Fatigue strength assessment of large bolting assemblies using the notch-strain approach

Due to trends such as increasing tower heights, optimizing the design of wind turbine towers with regard to their fatigue strength is becoming increasingly important. For this reason, all available fatigue strength potentials must be used. To be able to make use of existing fatigue strength potentials of bolting assemblies used for example in the tower flanges, the new revision of Eurocode 3 part 1-9 (FprEN 1993-1-9 (2024-02)) considers more differentiated detail categories, taking into account size effects, the thread manufacturing process as well as the effect of zinc coatings. However, incorporating positive or negative effects on the fatigue strength beyond the scope of the specified detail categories is difficult, since the underlying nominal stress concept requires additional tests for any additional influencing parameter. This is a particular challenge for large bolting assemblies of nominal diameter d ' 36 mm, like those used in current tower designs, as the experimental investigation of individual influencing parameters is costly and time-consuming. Compared to this, the notch-strain approach allows for the consideration of individual parameters based on the analytical or numerical calculation of local stresses and strains as well as on small-scale base material tests. In theory, changes in the bolt material, thread manufacturing process, bolt geometry, preload level, loading scenario and surface layer can be considered in one or more subsystems of the concept without the need for tests on the full-scale bolting assembly. However, the accuracy of the notch-strain approach strongly depends on the accuracy of the base material characterization as well as on the selected modelling approaches in the different subsystems of the concept and is still a subject of research. In this context, the ongoing public funded IGF research project No. 01IF22748N, Hinrichs et al. (2025), regarding the effects on the fatigue strength of large bolting assemblies (d ' 36 mm) delivers extensive test results and offers a large database for the validation of corresponding calculations using the notch-strain approach. The methods for the implementation, the results of the validation as well as the conclusions for the further development of the notch-strain approach for application to large bolting assemblies are presented and discussed in this paper.