In this study, we focused on the cyclic re-tightening processes of large tower flange bolts used in the assembly of offshore wind turbines, which have been increasingly introduced in Japan in recent years. To investigate the behaviors of bolts when the bolt deformed plastically due to repeated in the re-tightening process, we conducted the re-tightening experiments using M12 bolts, which represent a 1/6 scale model of the actual M72 bolts used in practice. We performed the re-tightening experiments seven times, excluding the initial tightening, in increments of 10 kN from 0 kN to 90 kN. After each re-tightening, the clamp force was recorded for 60 seconds to measure decreases of the clamp force. The experiment was conducted on five bolts under the same conditions. The results showed that the clamp force slightly decreased if the bolts deformed plastically in the re-tightening process. The decrease in clamping force saturated gradually, and their amount in the clamp force reduction depended on the clamping force in the re-tightening process.
HASHIMURA et al. (Wed,) studied this question.