Principal Stress-Based Equation for Multi-Axial Fatigue Analysis of Preloaded Threaded Fasteners

Abstract

A novel principal stress-based high cycle fatigue (HCF) model is proposed for preloaded threaded fasteners under cyclic tensile-shear loads. The model uses the fastener principal stress amplitude in order to construct a singular multi-axial S–N curve from the conventional uniaxial S–N curve with zero mean stress of bolt along with some experimental data. An material testing system (MTS) fatigue testing system is used first to generate the fastener preload by applying a direct tensile-shear load using a special fixture. Subsequently, the same system is used for applying combined cyclic tensile-shear loading of the fastener at various levels of mean stress. Results show that for the same level of axial stresses, the multi-axial loading would significantly reduce bolt fatigue life as compared to that of uniaxially loaded bolt. Moreover, only one S–N curve would be able to predict the multi-axial HCF of preloaded threaded fasteners, when the maximum principal stress amplitude is used. Detailed discussion of the proposed model results and test data are provided.