Fatigue Life Evaluation of Welded Joints in OSD for Railway Bridges Considering Welding Residual Stress

An orthotropic steel deck (OSD) has a complicated structure, and its fatigue life is mainly determined by various welding details. Fatigue assessment of deck-to-rib welded joints under long-term train loads is an important concern for engineers. Using the stress range–number of cycles (S-N) curves that are recommended by existing specifications, it is difficult to consider welding residual stress. As a type of initial stress, welding residual stress reduces structural fatigue life by influencing the mean stress of the external cyclic load. However, the effect of mean stress is often overlooked by traditional welding fatigue theory. In this paper, a full-range S-N curve model of steel fatigue resistance integrating welding residual stress is proposed by adopting the Soderberg formula to equivalently transform the stress amplitude and mean stress. The Nanjing Dashengguan Bridge, a six-line railway steel arch bridge and the longest steel arch bridge in the world, is used as an example to demonstrate the fatigue life evaluation procedure of the proposed model in detail. First, the distribution of the welding residual stress of deck-to-rib welded details is obtained based on a refined finite-element model (FEM), and its accuracy is verified experimentally. Second, the partial welded joint model is embedded into the whole multiscale FEM of the steel deck. The structural/real stress spectrum of the welded details under a passenger/freight train is obtained by integrating the coupling effect of the train load stress and residual stress. Finally, fatigue life evaluation of the deck-to-rib welded details of the Dashengguan Bridge is carried out based on the proposed S-N curve model, integrating the residual stress and annual train volume. The results show that (1) the longitudinal residual stress (${\sigma }_Z$) on the top and bottom surfaces of the deck vary with its welding direction respectively (perpendicular or parallel); (2) the maximum value of the real stress under the coupling effect of train load stress and residual stress is less than the yield strength ($f_y$), so the influence of mean stress on the fatigue life of welded joints should still be considered; and (3) the fatigue life calculated by the proposed model is more accurate and conservative than a common standard. In summary, the proposed model provides a basis for determining whether the Dashengguan Bridge can allow freight trains to pass.