Welded joints exhibit fatigue failure potential from weld geometry and characteristics of the heat affected zone. In order to counteract fatigue, structures and components require larger thicknesses resulting in heavier designs exhausting the finite natural resources. We hereby introduce a novel post-weld treatment, which postpones or even prevents fatigue failure of the welded connection. A Cu/Ni nanostructured metallic multilayer (NMM) is applied via electrodeposition and a 300 % - 600 % increase in usable lifetime compared to the untreated weld is observed. A FAT class 190 with a slope of k = 6 is proposed for the design of NMM treated butt welds.

Fatigue tests of welded S355 J2 steel type-E specimen according to DIN 50125, comparing the NMM treated welds (Fig.1d) to untreated (Fig.1a) and HFMI post-weld treated welds (Fig.1c), reveal a superior fatigue strength. The S-N curve of the NMM treated weld corresponds to a FAT class 190.

One example of a steel structure subjected to cyclic loading are support structures of offshore wind turbines, where fatigue is caused by water waves, wind and blade rotation. The most common foundation used is the monopile. Monopiles are being built in wind parks around the world to secure sustainable energy production. The design of monopiles is highly governed by fatigue. Wall thicknesses of up to 150 mm are necessary because of fatigue criticality at the circumferential welds which hold the monopile segments together. Fatigue limits the lifetime of the support structure to approximately 25 years. Decommission of monopiles and re-erection are detrimental to the sea flora and fauna. Without the fatigue considerations in the design process, the monopile could be designed substantially thinner, which is investigated herein. Thereby leading to beneficial implications for the carbon footprint, the manufacturing process and the service lifetime of offshore wind turbines.

A case study investigates NMM treatment of all circumferential welds of a 15 MW reference wind turbine monopile foundation, assessed in a hydrodynamic and aeroelastic finite element analysis. Complete elimination of the fatigue criticality of all welds, extension of usable lifetime up to 600% and additionally a 28% weight reduction of the structure are identified (Fig. 2).