Wheel Under Rolling Contact

Fatigue damage of railroad wheels under rolling contact continues to be a basic issue in rail transportation.As the main fatigue damage of railroad wheel,rolling contact fatigue results in shattered rim and tread peeling.These fatigues pose significant safety issues and are difficult to prevent.Thus both failures have been the focus of intensive wheel and rail failure study.Due to the increasing engineering requirements for service reliability of metal materials used in wheel/rail systems,it is important to develop an enhanced understanding of these failure types.In the study of this paper,the mechanisms about crack initiation and growth in railroad wheels under rolling contact loading was studied by experimental and simulation methods.Firstly,the failure mechanisms about the fatigue damage of railroad wheel was studied by the analysis of materials and fracture appearance shows.The fracture appearance of shattered rim and tread peeling have obvious fatigue features and the microstructure of fatigue crack have clearly changed.The ultra-high cycle fatigue behavior of ER8 wheel steel were studied with the ultrasonic fatigue testing and the failure mechanism was further analyzed by comparing with the microstructure and fracture appearance between contact fatigue damage and ultrasonic fatigue testing.It was found that the service mileage of a railroad wheel which have developed shattered rim and tread peeling is usually between 200 and 1200 thousangd kilometers and that numbers of cycles is in excess of 107 times and was established by analyzing recent the railroad wheel failure cases involving shattered rim and tread peeling.It was demonstrated that the fatigue crack initiation depth of peeling is less than 10 mm from the tread,and shattered rim between 10 and 25 mm from the tread.A three-dimensional elastoplastic finite element model of wheel-rail rolling contact was established by ABAQUS software to analyze the contact stress distribution of the railroad wheel and study the influence of brittle inclusion and cavity defect inside subsurface of the railroad wheel on contact stress.The simulation results show that the maximum rolling contact stress develops in the shallow surface layer of the railroad wheel and the stress concentration occurs of brittle inclusions and cavity defects in the subsurface under the rolling contact load,but the stress concentration position is different.When the size of brittle inclusions and cavity defects increases and is also close to the wheel-rail contact point,the stress concentration value will increase.Combined with the test results of wheel fatigue damage failure,the fatigue crack initiation mechanism of shattered rim and tread peeling is elucidated.Based on finite element model using FRANC3D,three-dimensional crack propagation model was established on basis of the ultra-high cycle fatigue and fracture mechanics and the results of railroad wheel fatigue damage failure analysis.The rolling contact fatigue crack propagation mechanism and the railroad wheel fatigue damage remaining life are analyzed and the influence of initial crack size,location and shape on fatigue crack life are considered.The results of crack propagation calculation show that the fatigue crack propagation under rolling contact load is dominated by type ?.The initial crack propagation period occupies the main stage of fatigue crack propagation life and the initial crack size has a great influence on fatigue crack growth life.In addition,the position and the surface area of the initial crack are also an important influence on fatigue crack growth life.