Further development of detail categories for bolt‐assemblies subjected to normal stress in steel constructions

The fatigue strength of bolts is determined not only by the geometric design but also by production‐related influences such as rolled before heat treatment, rolled after heat treatment or even hot‐dip galvanising. For the bolts of steel constructions considered in this paper, these production‐related influencing factors are considered very differently in the evaluation of the fatigue resistance, depending on the used respective standards. This is for example done according to the still valid version of EN 1993‐1‐9 in the unified form of detail category 50, taking into account the size effect, but independent of the other influencing factors mentioned before. This is not least due to the very limited available experimental database and test documentation. The authors address this topic in this paper, especially for large‐size bolts used in large quantities in wind turbines. This article presents an evaluation of results from 22 fatigue test series (n = 411 tests) on 3 different high‐strength bolt assembly systems with varying manufacturing conditions. The data base is covering a diameter range between M12 and M72 and focusses on test series with high mean stress in a representative order of magnitude for practical application of the bolts. For their further evaluation, the findings described in the technical literature are considered in relation to the different tested manufacturing conditions. The additional comparison of the test results with the current design codes serves to a better classification of the results. The authors give a proposal for the adjustment of detail categories of bolt assemblies subjected to normal stress.