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Our previous study described loosening-fatigue tests under small transverse vibrations that had been performed to elucidate loosening-fatigue mechanism in the long life region. In this study, the influences of various factors of a bolted joint on loosening-fatigue characteristics under transverse vibration have been investigated. Results show that bolt property class has little influence on the transverse loosening-fatigue limits. Results also show that the transverse loosening-fatigue limit is reduced if a bolted joint which has been tightened to within the plastic region is subjected to transverse vibration. And if the grip length and the engaging length are long, the apparent transverse loosening-fatigue limits also decrease.

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.

If a bolted joint is subjected to axial vibration, the internal force the bolt receives is determined based on the load factor that is a function expressed by bolt stiffness and clamped part stiffness. The stiffness of the bolt and clamped part are generally calculated based on VDI2230, but if the structure of the clamped parts is complicated or made of multiple materials, we have to calculate the clamped part stiffness using FE analysis. FE analysis is very useful for calculations of clamped part stiffness. However it is not easy to conduct FE analysis for all parts and structures. A new method to detect clamped part stiffness of bolted joints was developed in this study. The proposed method is based on a clamp force detection method that we had already proposed. In the method, a protruding bolt thread portion is first pulled while holding down the nut’s upper surface and the displacement at the pulling point is measured.