On fatigue strength of large-size bolt-assemblies in steel constructions under consideration of manufacturing and surface condition related impacts - part 2: evaluation of fatigue tests

Manufacturing related impact factor significantly affect the fatigue strength of bolt assemblies used in steel constructions. The effects are taken into account very inconsistently in the design bases, depending on the normative reference used. Especially for bolt assemblies with large diameters, as they are commonly applied in large numbers at towers for wind energy plants, only a very limited experimental background for assessment of relevant impact factors has been available in the past. This paper presents and evaluates results from 20 different fatigue test series, most of them conducted in the recent past on three different high-strength bolt assembly systems with varying manufacturing conditions. The data base covers a diameter range between M12 and M64 and focuses on test series with high mean stress in a representative order of magnitude for practical application of the bolts. Based on the knowledge given in the relevant literature, the test results are evaluated regarding the impact of different manufacturing related effects. Furthermore, the results are compared to the specifications of the relevant international normative guidelines. In addition to the results of fatigue tests, supplementary metallographic examinations of the fracture surface shape and surface-layer condition behavior are taken into account. On this basis, the paper derives recommendations for future design considerations and gives an outlook on further expedient investigations.