A novel fiber optic bolt loosening monitoring sensor system for aircraft bolt joints

A novel fiber optic bolt loosening monitoring sensor is proposed and applied to an experimental model of aircraft lug assemblies. The lug assembly is fabricated with a stainless steel lug, a composite sandwich panel, and 10 carbon-steel bolts, nuts, and washers. The bolt loosening monitoring system is configured with a single-mode fiber and an optical time-domain reflectometer. Sections of the single-mode fiber line are mounted on the respective 10 bolt/nut joints and function as bending sensor nodes to detect bolt loosening events. Then, the other end of the single-mode fiber is connected to the optical time-domain reflectometer, to measure optical loss in the backscattering light attenuation trace, induced by the macro-bending of the sensor nodes. In addition, the bolt loosening angle and locations can be determined, by analyzing the optical time-domain reflectometer traces. The experiments for multi-bolt monitoring demonstrate that the single single-mode fiber line incorporated with optical time-domain reflectometer is able to detect simultaneously which bolts were loosened with an angle resolution of 1.45°, even under temperature variation. Since the optical time-domain reflectometer and SMF combination allows monitoring tens of kilometers, just one system can monitor thousands of bolt joints. Therefore, numerous bolt joints in large structures can be economically monitored in real time.