Monitoring torque in bolts using an embedded fiber Bragg grating sensor

Abstract

In this study, we embedded a fiber Bragg grating (FBG) sensor into a bolt and experimentally examined whether the wavelength in the FBG changes in response to small elongations of the bolt caused by an acting torque. The FBG sensor was fabricated and embedded in the bolt at different depths and positions to investigate whether these factors directly affect the sensitivity. M20 bolts were used in this study. The greatest sensitivity (0.00298 nm/N-m, R2 = 0.9984) was obtained from the bolt in which the FBG sensor was embedded 7.0 mm from the center of a completely penetrated bolt. The findings suggest that the deeper the embedding depth of the sensor and the closer its position to the bolt surface, the higher the sensitivity. The FBG sensor is suitable for the measurement and long-term monitoring and control of bolt torque.