Theoretical and experimental study on FBG bending sensor for quantitative monitoring of bolt looseness

Abstract

A quantitative monitoring method for bolt looseness using an elastic substrate fiber Bragg grating (FBG) bending sensor is presented in this paper, which is achieved by measuring the wavelength shift of the FBG sensor response to the bending curvature change induced by bolt loose angle variation. The theoretical sensing principle is established based on the deformation model of an inextensible elastic beam under prescribed end rotations. The influence parameters on the FBG bending sensor are investigated numerically under the bolt loose angle variation from 0° to 15°, and the position on the sensor with maximum curvature difference varying linearly to bolt loose angle variation is obtained and selected to arrange the FBG. Based on the theoretical and numerical analysis, a general solution of the wavelength shifts of the FBG bending sensor versus bolt loose angle under an optimal initial installation condition (i.e., d/l=0.9, and s/l = 0.85) is established for quantitative monitoring bolt looseness. The feasibility and performance of the proposed method are experimentally verified and the maximum error between numerical and experimental results is less than 6.19%. The results indicate that the proposed method can quantitatively monitor bolt loose angle with high resolution (about 0.005°), also prove that the theoretical derivation in this paper and the proposed design criteria can theoretically guide the monitoring of bolt looseness using FBG bending sensor.