Bolted flange connections for wind power generations and chemical plants are subjected to repeated bending moments due to wind and earthquake loads. It is known empirically that the bolts loosening in the bolted connections occur. Thus, it is necessary to examine the bolt loosening in the bolted flange connections under repeated bending moments. The objective of the present paper is to clarify the mechanism of bolt loosening in bolted flange connections subjected to repeated bending moments and to discuss on a solution for preventing the loosening of the bolted flange connections subjected to repeated bending moments, too. The loosening mechanism of the bolted flange connections is examined using the FEM calculations and the experiments. In the loosening experiments using the specified hexagon nuts, it is observed that the loosening of the bolts in the connection occurs due to the permanent-set of the bolts and the movement of the bearing surfaces in the flange due to the external bending moment. In the FEM calculation, it is examined that the bolt loosening in the bolted flange connections qualitatively. In addition, it is found that as the bolt preload increase, the bolt loosening decreases. From the FEM results and the experimental results, it is found that the most effective part for preventing the loosening of the bolted flange connection is the eccentric nut.

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