Initiation of Fatigue Cracks in Bolted Joints

In this paper, bolted joints were subjected to fatigue testing under load control with R-ratio equal to zero. Fatigue fracture surfaces show a crescent moon geometry for short cracks turning into quasi-straight fronts for long cracks. Fatigue cracking usually starts on the bolt's first notch root inside the bolted joint, secondary cracks appearing sometimes with smaller size and a greater angle (in relation to the transversal section) in consecutive inner threads. The starting places, as well as the initial geometry of the cracks, are explained according to the distribution of the equivalent plastic strain in the bolt.