A study of loosening and fatigue failure of bolted joints under transverse vibration

Loosening and fatigue failure of bolted joints are the two most important problems for vehicles that are subjected to vibration loading. Although many fatigue tests have been performed under axial vibration, there have been few tests under transverse vibration. In this study, loosening-fatigue tests under small transvers vibrations have been performed to understand bolt behavior for the long life region. Results show that if a bolt has loosened within 10 ²∼10³ vibration cycles, damage such as crack nucleation at the root of the first thread is not observed. However if loosening does not occur until approximately 10⁴∼10⁶ cycles, then a crack is observed at the root of the first thread in all experiments. Loosening occurs due to loss of axial stiffness caused by crack propagation which then leads to bolt rotation. The results also show that the loosening-fatigue life does not depend on the initial clamping force although the loosening-fatigue life significantly depends on the transverse vibration force.