P 1091 – Development of a concept to assess the loss of preload in preloaded bolted connections under fatigue loading
With the aim of increasing their resistance and stiffness, bolted connections are preloaded. Furthermore, a significant increase of fatigue strength under tension or shear loading is being achieved. As a result, the use of the preloaded bolted connections in structures with dynamic loading is mandatory. The preload losses that occur during the entire lifetime of a structure lead to loosening of the mentioned connections and consequently pose a risk to their serviceability or even the load capacity. For this reason, the estimation of potential preload losses as well as their consideration during design is important.
In terms of this project systematic investigations were carried out on preloaded bolted connections of categories B/C and E according to EN 1993-1-8. Based on relaxation, quasistatic and fatigue tests, the specific preload losses were determined separately, evaluated and extrapolated on the intended service life of structures. The investigations were based on different types of M20 bolting assemblies, tightening methods as well as the clamping length ratios and surface treatments.
Based on the experimental results, a critical review of current regulations was possible. The experimentally observed system reserves resulted from the tightening process and the preload losses revealed, that the use of direct tension indicators is only advisable for preloaded bolted connections at serviceability limit state due to high preload losses. The determined preload losses for the investigated preloaded bolted connections of category E and the clamping length ratio lk/d 5 lied within the range of 10 % and therefore met the area for a possible preload loss, which is implicitly covered by the tightening methods acc. to EN 1090-2 (noconsideration of the Lock Bolts). Independent of the category of the bolted connections and their surface treatments, the preloaded bolted connections with lk/d 2 showed preload losses above 10 %. On this occasion, comparatively thin coatings in the faying surfaces led to preload losses that averaged up to 20 % regarding the intended service life. The combined method turned out to be the only tightening method, which possessed enough system reserves for a possible preload loss even for coated surfaces, so that the normative minimum preloading force stays fulfilled after the service life of 50 years.
The observed results provide a fundamental knowledge for the consideration of the preload losses during design in the future.
The research project (IGF-Nr. 18711 BG) was carried out at Institut für Metall- und Leichtbau der Universität Duisburg-Essen und an der Fraunhofer-Einrichtung für Großstrukturen in der Produktionstechnik IGP, Rostock. FOSTA has accompanied the research project work and has organized the project funding from the Federal Ministry of Economics and Technology through the AiF as part of the programme for promoting industrial cooperation research (IGF) in accordance with a resolution of the German parliament.
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