Preloading force monitoring and looseness identification of bolt connection using washer-type buckypaper sensor

Glass fiber-reinforced polymer composite bolted structures, as an all-composite connection method, offer significant weight reduction advantages. However, the structural failure caused by preload attenuation has become a major safety hazard threatening engineering applications. To address this issue, this study proposes a novel method for structural health monitoring of composite bolted joints based on a washer-type buckypaper (BP) sensor. The feasibility and load variation synchronization of this new washer-type BP sensor for monitoring bolt preload changes were verified through multistage continuous loading–unloading comparative tests. Static tensile tests, vibration characteristic tests, vibration tests under different acceleration levels (20, 40, 60, and 80 g), vibration fatigue tests (58,000 cycles), and simulated loosening tests were conducted on composite bolted joint specimens equipped with this new washer-type BP sensor. The results demonstrate that the sensor exhibits excellent load response synchronization, high sensitivity, and outstanding stability. It enables in situ monitoring of preload force in composite bolted structures, providing real-time response to preload variations under both static and dynamic loading conditions. This offers a novel solution for full-life cycle loosening health monitoring of composite bolted joints.