A Study to Predict Fatigue Limits of Bolted Joints under Transverse Vibration

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. Experiments to investigate the relationship between the apparent and the real fatigue limits were conducted using commercial M10 hexagon head bolts. In the experiments, three separate tightening conditions of a bolted joint were combined with various grip lengths and engaged thread lengths. The results showed that the relationship between the apparent fatigue limit and the real fatigue limit depends on the bending moment at the root of the first thread. The bending moment is determined as the product of a coefficient, which is determined from the tightening condition, multiplied by the grip length and the transverse vibration force. The coefficient was determined experimentally in each tightening condition because the apparent fatigue limit can be predicted if the coefficient can be known. From the results of experiments to determine the coefficients, it has been seen that the apparent fatigue limit can be predicted from the real fatigue limit of the bolt material.