Residual Stress-Based Fatigue Design of Welded Structures

Fatigue design concepts for welded structures generally consider residual stresses as a factor affecting the mean stress influence. Residual stresses are mostly interpreted as mean stresses. In addition, high tensile residual stresses are conservatively assumed, resulting in maximum effective load stresses from fatigue loading in the order of the yield strength. The consequence of this is that additional static load stresses may have no influence on the resulting fatigue strength because the local effective mean stress (residual stresses and load mean stresses) is always high. The related evaluation concepts neither distinguish between different steel grades nor between different origins and amounts of possible residual stresses in welded joints. The real magnitude of existing residual stresses is also usually not considered, because in practice, usually no explicit knowledge of the residual stresses at critical sites of a construction is available because residual stress measurements are not state of the art in welding practice. For an explicit consideration of residual stresses in design concepts, the sign, the initial amount, and especially the amount of the residual stresses after a load induced relaxation must be considered. Therefore, the steel grade and the condition of the material are of great importance, as well as the local stress condition influenced by welding-induced notch geometry. The article shall give an overview about the state of the art of consideration of residual stresses in fatigue design concepts for welded structures and the background of their development. Finally, a new approach shall offer a possibility for an enhanced consideration of residual stresses in design concepts based on the explicit knowledge about the effective residual stresses that can actually be observed with different measurement concepts.