Research On Fatigue Strength Analysis Method For Welding Structure Of Car-body Of Railway Freight Car

The Car-body of railway freight car is subjected to a variety of alternating loads,and its welding area shows significant multiaxial stress characteristics.The traditional fatigue strength assessment is usually carried out by the method recommended by AAR M-1001,which requires the multiaxial stress based on the maximum principal stress criterion to be equivalent to the uniaxial stress.Along with the freight car running speed and axle load increases continuously,applying this method to welded structures subjected to multiaxial variable amplitude loading will make the analysis results biased towards danger,so the existing welded structures fatigue strength analysis methods of the Car-body of railway freight car are investigated.It has important engineering application value.According to the method recommended by IIW and FKM,the multiaxial fatigue strength analysis method of welded structure is constructed by taking into account the influence of the variable amplitude stress cycles and the non-proportional loads,it is based on the determination of stresses in local coordinate systems.Firstly,the weld beam element and the weld reference beam elenment are established in the weld area;Then,according to the corresponding position of the node of the weld beam element and the node of the weld reference beam element,the direction cosine of each coordinate axis of the local coordinate system under the overall coordinate system is calculated;Finally,the local coordinate system and the nodal stress components under the local coordinate system are determined.Two methods both take into account the influence of the variable amplitude stress cycles,and introduce the critical cumulative damage under the variable amplitude to calculate the equivalent constant amplitude stress of each stress component.The difference is that the IIW method requires linear superposition method to obtain the nodal stress time history on the basis of the time history and the stress response of the structure under various loads.Finally,the equivalent constant amplitude stress under various loads is calculated and the non-proportional loading critical cumulative damage are introduced for analysis.The FKM method calculates the Do U of each stress component under various loads when the loads are independent with time,and evaluates the structural fatigue strength by linearly adding the Do U under various loads.Comparing the calculation results of different methods,there are differences in the analysis results between the two methods due to different strength theories,differences in S-N curves,and differences in non-proportional loading methods.The error of the calculation results under different stress systems is small.Evaluation using a multiaxial stress method will yield conservative results.In this paper,the fatigue strength of welded structures is analyzed according to the equivalent structural stress method.Taking the C_70E gondola as the object,the calculation process of calculating the structural stress by the nodal force method based on the shell element is determined.Firstly,the local coordinate system is defined and the coordinate transformation matrix is established to transform the nodal forces in the global coordinate system into the local coordinate system.Then,the transformation matrix of the nodal force and the linear force is established and the structural stress is calculated after converting the nodal force into a linear force.Considering the effect of load mode and plate thickness,the structural stress is transformed into equivalent structural stress range based on the load spectrum given by AAR M-1001,and the fatigue analysis is carried out.Finite element models of angular joints and lap joints were established to verify the effect of mesh size on structural stress.The fatigue strength of key welds of C_70E Gondola carbody is analyzed,and the results show that the Do U is less than 1.The results calculated by the equivalent structural stress method are smaller than those calculated by the multiaxial stress method,and analysis results are biased toward danger.Based on BP neural network and genetic algorithm,the fatigue strength of weld joint between upper cover and floor of sleeper beam of C_70E Gondola carbody was optimized.Firstly,the orthogonal test method is used to determine the test samples.Secondly,BP neural network is used for fitting.Finally,the optimization structure of pillow beam is determined by genetic algorithm.The Do U of the optimized structural fatigue strength is effectively reduced.