The fatigue strength of structural steel bolts is significantly affected by manufacturing impacts. These impact factors are taken into account very inconsistently in the design rules, depending on the set of rules used in each case. In particular for large-diameter bolts, such as those used in large numbers in wind energy plants, an experimental basis for evaluating various impacts was only available to a very limited extent in the past. This paper evaluates the results on three different high-strength bolt-systems with varying manufacturing conditions. The test background covers a diameter range between M12 and M64 and focuses on tests with high, representative mean stress levels. In addition to the results of fatigue strength tests, fractographic investigations of fracture pattern growth are also used.
In the last few years, the reduction of electricity generation costs - Levelized Cost Of Energy (LCOE) - has gained massive importance in wind energy. Effects of LCOE optimization include both significantly faster increases in rotor diameters and turbine ratings, as well as pressure to generate ever more cost-effective plants. The tower has always played an important role in the total cost of a plant, which is about 20-30%. Accordingly, optimized and lightweight tower structures are important (Schaumann, et al., 2021; Lange, et al., 2017).
Tubular steel towers are still the dominant tower concept. In the course of increasing hub heights, in order to make even low wind sites usable for wind energy, a soft-soft design for cost-effective tubular towers with a hub height of more than 120 m is now state of the art. This trend, as well as the fact that wind farm project developers are increasingly demanding lifetimes of more than 20 years, means that fatigue strength is becoming more and more important for tower dimensioning. This is also evident in the ring-flange dimensioning. Whereas a few years ago the extreme loads were usually decisive for the flanges, nowadays more and more ring-flange connections are dimensioned for service loading (Wegener, et al., 2020). The lower flanges, where large bolt diameters are used, are particularly affected by this. Since the ring-flange connection represents a neuralgic point in the tubular tower design not only from the dimensioning point of view but also from the cost point of view, the fatigue strength of connections with large-size bolting assemblies has meanwhile become a decisive factor for cost-effective towers. The fact that the fatigue strength of bolts in general is becoming increasingly important is also shown by the growing number of papers on this topic in recent times (Bartsch, et al., 2021), (Feldmann, et al., 2020), (Maljaars, et al., 2021).
