Experiments And Theoretical Models On Ratcheting-fatigue Interaction Of Wheel And Rail Steels

The development of railway transportation is in a new and vigorous time in China at present.The wheel-rail service environment is changing more and more rigorous with the increasing of vehicle axle load,traffic density and the total weight of transit.In previous studies,experiment and modeling were carried out only in one kind of material of wheel or rail steel,few investigations on wheel and rail steels were discussed simultaneously.It is necessary to carried out experimental investigations on ratcheting-fatigue interaction of U75V rail steel and ER7 wheel steel firstly.And then the cyclic features and the effect of ratcheting-fatigue interaction on the fatigue life was obtained.Moreover,a cyclic plastic constitutive model was established to accurately describe cyclic features and ratcheting behavior,and finally a fatigue failure model was constructed to reasonably predict fatigue life of U75V rail steel and ER7 wheel steel.Research results can not only be used to assess the fatigue failure of wheel and rail steels,but also provide important means of evaluation for the performances of wheel and rail materials in service.The main contents of this work are summarized as follows:1)Experiments of U75V rail steel and ER7 wheel steel were carried out to investigate the cyclic features,ratcheting behavior and low-cycle fatigue under both strain-and stress-controlled loadings at room temperature.It was found that the U75V rail steel shows strain amplitude dependent cyclic softening feature and reaches a stable value after about 80th cycle with the increasing number of cycles,and it is easier to reach the stable state as strain amplitude is lower.The mean stress,stress amplitude and stress ratio have a significant influence on the ratcheting behavior of materials,and the ratcheting strain increases rapidly and then reaches a quasi-shakedown state.The ratcheting strain rate can affect fatigue life of rail steel remarkably.The ER7 wheel steel shows strain amplitude dependent cyclic softening feature with the increasing number of cycles and reaches a stable value after about 30th cycle when the strain amplitude is less than 0.6%.However,it shows cyclic hardening feature but immediately reaches a stable value when the strain amplitude is beyond 0.6%.2)To describe the cyclic plastic deformations of U75V rail steel and ER7 wheel steel quantitatively,a strain amplitude dependent cyclic plastic constitutive model was proposed based on existed elastic-plastic constitutive model,an evolution equation of saturated stress amplitude related to the accumulated plastic strain was introduced into the isotropic softening rule,and a new nonlinear kinematic hardening rule was established by introducing an ratcheting parameter depending on accumulated plastic strain to control the initial accelerated and then quasi-shakedown states of ratcheting evolution.Based on the derivations of implicit stress integration and the consistency tangential stiffness matrix,the cyclic plastic constitutive model was implemented into finite element software to simulate the cyclic features and ratcheting behavior of U75V rail steel and ER7 wheel steel.The comparisons of experimental and simulated results of U75V rail steel and ER7 wheel steel show that the proposed model can predict the strain amplitude dependent cyclic softening and ratcheting behaviors well.3)A new fatigue failure model considering the effect of both strain energy density and ratcheting strain rate was proposed based on evaluations of fatigue failure life prediction models.The proposed fatigue failure model was used to predict fatigue lives of U75V rail steel and ER7 wheel steel.Results show that the proposed model can predict fatigue lives of two materials well.