Fatigue analysis of corroded welded offshore steel joints.

Abstract

The fatigue strength of steel structures can decrease significantly when corrosion occurs. Pitting corrosion in particular can lead to locally high stress concentrations and may interact with existing stress concentrations from weld seams. Particularly in case of offshore support structures, which are exposed to corrosive environment and include several welded connections, this issue becomes relevant. Hence, in this study, butt- and fillet welded joints of S355 ML structural steel were exposed to accelerated corrosion in a salt spray chamber and then tested for fatigue strength. In order to investigate the long-term behaviour, the specimens were stored for 12 months. Additionally, base material specimens were investigated as reference. All specimens were clean-blasted accord- ing to the requirements for offshore steel structures. Prior to the fatigue tests, the specimens were 3D-scanned to investigate the stress concentrations on the basis of 3D-models. In addition, the fatigue tests were compared with the results of one-month corroded specimens from previous tests. It was shown for all specimens that the fatigue strength decreased after 12 months compared to the uncorroded reference tests. However, the fatigue reduction was different for the different geometries. The highest reduction was observed for the base material, followed by butt-welded joints. The smallest reduction was observed for fillet-welded joints. It can be concluded, that higher notch sharpness in the uncorroded state lead to lower re- duction after corrosion exposure. A joint evaluation of all specimen geometries showed that there is almost no difference between the geometries, thus an equalization effect can be assumed. The comparison of the crack locations with the numerical prediction showed a match for approx. 60% for welded specimens, which confirms the influence of the notch stress concentrations. The results show that a generalized reduction of the fatigue strength, without consideration of the specimen geometry (notch class) according to the guidelines, is not ap- propriate and therefore should be revised for a more accurate design of offshore support structures