A comprehensive review and analysis of S355 fatigue crack growth rate data for offshore wind applications: Air environment

The fatigue crack growth rate recommendations in BS 7910 standard are based on experimental data that are nearly three decades old and derived from a wide range of steels, hence limiting their accuracy for specific applications. This paper addresses this limitation by compiling and validating a contemporary fatigue database for S355 structural steel, which is widely used in offshore wind turbine support structures. The focus of the study is on fatigue behaviour in the air environment, which directly applies to wind turbine towers and other parts of the support structure above the splash zone and additionally serves as the benchmark for subsequent analyses under environmental degradation conditions. The collected S355 fatigue database was subjected to comparative analyses using both bilinear and simple linear regression models as well as a LEFM validity criterion. The obtained results were assessed against the BS 7910 recommended curve. Based on the available S355 data at R = 0 and R = 0.1, the analyses revealed a potential non-conservatism in the BS 7910 low- R bilinear curve within the lower stress intensity factor range, with up to 10 times faster crack growth observed between Δ K values of 200 and 300 N/mm 3/2 . In the same region, the simplified single-slope laws of BS 7910, DNV, and IIW remain conservative relative to the S355 data. The simple linear regression model was found to be the most appropriate fit across all classified S355 datasets, in contrast to the commonly adopted bilinear approach. The influence of material microstructure and specimen geometry on the fatigue crack growth behaviour was found to yield minimal variation. However, slight differences were observed between different S355 subgrades which warrants further investigation, particularly given the increasing use of Thermo-Mechanical Controlled Process S355. Overall, this study demonstrates that a linear model for S355 in air is sufficient compared to a bilinear model and highlights the need to incorporate offshore steel grade specific factors into future life assessment guidelines, to optimise design and enable life extension of the critical offshore wind infrastructure. A comprehensive openly accessible database of all included specimens is published alongside this paper.