Research On Fatigue Life Prediction Of Ring Welding Joints

The application of stainless steel carbody on the urban rail vehicle is promoted by the development of modern welding technology. New welding methods such as ring welding, plug welding and fillet welding are widely used in the manufacturing of carbody. The vehicle was subjected to a variety of alternating loads during operation and fatigue failure is most likely to occur in the weld joint of carbody, therefore, accurate assessment of the fatigue life of the joint is the foundation to ensure the operation safety of vehicle. The traditional nominal stress method is not applicable to the fatigue estimation of those new welded joints for the limit of joint type and load form, while conducting of fatigue test would be a time consuming and costly process.Aiming at this engineering problem, two advanced fatigue analysis methods, the master S-N curve method and fatigue crack propagation method were proposed for the fatigue life analysis of ring welded joint in this study. Firstly, the structural stress under different fatigue failure mode was calculated and fatigue life was predicted based on the corresponding master S-N curve. Secondly, a comparative analysis between the life prediction result and experimental result was conducted. Finally, the fatigue crack propagation analysis was conducted using the fracture mechanics, and a simplified fatigue crack propagation life calculation method based on the Paris law was proposed.Research shows that structural stress calculation and life prediction based on weld root failure provides higher accuracy for the welded joint failing at weld root failure and weld toe. The mesh of hotpot should be as fine as possible since the evaluation of weld root failure using structural stress method requires a relatively high meshing quality. The fatigue life prediction result by the master S-N curve method with a relatively complex structural stress calculation process provides an accurate result, thus, this method is recommended in the fatigue estimation of structure which requires high fatigue safety. The crack propagation analysis process in ring weld indicates that the crack shape tends to stable during the crack propagation, so the crack propagation simulation using a simplified one freedom degree model would not introduce too much error. The simplified fatigue crack propagation life calculation method proposed in this study shows a relatively low accuracy compared with the master S-N method, but it offers a rapid and relatively simple engineering evaluation method for the calculation process.