Mean stress correction in fatigue design under consideration of welding residual stress

Abstract

The fatigue strength of welded steels is affected by the applied load mean stress and the residual stress in the vicinity of the weld. The mean stress correction in fatigue design concepts used for welded structures commonly distinguishes between three subjective generalized residual stress conditions, “low, medium, and high” tensile residual stress. This qualitative treatment of residual stress leads to imprecise evaluation of residual stress effects, in particular when compressive residual stress is present or high-strength steels are applied. The objectives of the underlying study are to emphasize the interaction of load mean stress with residual stress and to provide an approach for the combined treatment of those stress components in the nominal stress concept. The principles of mean stress and residual stress effects on fatigue are presented and discussed. Furthermore, the role of residual stress relaxation is emphasized and cyclically stabilized local residual stress is combined with mean stress to effective mean stress. The fatigue design concept of local endurance limits and effective mean stress is introduced for the quantitative evaluation of residual and mean stress effects. Finally, the effective mean stress approach is applied to longitudinal stiffeners made from different steel grades containing various residual stress conditions. It is shown how design S-N curves can be adjusted based on quantitative effective mean stress. Finally, an improved bonus factor concept based on effective mean stress is presented, which allows a mean stress correction under consideration of the residual stress condition.