Assessment of fatigue strength of steel butt-welded joints in as-welded condition – Alternative approaches for curve fitting and mean stress effect analysis

• Validation of the notch stress fatigue strength of butt-welded joints.
• Re-analysis of the fatigue data using two different methods.
• The accuracy of fatigue strength predictions.
• The effect of applied stress ratio – linear behavior.
• The effect of applied stress ratio and residual stresses – local stress ratio.

Experimental fatigue data for butt-welded joints in as-welded condition and under constant amplitude tensile loading (secondary bending included) were analyzed using the nominal stress system and the notch stress system. Two approaches were used; a standard fitting procedure and minimization of the sum of squared perpendicular distances from a line with a fixed and free slope. In all cases, the latter method gave better agreement between the experimental and predicted fatigue life and fatigue strength. The analyses showed both with all broken specimen data included and with reduced data that the FAT225 curve, as recommended by IIW, might be too optimistic for the notch stress approach in the case of butt-welded joints in as-welded condition. It was also found that use of the local stress ratio instead of the applied stress ratio might explain many issues concerning current observations and apparent inconsistencies in reported literature.