A Multiplexed Fiber Bragg Grating Angle Sensor for Bolt Loosening Monitoring

Bolt joints are widely used in large-span truss and bridge structures. The integrity of bolt joint is paramount to the safety of the structure. For distributed multiple bolt joints in large-span structures, a multiplexed angle sensor based on fiber Bragg grating (FBG) for bolt loosening monitoring is proposed. Two FBGs characterized by different wavelength values are symmetrically arranged inside the specially designed sensor structure for the measurement of the bolt loosening angle in terms of their wavelength shift difference. The detail structure design of the FBG angle sensor with temperature self-compensation, automatic reset, and simple reuse is presented. The theoretical and numerical analyses are adopted to derive the measurement principle and optimize the structure of the sensor. Experimental results show that the measuring sensitivity of the FBG angle sensor prototype is about 109.6 pm/° with excellent linearity in the measuring range from 0° to 20°, and the maximum resolution of the bolt loosening angle obtained is 0.38°. The proposed sensor has stable static characteristics and can be expediently reused and developed into a quasi-distributed sensing network, which offers a new approach to real-time quantitative bolt loosening monitoring for distributed bolt joints.