Bolt loosening fatigue tests under small transverse vibrations have been performed to understand the behavior in the long life region. The current study is focused on the loosening-fatigue mechanism and on the estimation of nominal stress at the root of the first thread caused by transverse vibration. Results show that if a bolt has loosened within 103∼104 vibration cycles, damage such as crack nucleation at the root of the first thread is not observed, and loosening is due to bolt rotation. However, if loosening does not occur until approximately 105∼106 cycles, then a crack is observed at the root of the first thread in the all experiments. Loosening occurs due to loss of bolt stiffness caused by crack nucleation and propagation which then leads to bolt rotation. Results also show that the initial clamping force above a threshold level does not significantly influence the loosening-fatigue life of a bolted joint. The amplitude of the transverse vibration force is the most significant influence on the loosening-fatigue life.
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