Corrosion fatigue load frequency sensitivity analysis

• A study of crack growth behaviour of S355J2+N steel tested in air and laboratory simulated seawater was described.
• Back face strain regression model was derived and compared with other BFS relations to obtain reasonable agreement.
• Crack growth rates in seawater revealed ERF of 2 and 4 at lower and higher values of stress intensity factors.
• The behaviour of S355J2+N steel was similar and in some cases better than other types of offshore structural steels.
• The fatigue growth behaviour of S355J2+N steel in air and seawater was in accordance with the BS7910 recommendations.

This paper presents experimental assessment of crack growth rates of S355J2+N steel in a corrosion fatigue environment similar to what is experienced on offshore wind farm monopile structures under various cyclic load frequencies in order to assess the effect of cyclic frequency of the applied loading within a frequency range pertinent to the structure. Fatigue crack propagation behaviour in this test programme is evaluated through fatigue tests on six compact tension test specimens in air and in laboratory simulated seawater under free corrosion condition. Fatigue crack lengths were monitored by back face strain (BFS), DCPD and ACPD. A regression model was derived through the BFS method to express strain values as a function of crack length to width ratio. The effectiveness of BFS method is particularly demonstrated in the simulated marine environment. Within the range of test frequencies, crack growth rates in simulated seawater when compared to the equivalent air test revealed environmental reduction factors of 2 and 4 at lower and higher values of stress intensity factors respectively. Significant difference in the results of the seawater test frequencies is discussed.