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The clamping force of a bolted joint greatly influences its performance. But the clamping force is seldom checked after tightening. In this study, a new method to detect the clamping force after tightening has been proposed. In the proposed method, the clamping force is detected by pulling the bolt threads which protrude through the nut. Clamping forces of pre-tightened bolted joints were detected experimentally by the proposed method. Results show that the proposed method can detect the clamping force within to ±10% accuracy. Furthermore, a prototype wrench to detect clamping force has been developed using a commercial torque wrench. Results of the verification show that this wrench can also detect the clamping force within to ±10%.

Fatigue failure of bolted joints is a very serious problem for vehicles that are subjected to vibration loading. Fatigue characteristics of bolted joints under transverse vibration have been investigated in our previous study. According to the results, apparent fatigue limits (the highest amplitude of transverse vibration force which can be applied to the bolted joint without generating fatigue) differ significantly according to tightening conditions although real fatigue limits of bolts are the same if the property classes are the same. The difference in apparent fatigue limits is due to changes in distribution of bending moment applying to the bolt due to tightening conditions such as grip length and engaged thread length. In this study, the relationship between the apparent fatigue limit and the real fatigue limit has been experimentally revealed and a method to predict the apparent fatigue limit using the real fatigue limit has been developed.

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.