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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.