Design and analysis of bolt loosening angle sensor based on surface-encapsulated FBG of elastic curved beam

This paper presents a novel bolt loosening angle sensor based on the elastic curved beam and Fiber Bragg Grating (FBG). The sensor offers the advantages of ensuring long-term operation in harsh environments, preventing damage to the bolt structure, and resolving wiring issues in blind holes. The sensor operates on the principle that the FBG encapsulated on the elastic curved beam responds to the bending curvature change caused by the variation in bolt loosening angle. Specifically, this response is monitored by measuring the wavelength shift of the FBG. To verify the reliability of the theory, a prototype of the sensor was fabricated according to the optimized installation parameters of the beam and FBG position. Based on the fabricated prototype, the theoretical angle sensitivity of the sensor was calculated to be 177.8 pm/°. The test results showed that the sensor not only demonstrates high sensitivity but also possesses the potential to monitor the subtle variations in bolt loosening angle during the loosening process. The experiments revealed that the sensor responds rapidly and reliably, effectively capturing the progressive and complex angle changes during the bolt self-loosening process.