A Method for Identifying Loose State of Bolt Groups in Composite Lapped Panels Based on Fiber Optic Strain Sensing

In the service process of aerospace composite lapped panels, the bolted connection structures may become loose due to the action of vibration and fatigue loads. Real-time monitoring of the bolt tightening state is of great significance for ensuring the reliability and safety of the lapped structure. To this end, this paper develops a method for identifying the loosening state of a group of bolts based on strain modal parameters, and proposes four quantification indices for the loosening state of bolts, which are based on the differences and relative change rates of strain modes of different orders. By constructing a threshold for identifying the loosening state of bolts based on strain modal parameters, precise identification of the loosening state of multiple bolts is achieved. A sensitivity evaluation index for loosening identification is defined, and the identification effects of bolt loosening states are presented based on four indices, including the differences and relative change rates of strain modes of different orders. On this basis, a monitor and identification system for the loosening state of the bolt group in composite lapped panels was constructed based on quasi-distributed fiber optic strain sensing. This system overcomes the difficulty of synchronous monitor and identification of the loosening state of multiple bolt connections on the lapped panel. The research outcomes can provide technical support for identification of bolt group loosening states during the manufacturing, assembly, and service stages of aerospace equipment, aiding their maintenance and inspection.