| As a key component of large steel structures,bolted joints have a wide range of applications,a large number of uses,and high monitoring difficulty.The connection status of bolts is related to the safe operation of the entire structure.Once bolt loosens,external loads can cause them to loosen even faster.If not repaired in time,it can cause structural safety accidents.Therefore,monitoring the connection status of bolts(expressed as bolt pre-tension)is of great significance,especially for the numerous bolts distributed in important structures.This paper proposes a method for monitoring the bolt connection status based on the detection of the bolt loosening angle using Fiber Bragg Grating(FBG)sensors.This paper focuses on the bolt loosening problem in bolted joint structures.The main research contents of this paper are as follows:(1)The theoretical relationship between bolt pre-tension and bolt loosening angle is studied,and the theoretical relationship formula is derived.Experiments are designed to verify the correctness of the theoretical relationship.The experimental results are in good agreement with the theoretical analysis results,which proves that the connection status of bolts can be determined by monitoring the changes in the bolt loosening angle.(2)A bolt loosening monitoring method based on FBG sensing is proposed.By using FBG sensors to monitor the changes in the loosening angle of bolts during the bolt loosening process,the connection status of bolts can be monitored.This method uses the rotation of the conversion structure driven by the bolt loosening to convert the bolt loosening angle into a linear displacement acting on the optical fiber.The resulting deformation of the optical fiber is measured by measuring the wavelength shift of the FBG,and the quantitative relationship between the wavelength shift and the bolt loosening angle is established to monitor the loosening angle of bolts during the bolt loosening process.A conversion structure is designed based on this measurement principle,and the conversion structure is optimized to make the proposed measurement method have a reasonable range and sensitivity.Experimental devices are designed to conduct experimental studies,and the experimental results show that the sensor has good linearity and sensitivity within the range,and can effectively measure the bolt loosening angle,with a minimum detectable bolt loosening angle change of 0.38°.(3)Based on the sensor principle,a cascaded fast disassembly FBG sensor is designed,and its structural performance and operating process are analyzed and explained.A temperature compensation experiment is designed and temperature compensation experiments are conducted.The experimental results show that the FBG sensor has good temperature compensation function and can operate normally even when the external temperature changes.Finally,the static characteristic indicators of the FBG sensor are analyzed to evaluate the sensor’s performance. |
