PSI - Issue 75

Fritz Wegener et al. / Procedia Structural Integrity 75 (2025) 363–374 Wegener et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction

Bolting assemblies are one of the most frequently used machine elements. In areas such as wind energy, increasing hub heights of modern wind turbines require high-performance fasteners that are optimized as far as possible in terms of their strength properties. Meanwhile, the fatigue strength verification has become a design driver for the flange connections. The fatigue strength of bolting assemblies is usually verified according to the so-called nominal stress concept, e. g. according to Eurocode 3 (EC3), EN 1993-1-9 (2010-12). As the consideration of new relevant influencing parameters on the fatigue strength within the scope of this approach is associated with a high level of e ff ort due to the additional tests required, alternative concepts such as the notch-strain approach are of increasing interest. Regardless of which concept is used, it must be possible to evaluate as many of the influencing parameters on the fatigue strength of the bolting assembly as possible due to the increasing requirements on the fasteners. According to Wiegand et al. (2007), these e ff ects can be categorized in three system levels: increasing the local component strength, reducing the local stress and reducing the acting bolt loads. This article focuses in particular on the local component strength, which is influenced, for example, by the nominal bolt diameter, the time of thread production (before or after heat treatment) and surface layers like hot-dip galvanizations. These influencing parameters are already taken into account in the current draft FprEN 1993-1-9 (2024-02) of EC3 and can also be included in the analytical assessment of fatigue strength according to the notch-strain approach. However, the application of the notch-strain approach to bolting assemblies is still a subject of research. In this regard, the article aims to compare calculations based on the notch-strain approach with the results of experimental investigations. Furthermore, the possibilities for taking into account the e ff etcs also contained in the EC3 draft are shown and evaluated with regard to their performance.

2. State of the Art

2.1. Fatigue strength verification using the nominal stress concept

The most commonly used fatigue strength verification method for bolting assemblies is the nominal stress concept, see Glienke et al. (2021a). In this method, an acting nominal stress, which is defined on the stress cross-section A S of the bolt, is compared with a corresponding resistance value. The type of resistance value depends on the respective standard. For machinery, for example, the established guideline VDI 2230-1 (2015-11) is dominated by the verification of endurance limit, so that bearable stress amplitudes at the fatigue endurance limit σ AS are provided. For steel structures and the associated FprEN 1993-1-9 (2024-02), the fatigue strength is given by the so-called detail category (DC) ∆ σ C , which is defined as the characteristic reference value of fatigue resistance at N C = 2 · 10 6 stress cycles, see Figure 1 (a).

Fig. 1: (a) Definition of S-N curve acc. to FprEN 1993-1-9 (2024-02); (b) Detail categories for selected bolting assemblies acc. to FprEN 1993-1-9 (2024-02).