Study On Fatigue Reliability Of Welded Bogie Frame For Railway Vehicle

In order to decrease the weight of railway passenger vehicles, the welded steel structures are used for the bogies in our country. However,with the raising of operation speed,and increasing of loads and frequency ranges, and especially the complicated types of weld joints, the load conditions of railway bogies become even worse. Because of the exsitence of weld imperfections of weld joints, such as geometry irregularity, porosity, welding cracks, residual stress, deformation and the changing of material characteristic in heat affected zones, the weld joints are turned to be the main areas of structure failure. In the previous vehicle operation, the fatigue problems of welded bogies have occurred as the increasing of speed.Therefore, higher safety and reliability of bogies in operation are required. In this paper, the fatigue relability of welded bogies are carefully studied and the work is as follows:(1) From the viewpoint of integration of design and manufacture, the formation mechanisms of weld imperfection have been studied. The thermo-elastic-plastic FEM and inherent strain methods have been applied to predict the welding deformation of T-joint, also the comparative analysis between two methods has been carried out. Meanwhile, the bogie welding deformation is simulatied by using the inherent strain method. To assure the bogie fatigue strength and relabiltiy, the measures of eliminating welding imperfection are proposed.(2) Based on the theory of multi-axial fatigue, the bogie stress states under standard loads are analyzed. Meanwhile, three methods (maximum principal stress, Mises equivalent average stress and Sines average principal stress) are used to transform the multi-axial stress to uniaxial stress. The fatigue strength is evaluated and compared based on the Goodman curves recommended in ERRI B 12/RP 17 and JIS E4207. It is concluded that the method of maximum principal stress is suitable to analyze the multiaxial fatigue strength of bogie frame.(3) Rigid-flexible coupling multi-body dynamics has been used to build the bogie frame model of a passenger car. Firstly the flexible model of bogie is built in FEM software, and then the entire vehicle dynamics model is established in multi-body dynamics software. In the rigid-flexible coupling model, the bogie frame is built as flexible body, and other parts are taken as rigid. The dynamical response of bogie is obtained by time integation calculation. Based on the load time history of bogie, several methods which convert the load spectrum to stress spectrum are investigated. Meanwhile, the stress results of transient kinetic analysis and polynomial fitting method are compared, and also their deviation is analzed. At last the polynomial fitting method is selected to calculate the stress spectrum of fatigue life. Based on WAFO rain flow skill, Miner damage theory, nomial stress S-N curve of welding joint recommended by IIW and crack propagation curve of BS7910 standard, the fatigue life and crack propagation life of bogie frame are analyzed. The results show that the life of bogie can satisfy the requirement of design.(4) Based on the dynamic design methodology, the modal analysis method is used to research the fatigue strength problems which caused by the vibration of structure. The modal superpostion method is applied to analyze the influence of structure modal on the fatigue life. According to the analysis, the dynamic performance and fatigue relablity of bogie can be improved.(5) The reliability of railway vehicle bogie is affected by lots of factors. Therefore, finding out the main factor is very important to assure the reliability of bogie. In this paper, the parametric finite element model of a bogie frame is established by using the probabilistic analysis method.The material properties, geometric parameters and loads of the bogie frames are taken as the random variables and their distributed parameter values are obtained through statistical analysis. The failure state functions of the fatigue strength reliability are established by the allowed fatigue strength value and the maximum principal stress of the parent material or weld in fatigue weak location of welded bogie frame. The reliability and parametic sensitivities of random variables are computed by Monte Carlo method, which provides the reference for the bogie design.(6) The accelerated life testing methods are proposed to solve the problems of large cost and long testing period for high-speed train reliability tests. Meanwhile, the whole system life is predicted by sub-system which is verified by bogie frame experiments. In addition, the fatigue life of bogie frame is predicted by failure model of welded joints. Finally the fatigue experiments of bogie and some welded joints are carried out, and the accleration factor of fatigue experiment is calculated.