Analysis On Rolling Contact Fatigue Of Heavy Haul Rail U78CrV Considering Ratcheting Behavior

With the rapid economic development in China at present,the transportation of bulk goods and energy supplies has been becoming increasingly frequent,and the heavy haul railways in our country are faced with increasingly more and more challenges and pressures.The rail service environment is changing more and more rigorous with the increasing of vehicle axle load,traffic density,running speed and the total weight of transit.To solve this problem,it is necessary to select the main rail material U78 CrV from the heavy haul railways in China at present,and carried out experimental investigations on cyclic features,ratcheting behavior and fatigue life of it.A series of the mechanical parameters could be identified based on the results of the experiments and the improved Abdel-Karim-Ohno model,and this improved constitutive model was incorporated into ABAQUS finite element method(FEM)code as a user material subroutine to simulate the experiment results which could verify this constitutive model of rationality.Meanwhile,several common fatigue failure methods were selected to predict the fatigue lives of U78 CrV rail steel,and the prediction results and the experiment results were compared to choose out the best fatigue failure method.At last,the 3D rolling contact rail model was established,besides,the UMAT and Dload were used to define the material properties of the rail and the hertz contact pressure filed distribution.The effects of axle loads and rolling times to the stress filed,the equivalent plastic strain and the shape of the hertz contact spot were discussed,and the fatigue failure position could be predicted at last.These simulation results could provide important references to ensure the service safety and prolong the life of the rail.The main researches of this thesis are listed as follows:(1)Low cycle fatigue tests of heavy haul rail steel U78 CrV under single tension,strain and stress control cycles at room temperature were carried out.The results of strain cycles experiments showed that U78 CrV exhibit obvious cyclic softening characteristics at an initial period of about 100 cycles,and the softening degree increased with the increase of strain amplitude.Stress cycles experiments showed thatthe ratcheting behaviors of materials were closely related to average stress,stress amplitude,stress ratio and the loading history of high stress amplitude,and the ratcheting strain of the materials accumulated at the initial period of 100 cycles,but the ratcheting strain rate would become to a steady sooner and the strain would grow linear until fatigue failure.The fatigue life of materials under loading conditions was shortened by the increase of strain amplitude under the strain cycles,while the fatigue life of materials was shortened by the increase of stress amplitude and preloading cycles of high stress amplitude under stress cycles.The effects of stress ratio and average stress on the fatigue life of materials were not single positive correlation.(2)The mechanical parameters determined by the cyclic experiments results and the UMAT were combined to make the improved Ohno-Abdel-Karim constitutive model transplanted into the ABAQUS.The processes of uniaxial tension experiment and cyclic deformation experiments of U78 CrV were simulated,and the rationality of this improved constitutive model was verified by comparing the results of the simulated and experimental curves.(3)The fatigue lives of U78 CrV under different working conditions was predicted by using several existing fatigue failure models.The predicted results and experimental results were processed by linear regression,and the predicted effects of these models were evaluated systematically.The results showed that SWT parameter method model and Manson-Coffin model couldn't predict the experimental results well,while energy method model and Morrow model could predict the fatigue lives of U78 CrV well.(4)A three-dimensional rail model was established,and the effects of axle loads and cycles on the stress field,equivalent plastic strain and contact spot shape of75kg/m rail were discussed.The results showed that under a certain friction coefficient and creep rate,the increase of the axle loads and the number of load cycles would significantly increase the equivalent stress and the equivalent plastic strain of the rail,and the increase of the axle loads would significantly increase the area of the contact spot also.The location of the equivalent stress and the equivalent plastic strain of the rail both occured in the sub-surface layer of the rail.The position of themaximum equivalent plastic strain would move downward and the equivalent plastic strain accelerates to accumulate with the increase of the number of loading cycles.By discussing the change rules of the stress field and the equivalent plastic strain,the location of the fatigue crack initiation and the fatigue life can be reasonably inferred and predicted.