Research On Crack Initiation Cause And Fatigue Propagation Characteristic Of Wheel Plate Hole For Freight Car

As the main service wheel of the freight car, about 20% 840D wheel, which was equal to 700 thousand, appeared crack around the plate hole, and the number is getting greater. It has affected severely the railway transportation safety. So it has become imperious to clarify the cause of the fatigue crack of 840D wheel for freight car and the safety being used sequentially. Based on the fatigue failure of 840D freight car, the crack initiation condition, crack propagation condition, crack propagation law and the residual life of the crack around plate hole have been studied in the thesis. The initiation and propagation mechanisms of plate-hole crack have been discovered. It has provided a theoretical basis for making a reasonable and scientific inspection periodicity and managerial method.The primary work and remarks of the thesis are as follows:According to abundant investigation and statistic data of the crack around the plate hole, the thesis has analyzed the form and the size distribution of the crack. The relation between the crack percentage and the rim thickness, the service time, the location of plate hole has been concluded. The result determined the wheel geometry factor for studying the crack initiation and propagation.The mechanical property, the hardness, the macrostructure and the chemical constitution of the plate material from the wheel with typical plate-hole crack have been tested. The test result showed if the plate material had defect leading to the crack. By means of metallographic analysis, stereoscan photograph, energy spectrum analysis, the macroscope, microscopic fractography and metallographic structure of the typical crack has been analyzed to indicate the plate-hole crack property, the crack source, the crack propagation direction and the variation rule of the crack tip The analysis results have established a basis for further analyzing the cause and propagation law of the crack.Based on the standard of UIC510-5, the boundary condition under mechanical load has been determined. Applying friction power method, the heat input during braking has been determined. The temperature field under braking thermal loading and the radial stress under the mechanical and thermal braking loads has been analyzed by finite element method. The result provided load condition for evaluating fatigue strength and predicting the life.The fatigue strength of plate hole under mechanical load has been analyzed applying the Haigh-Goodman fatigue limit curve. After converting the zero-tensile stress cycle into symmetrical cycle with Goodman equation, the plate-hole strength under braking thermal load was evaluated by using the S-N curve under symmetrical cycle of plate material. The stress superimposing method was put forward for solving the stress suoerimposition under mechanical and thermal loads. The model of predicting crack life was built by nominal stress method and Miner theorem. The crack life was predicted for evaluating the fatigue strength under integrated effect of the mechanical stress and thermal stress.The mechanical load spectrum and the thermal load spectrum has been determined, and the model for estimating plate-hole crack initiation life was established applying Morrow equation to take into account the influence of the mean stress. The initiation life under the superimposition of the mechanical load and the thermal load was evaluated. The influence of the rim wear and the plate-hole offset toward the rim on the crack initiation life was analyzed, and the load condition making the crack initiate was also determined.The plate-hole crack stress intensity factor and its corresponding variation law with the crack size under the mechanical load and braking thermal load was studied applying the 1/4 node displacement method and the sub-model technique. The influence of the rim wear and the plate-hole position was taken into account. It established a basis for studying the plate-hole fatigue crack propagation law and the rest life.The superimposition method of plate-hole crack stress intensity factor under the joint action of the mechanical load and the thermal braking load was put forward. The plate-hole fatigue crack propagation model was established applying the Forman equation to take the influence of stress ratio into account. The crack propagation law under different load was studied to determine the propagation load condition. The crack propagation life of middling rim wear wheel with normal plate-hole and plate-hole offset toward rim was calculated. The evaluating result was in agreement with the tracking test result basically. It provided a theoretical and trial basis for judging the wheel being used continually or not.