An important issue is how to determine a bolt preload of a bolted joint taking an external axial load into account. The following two methods for determining the bolt preload are discussed in the present paper. In the first method, the bolt preload is determined such that the stress at the bearing surfaces is lower than the yield stress of the clamped parts taking account of the load factor. In the second method it is determined such that the stress at the bearing surfaces is higher than the yield stress of clamped parts. Thus, firstly the load factor to obtain the increment Ft in the axial bolt force when an external load is applied to a bolted joint is investigated using axi-symmetrical theory of elasticity, FEM analyses and experiments. The analytical results are in a fairly good agreement with the FEM and the experimental results. Then, the contact stress distributions and the plastic displacement distributions at the bearing surfaces under external loads are examined using elasto-plastic FEM analyses. Experiment to measure the load factor and the reduction in the axial bolt forces were carried out. The prediction for the reduction in axial bolt force of the joint under external load is fairly consistent with the experimental result. In addition, in the first method for determining the bolt preload, it is found that no plastic deformation is occurring at the bearing surfaces and no reduction is occurring. It is noticed that a higher bolt preload according to the second method shows a small reduction in axial bolt force. Discussion is made on the critical stress at the bearing surfaces.
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