The effect of mean stress is positive for alternating load. For R > 0 the fatigue strength is reduced for parent material. For welded joints is considered as independent of the R-value.
The tensile residual stresses are as high as the yield strength for welded joints is the common design assumption. Even if there in theory is significant effects on e.g. crack closure.
The stress alternation factor, for different stress ratios, describes the effect of residual stresses. Note that the stress alternation factor is to be set to 1,0 if the residual stresses are above approximately 50 MPa. Otherwise the values are dependent of the R-value and if the design is in parent material or if it is welded.
When a part of the load cycle is compressive stress relief treatment gives significant increase in fatigue strength, crack closure reduces the effective stress range when the residual stresses are removed. Stress relief treatment to a welded structure does not have any effect on fatigue life unless some part of the stress cycle is compressive. The stress alternation factor is used to consider this. Note also that for post treated welded joints stress relief does not improve the fatigue strength.
Picture 1: Stress alternation factor