Abstract
SI grade 12.9 high strength steel bolts were used to investigate the fatigue behavior of bolt threads rolled before/after heat treatment using two different thread profiles and two different high preload values. Bolts were 3/8 UNRC-16 (coarse) and 3/8 UNRF-24 (fine) and preloads were taken as 75 and 90% of proof stress. Axial forces were applied through the nut. Axial and transverse residual stresses near the thread root were measured using X-ray diffraction. Most fracture surfaces contained crescent-shaped cracks and SEM evaluation indicated all fatigue crack growth regions contained multiple facets with no striations. Rolling after heat treatment caused little to no increase in fatigue resistance for the coarse threads. However, the fine threads finite fatigue life increased by factors of 2 to 5 and fatigue strengths based on Sa at 107 cycles increased by 40 and 50% for the 90 and 75% of proof stress preload, respectively. These are significant increases considering the very high preloads or R-ratios. The greater preload gave better fatigue resistance when based on the maximum stress, Smax, and lower fatigue resistance when based on the alternating stress, Sa Based on Smax, fatigue strengths at 107 cycles for 90% of preload were equivalent to or greater than the actual proof stress. Smax at 107 cycles for fine threads rolled after heat treatment with 75% of proof stress was also greater than the actual proof stress. The above results can be attributed to different thread root radii, thread region smoothness, original residual stress from rolling after heat treatment, and residual stress relaxation from the high preloads.
