Our previous study showed that 1700 MPa-class ultra-high-strength bolts had loosened by approximately 8% during outdoor exposure test in Okinawa. This study investigated the damage to the bolts and nuts retrieved from the bolted joints that were subjected to the exposure test, in order to clarify the cause of the bolt loosening. The damage investigation showed that the bolt loosening was due to the delayed fracture occurring in the middle section of nut. It was considered that the suppression of thread stripping following the delayed fracture prevented complete breakage, which instead resulted in the bolt remaining in a loosened state.
