Mean stress effects on the corrosion fatigue crack growth rates in offshore steel structures

Corrosion fatigue has a detrimental influence on structures, particularly those used in harsh marine environments. Offshore structures for renewable energy applications have significant volume of manufacturing and developing to use in harsh marine environment and are highly dynamic. However, the design requirements and structural details of offshore wind support structures are significantly different compared to oil and gas structures, due to environmental aspects and loading regimes on the support structures. There is therefore a requirement to update corrosion fatigue knowledge and databases in order to assist operations and designers to optimise structures with respect to fatigue strength and cost. In this research, fatigue crack growth in the S355J2+N steel used for offshore wind monopile fabrications was investigated in air and free corrosion conditions. Tests were conducted on middle tension (MT) specimens under tension-compression loading conditions at R ratios of (0.1, -0.25, -0.5 and -1.0). The artificial seawater used for the investigation was prepared according to ASTM D1141 standard and was circulated past the specimens through a purpose designed and built corrosion rig at a rate of 3 l/min, at a temperature range of 8-100C and pH range of 7.8-8.2. Crack propagation gauges were used to monitor crack growth rate in seawater. Fatigue crack life was significantly reduced when the absolute value of the negative R ratio increased in both environments. The experimental results show that the compressive stresses have a detrimental effect on the crack growth rate. Two new theoretical models are proposed to account for the effect of negative R ratio on crack growth rate behaviour in air and seawater, and are found to correlate well with the experimental results as well as with the other models of the effect of the negative R ratio on crack growth rate behaviour. Thesis pdf available on request.