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In this study, tightening tests for four types of commercial bolts made of nonferrous materials were conducted to investigate torque coefficients that show the tightening accuracy. These four types of bolts were made of AZ31 and AZX912 magnesium alloy, A5056 aluminum alloy and TB340 pure titanium respectively. Materials for the clamped parts corresponded to the bolts: AZ31 magnesium alloy, A5056 aluminum alloy and TB340 pure titanium. These clamped parts were tightened by bolts and nuts of the same material except when using AZX912 bolts, in which case AZ31 nuts and AZ31 clamped parts were used. Tightening tests using A5056 clamped parts with magnesium alloy bolts were also conducted to verify the usefulness of the magnesium alloy bolt in the tightening of aluminum parts. Tensile tests for all bolts were conducted to investigate the difference between their tensile strength and tightening strength.

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.