Tubular steel towers of wind turbines are fatigue loaded structures that are exposed to time-variant stresses because of turbine operation. A critical point within the tower structure are the ring flange connections, which connect the individual tower sections with preloaded bolts.

This article publishes two procedures for verifying the fatigue safety of bolts in ring flange connections with the help of experimental und numerical investigations under consideration of bending effects. The authors point out the problem on the basis of experimental and numerical investigations and give recommendations for future flange design.


There are two procedures for verifying the fatigue safety of the bolt. As a first option, the use of a method is presented which allows the numerical calculation of the tensile and bending stresses in the cross-section of the thread area of the preloaded bolt, whereby possible flange flatness imperfections are to be considered. The resulting total stress from axial load and bending moment in the bolt has to be compared with the fatigue strength of the bolt subjected to axial stress. This procedure corresponds to the concept used in mechanical engineering according to Part 1 of VDI standard 2230. As a second option, an experimentally validated method is presented, which only requires the calculation of the axial tensile stresses - from preload and external load - in the cross-section of the thread area. The normal stress determined by this simplified calculation method according to SCHMIDT/NEUPER is compared with a fatigue strength determined for the special case of the L-flange. This fatigue strength (eccentric) implicitly contains the influence of bending. The paper presents a calculation tool which was developed as a source code based extension for ANSYS-Workbench (ANSYS-WB) for a parametric design of ring flange configurations. Subsequently, the consequences for the verification of fatigue safety, for the numerically determined normal load and bending moment in the preloaded flange bolted connections are discussed. For this purpose, new experimental investigations on L-flange test specimens with typical steel construction bolts (system HV) and IHF bolt systems with threads rolled after heat treatment are presented. The innovative contribution consists, on the one hand, in the re-evaluation of the fatigue damage relevance of bolt bending components from eccentric loading on the fatigue strength and, on the other hand, in the derivation of a detail category of eccentrically loaded bolted joints considering manufacturing and surface layer influences. In recent years, the reduction of the levelized cost of energy (LCOE) has become increasingly important in the wind energy sector. One effect of LCOE optimization are larger rotor diameters and higher rated power of the turbines as well as the economic pressure to realize more cost-effective plants. The tower has always played a major role in the total cost of a plant, accounting for around 20-30 % (Hau, 2016). Supporting structures of wind energy turbines on- and offshore are exposed to constantly changing loads during turbine operation. Fatigue strength is therefore of decisive importance. For the ring flange connection shown in Fig. 1, which represents the traditional connection form between the individual support structure sections (of tubular steel towers as well as between monopiles (MP) and transition pieces (TP) in the substructure), a paradigm shift can be seen. While the flange connections of older turbines are governed by extreme loads, modern wind turbines with high rated power increasingly have flanges for which the fatigue strength verification determines the flange design. With the development of computer capacities, it is currently possible to simulate the transfer behaviour of the external tensile stress in the tower shell to the preloaded multi-bolt connection in a computationally efficient manner using the finite element method (FEM) on the basis of suitable modeling strategies, taking into account the flange gap imperfections (Wegener et al, 2019; Wegener et al, 2020). Based on the state of the art, an alternative verification method for the fatigue strength of bolted connections in ring flanges for wind turbines will be discussed.

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