Fatigue life improvement by slight pitch difference in bolt-nut connections

The bolt-nut connections are widely used in various fields. In this paper, a slight pitch difference is introduced between the bolt and nut in order to study the effect on the fatigue performance. Here, we consider that the nut pitch is a few microns larger than the bolt pitch. Fatigue experiments are conducted for three kinds of specimens with different levels of pitch differences. The obtained S-N curves show that the fatigue life is extended to about 1.5 times by introducing a suitable pitch difference. According to the detailed investigation on the fractured specimens, it is found that the fractured positions and the crack configuration are totally different depending on the pitch difference. The mechanism of the fatigue strength improvement is discussed in terms of the stress amplitude and average stress at each bolt thread