Thread effect on fatigue behaviour of high-strength bolt steels

High-strength bolted connections are extensively used in structural applications, including offshore wind energy applications. In this context, the fatigue behaviour of high-strength bolt steels with strength classes 10.9 and 12.9 was investigated. Bolt steel 12.9 demonstrated superior quasi-static mechanical strength properties but lower ductility than 10.9. A progressive improvement on the fatigue life was found considering experimental results from smooth specimens of 8.8 bolt steel to 12.9 bolt steel. To assess the effect of the thread on the fatigue life of 10.9 and 12.9 bolt steels, threaded specimens were designed. The stress concentration factor of these specimens was determined numerically and analytically. The higher fatigue resistance of 12.9 bolt steel is no longer observed in the presence of the thread, with the results for threaded specimens demonstrating a similar fatigue resistance for both steels. The Castillo and Fernández-Canteli (CFC) model was applied to characterise the fatigue life in the finite and infinite life regions. A higher notch sensitivity for the 12.9 bolt steel was observed. Through scanning electron microscopy (SEM) analyses of fracture surfaces, cases of crack initiation at inclusions were found in the smooth specimens. In the threaded specimens, crack initiation occurred at the root of the first thread, highlighting the effect of stress concentration at that region. Different propagation mechanisms were identified, with intergranular crack propagation observed in the 12.9 steel, which may help explain the higher dispersion observed in the results.