32. In-service Preload Monitoring of Bolted Joints Subjected to Fatigue Loading Using a Novel ‘MoniTorque’ Bolt

Bolted fastening is one of the oldest, most important, and most neglected aspects in engineering design of machines and structures. One of the biggest concerns is the bolt preload control and monitoring during and after the joint assembly. A survey of automobile service managers in the United States has shown that one-quarter of all service problems were traced to loose fasteners. Therefore, fatigue behavior of in-service bolted joints is of special interest for predicting structural integrity and residual lifetime of the structure. In this work, a novel technique ‘MoniTorque’ (Monitoring Torque) that embeds fiber Bragg-grating (FBG) sensors within the bolt shaft using temporary adhesives is used to monitor pre-load changes during fatigue tests of bolted joints. The inherent small size of the FBG allows precise monitoring without affecting the intrinsic properties of the bolt. Furthermore, temporary adhesives allow easy installation, removal and reuse of FBG sensor, thus converting a simple bolt into a reliable sensing element. Results show good correlation between the number of loading cycles and preload reduction. Overall, the ‘MoniTorque’ technique demonstrated in this work shows promise in cost-effective and reliable health monitoring of in-service bolted joints.