It is well known that tensile residual stresses from welding reduce the fatigue strength of welded joints. Fatigue design guidelines consider residual stress effects in the nominal stress approach by means of a mean stress correction. Fatigue enhancement factors for the manipulation of the design fatigue strength are provided in dependence of the residual stress conditions. However, these guidelines distinguish between "high", "medium" and "low" tensile residual stresses resulting in different mean stress correction factors. Until today no reliable criteria is given to structural engineers which of these three residual stress groups "high", "medium" or "low" apply to a given residual condition.
This work presents a model for the quantitative consideration of any residual stress value in fatigue design. The model was derived from fatigue testing of welded longitudinal stiffeners at different stress ratios and in different residual stress conditions. It considers cyclic residual stress relaxation and can be used for high and low strength construction steels. This work shows that the fatigue strength is a function of the effective mean stress under consideration of cyclically stabilized residual stresses and nominal load mean stresses.
It is commonly known that the fatigue strength of welded structures may be affected by load mean stresses. The current IIW recommendations for fatigue design of welded structures provide fatigue strength design values obtained at high tensile mean stresses, named FAT-classes (Hobbacher, 2009). These values can be used conservatively for all loading conditions but neglect the beneficial effect of lower mean stresses or compressive residual stresses. Residual stresses are rather covered by the simple approach of generally assuming high tensile residual stresses from manufacturing and mounting. Yet a more detailed consideration of residual stresses is possible only qualitatively. Designer must decide whether the structural detail of interest is covered by one of the three given residual stress groups "low", "medium" or "high" tensile residual stresses. The measure for the classification of residual stresses is commonly the yield strength of the material which leads to some specific problems.
