Fatigue Reliability Analysis And Optimization Of Three-Axle Bogie Frame

In recent years,with the continuous development of China’s railway freight toward the two goals of heavy-haul and fast,how to improve the carrying capacity of trains and ensure their safety has become an unavoidable problem in the process of railway construction.Compared with the two-axle bogie which is widely used at present,the three-axle bogie frame has increased the existence of the intermediate shaft,which has stronger bearing capacity and more complex and changeable stress condition.It bears the interaction of various vertical forces,longitudinal forces and transverse forces between the car body and the wheelset,and plays a vital role in the running quality of the train.Therefore,it is necessary to study and optimize it based on finite element analysis.In this paper,the freight three axle bogie frame is taken as the research object,the analysis for structure strength,weld fatigue life and reliability are carried out firstly based on the related standard,and then the lightweight design is achieved,and the influence of rib plates position and weld leg size error for the weld fatigue life are carried out finally.The structural strength of the frame and the fatigue strength of the base metal are checked based on the fourth strength theory and fatigue limit method.An analysis document for fatigue analysis was prepared by APDL language.The fatigue life of the main welds in the steel structures was evaluated and analyzed with the document by using the nominal stress method based on the British standard BS7608 for fatigue design and evaluation of steel structures.The results show that the strength of the frame and the fatigue life of the weld seam meet the design requirements and have a certain safety margin.The limit state equation for two kinds of failure modes are established by consider of the influence of dispersion characteristics for the analysis results such as load and geometry size,and then the reliability and sensitivity analysis for the static strength and the fatigue life are carried out by using the Monte Carlo method,and the variables,which are significant to failure modes,are determined.The lightweight model with the goal of minimizing the frame mass is established with the appropriate optimization variables are selected according to the results of sensitivity analysis,and the constraint of the reliability of the frame strength and fatigue life.DOE method was used to fit the response surface model of the target and constraint,and the fitting accuracy of the model was gradually improved by reducing the design space.The rationality of the selected design variables was verified by ANOVA.The size optimization of the framework is carried out under the premise of ensuring the accuracy of the response surface model.After the optimization,the thickness of each plate of the framework was reduced to different degrees.Finally,the weight of the structure was reduced by 188.6kg,accounting for4.3% of the total mass,and the lightweight design of the structure is achieved.The optimized frame is taken as the research object,the common problems of rib dislocation and welding foot size error in the process of welding and finite element modeling are discussed.A sub-model of the key weld structure is established based on the fatigue analysis results,and the influence of the position of the stiffened plate and the size of the weld foot on the fatigue life of the hazardous weld was analyzed respectively by using the nominal stress method and the equivalent structural stress method.The analysis results show that the displacement of the frame rib plate has little effect on the fatigue life of the welding seam,but the change of the size of the welding foot has a significant effect on the fatigue life of the welding seam,which mainly shows that the fatigue life of the welding seam decreases monotonically with the increase of the size of the welding foot.It is also proved that the equivalent structural stress method is more suitable than the nominal stress method for the analysis of local welds in a small range.In this work,reliability analysis is introduced into the fatigue life evaluation and lightweight design of the frame,and specific analysis and suggestions are given for the influence of the errors easily ignored in the welding process on the fatigue life of the weld seam.The analysis results are more reasonable,which has important engineering significance for the fatigue evaluation and optimization of similar welded structures.