Derivation of design S-N curves for butt welds in support structures for wind turbines

The fatigue life of welded connections in structures is normally calculated from bi-linear S-N curves presented in design standards, one part S-N curve for the largest stress ranges that can be determined from constant amplitude testing to the left of the fatigue limit and one part S-N curve for stress ranges lower than the fatigue limit that can be determined from variable amplitude fatigue testing. The second curve is governing the calculated fatigue damage for typical long-term stress distributions for marine structures subjected to cyclic loads from wind and waves. In this study fracture mechanics analyses are performed in a relative way to reduce much of the uncertainties present in such analyses to estimate a second part of the S-N curve as function of fatigue crack growth parameters, threshold stress intensity factor, geometry functions and the long-term stress range distribution. The analysis methodology is calibrated to fatigue test data from constant amplitude loading. Based on the performed analyses it is shown that the second part of the design S-N curve for as-welded butt welds can be lifted to a higher level than presented in design standards of today. For structures supporting wind turbines where the Fatigue Limit State is governing design, this may result in reduction in steel weight as compared with existing design S-N curves in standards. This information may also be useful for lifetime extension of existing structures.