Study On The Fatigue Life Of Welded Bogie Frame Based On Notch Theory

Under the overall background of high-speed and heavy-loading development of railway vehicles in China, the welding structure of vehicles is faced with the double tests of safety and light weight, thus the relevant problem of anti-fatigue design has been increasingly prominent. How to solve the conflict between structural safety and structural light weight has already become an important research topic. The key of welding structure's anti-fatigue design lies in reliable testing data and scientific and reasonable analysis methods.At present, Nominal stress method and structural stress method based on the overall design thoughts now still have lots of defects. For example, due to neglect structural influences of welding details, it is difficult to confirm nominal stress parameters and to match with the typical welded joints accurately. The structural stress method has the strong mesh-sensitivity at the notch of weld toe and weld root, which is easy to result in singularity of local stress and is impossible to do fatigue analysis at the weld root. In the paper, the author aims at analyzing local stress features of welding structure and introduces the more accurately and reliably equivalent notch stress method. By studying and discussing theoretical basis of the local method and engineering practice problems, it is finally applied to life prediction of vehicle welding structure reasonably, so as to improve accuracy of anti-fatigue design. In the paper, the author discusses the following aspects:1. Based on basic equation of Filippi notch mechanics and constraint effects of micro-structure in notch areas, the author deduces a general equation of fictitious notch radius under the plane stretch and shear models, studies notch constraint factors under different notch angles and root curve radius, and value of fictitious circle radius, etc., and elaborates feasibility principle of applying 1mm radius at weld notch in engineering application.2. Based on butt joints, T joints under the plane strain conditions, and 3D tube-plate joints, the author studies influence disciplines of local variables, such as plate thickness,angle of weld toe and throat depth, etc., on notch stress concentration factors under tensile,bending and torsion, and applies finite element method to calculate notch stress concentration factors on various testing welded joints.3. By combining uniaxial nominal stress fatigue test data of welded joints, such as S355J2, SS800 and 16Mn and corresponding notch stress concentration factor, the author applies least square method to establish the recommended notch fatigue life curve suitable for evaluating bogie welding structure and modifies the curve by adding influences of mean stress.4. According to multi-axial fatigue test data of different notch classes welded joints,the author applies multiaxial fatigue criterion, such as Von Mises, IIW and EESH, to establish the multi-axial notch fatigue life curve suitable for assessing multiple welded joints.5. Based on the systematic analysis methods of normal stress method, structural stress method and notch stress method, the author establishes three strength calculation models of welded box girder, applies three methods to calculate fatigue stress and accumulated damage of main weld. Calculation results indicate that the notch stress method has the obvious accuracy and reliable advantages in fatigue evaluation and optimal design of vehicle welded structure.6. The author establishes the overall model of frame structure and notch stress model of local welded joints, calculates accumulated damage in weak fatigue areas for frame's multiple welds based on bogie fatigue test's loading spectrum and the corresponding fatigue life curve, and further predicts notch fatigue lifetime of vertical damper seat based on random variable amplitude loading spectrum.