Novel model for mean stress effect on high-cycle fatigue performance of threaded fasteners

An experimentally validated fatigue model is proposed for the effect of mean stress on high-cycle fatigue performance of preloaded threaded fasteners. The proposed model is compared with commonly used models by Goodman, Gerber, Morrow, Soderberg, and ASME-Elliptic (ASME B106.1M:1985). Fatigue S-N curves are experimentally generated at various mean stress levels for both commercially available (CA) threaded fasteners (Class 10.9 and 12.9), as well as ultra-high strength (UHS) fasteners (Class 14.8, 15.8 and 16.8). Proposed model prediction and test data show that at high levels of fastener mean stress, the bainite microstructure of the ultra-high strength fasteners has a clear detrimental effect on fastener fatigue performance, as compared to the martensite microstructure of commercially available low carbon steel fasteners. Further discussion of test-validated model predictions and conclusions are provided.