Fatigue Strength Assessment Method Study Of Railway Vehicle Frame Welded Joint

With the rapid development of high speed and heavy load process of domestic and international rail transport, the service environment of train bearing structure has been increasingly worse, and the requirements of vehicle load bearing structure’s dynamic performance and reliability are also increasing. Bogie frame is the important bearing structure, which can not only transmit force of parts of the bogie, but also be the connection carrier of other components. Thus, the structure reliability of the welded frame will directly determine whether the railway vehicles can safely operate.The thesis systematically introduces the theory of fatigue strength assessment of weld seam, which includes nominal stress method, structural stress method and local method. The nominal stress method is the simplest and most widely used method of fatigue strength assessment of welded joints in the engineering application. Structural stress method includes surface stress extrapolation method, thickness linearization method and equivalent structural stress method. The local method can be classified into the notch stress assessment, the notch strain assessment and the critical distance method. Besides, a brief introduction of the design codes and standards of weld fatigue assessment have been made. The different characteristics of them are pointed out and the general steps of the fatigue evaluation process of them are described.The structural model of the frame is special treated basing on the fatigue failure mechanism of welded joint and the requirement of DVS1612 standard. Programmed by FORTRAN and APDL language, the quality grade classification of welded joint, the node coordinate information of the weld joint, and the direction of the joint in the whole coordinate system under different load conditions were then obtained from the ANSYS output CDB file. The direction of the different welds were automatic identified and the nominal normal stress and shear stress which associated with the weld orientation were then determined. By means of coordinate transformation, the nominal normal stresses longitudinal and transverse to the weld direction as well as the shear stresses longitudinal to the weld direction are obtained. The material utilization of the normal stresses longitudinal and transverse to the weld direction as well as the shear stresses longitudinal to the weld direction of all weld nodes were calculated, and the combination effect of the three were also investigated.This thesis takes the 160 km/h EMU welded bogie frame as research object, the structural model is special treated based on the principle of DVS1612 standard, and is dispersed in the ANSYS software. In order to obtain high precision stress analysis value, the regular shapes of cuboid or hexahedral elements were used to mesh the sheet structure, and the sheet is dispersed at least 3 layers in the thickness direction. The load cases are determined by standards UIC 615-4 and EN 13749, and the static strength is evaluated by the von_Mises criterion. The fatigue strength of the weld is evaluated by the method that transforms multi-axial stress into uniaxial stress and the DVS1612 method, the material utilization of weld nodes are calculated. The fatigue results of the two methods are compared. The results show as follow:When the fatigue strength of the weld seam is evaluated by the method that transforms multi-axial stress into uniaxial stress and the DVS1612 method, the critical positions tend to be consistent, and the most dangerous position is at the same place; but the material utilization of the DVS1612 method is relatively greater.