Multi-Axial Fatigue of Preloaded Threaded Fastener

Multiaxial fatigue performance of preloaded threaded fastener is investigated in this research. In order to study the effect of fastener tension decay on the fatigue performance of threaded fastener, an experimental study is carried out to provide an insight into the effect of cyclic multiaxial loading on the High Cycle Fatigue (HCF) life of preloaded threaded fasteners.;First, fastener tension decay is investigated for the effect of cyclic thermal loading on the fastener tension decay in single-lap bolted joints that are made of dissimilar-materials; that would affect the fatigue performance of the fastener.;Joint combinations include steel and lightweight materials such as aluminum and magnesium alloys, with various thicknesses. The range of cyclic temperature profile varies between --20 °C and +150 °C. Tests continued until the rate of fastener tension decay per cycle has become relatively small; that corresponded to ten consecutive ten-hour cycles. A computer-controlled environmental chamber is used for generating the desired cyclic temperature profile. Real time fastener tension data is collected using high-temperature load cells. Percent fastener tension decay is investigated for various combinations of joint materials, thicknesses, and initial preload level. Thermal and material creep finite element analysis is performed using temperature-dependent mechanical properties. FEA result has provided insight into interesting experimental observations.;Second, a special fixture is used for orienting bolts at different angles relative to the direction of the cyclic load applied by an MTS fatigue testing system. The orientation angle converts the external cyclic load to a combined cyclic axial and shear loading on test bolts.;A combination of three mean stress levels and three amplitude levels were tested at each angle to evaluate the fatigue performance of the bolts. Test data is used for creating S-N curves for various multiaxial load combinations. The effect of cyclic stress amplitude and mean stress level are investigated. Detailed discussion of the proposed model results and test data are provided.;Third, a novel principal stress-based high cycle fatigue (HCF) model is proposed for preloaded threaded fasteners under cyclic tensile-shear loads. The model uses principal stress amplitude to predict a single multiaxial S-N curve from the conventional uniaxial S-N curve with zero mean stress.