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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.

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.

Multi-material structures are demanded to reduce weight of vehicles. We have to reduce the weight of not only structural material but also joining elements to achieve multi-material structures. Some aluminum alloy bolts have begun to be used in the automotive fields recently. In our previous study, we investigated the tightening characteristics of Aluminum alloy A5056 bolt. The results showed that friction coefficients of thread surfaces and bearing surfaces are obviously different in comparison with those of steel bolt. The tightening strength, especially the proof clamp force, is very important to determine the target clamp force of bolted joint. However the proof clamp force of bolt is different from the proof tensile strength of bolt because the proof clamp force significantly depends on the friction coefficient of thread surfaces. Therefore we can easily know the proof clamp force for each bolt material if we can estimate the friction coefficients from the tensile strength.

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.