Fatigue behaviour of 12-month corroded offshore steel joints under accelerated salt spray exposure: an experimental and numerical analysis

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 the case of offshore support structures, which are exposed to a corrosive environment and include several welded connections, this issue becomes relevant. Hence, in this study, butt- and fillet-welded joints of 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 in a salt spray chamber. Base material specimens were investigated as reference. All specimens were clean blasted and 3D scanned prior to the fatigue 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 greatest reduction was observed for the base material from 282 to 122 N/mm², followed by butt-welded joints from 215 to 147 N/mm², and fillet-welded joints from 168 to 144 N/mm². As the fatigue strengths showed only minor difference after 12 months, an equalization effect can be assumed. The results show that a generalized reduction of the fatigue strength, in accordance with the guidelines, is not appropriate and therefore should be revised for a more accurate design of offshore support structures. Finally, numerical analysis based on 3D scans of the specimens was conducted and compared with the test results.