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A new analytical approach for describing fatigue load sequence effects

R.C. Dragt 1, S.T. Hengeveld 2, J. Maljaars 2,3

1 Department of Structural Dynamics, Institute for Applied Scientific Research (TNO), Delft, the Netherlands
2 Department of Structural Reliability, Institute for Applied Scientific Research (TNO), Delft, the Netherlands
3 Structural Design, Department of the Build Environment, Eindhoven University of Technology Eindhoven, the Netherlands

Load sequence effects in fatigue crack growth, such as retardation and acceleration due to a large overload (OL) and underload (UL), can dramatically change the (remaining) fatigue life. These effects can be determined using analytical and numerical tools, often trading accuracy for a steep increase in required calculation time. This paper describes a novel analytical model that includes these load sequence effects, with reasonable accuracy and with significantly less computation time as compared to numerical tools. This analytical description is based on observations from numerical simulation and fatigue crack growth experiments. The model uses simple and scalable equations for the OL effect on the crack openings stress. It also accounts for the effect of very low stress valleys, ULs, that can (partly) cancel this effect. This UL effect is treated independently of the OL. This adopted modular approach provides the model with the required flexibility to describe (semi-) variable amplitude signals. The model is calibrated with a set of experiments and compared against the experimental results and analytical models from literature.

Key words: Fatigue, load sequence effects, overloads, underloads, crack growth, offshore wind turbines