P 925 - Experimental and computational evaluation of the fatigue strength of bolts with large dimensions in steel construction under consideration of surface layer influences

Based on the experimental investigations presented in this report, the influence of hot-dip galvanising on the fatigue strength of large HV bolts is systematically investigated. The aim of the research project is to develop a design concept for large-diameter bolts and to extend and verify established standards and regulations for application to large-diameter bolts. In addition to the typically used geometry of the HV bolt, the special boundary conditions include in particular the hot-dip galvanising and the associated surface layer influences, as well as the operating load due to variable amplitudes under high medium voltage. The following sub-goals were achieved as part of this research project: - Experimental analysis and, based on this, description of the influence of the galvanised surface layer, both in relation to the load distribution (thread overlap) and the damage mechanism under typical operating loads - Development and testing of a mathematical evaluation model based on the local stress and using the thin surface layer model - Validation of the model and experimental testing of the fatigue properties of large HV sets by means of extensive fatigue tests with constant and variable amplitudes - Processing of the results obtained for technical application on the basis of current standards and regulations (nominal stress concept) The research results have made an important contribution to the reliable assessment of the fatigue strength of large bolted joints, taking into account the influence of surface layers. This increases the reliability of calculations, which enables further utilisation of load-bearing reserves. For manufacturers of HV bolt assemblies and engineering firms involved in design and construction, the area of large bolts represents a high-growth sales area. The expansion of the experimental basis and the establishment of an experimentally validated design concept as well as the expansion and validation of established regulations will create design certainty and further strengthen SMEs in particular.