Real-time stress and strain measurement in bolted flanges using FBG sensors for enhanced sustainability and leak prevention

Bolted flange joints are critical components in pressure retaining and structural systems, where maintaining proper preload is essential for sealing integrity and mechanical reliability. Conventional torque-based tightening methods often result in inconsistent preload due to frictional variations and the absence of real-time monitoring. This inconsistency arises because friction can vary from bolt to bolt, leading to uneven preload even when the same torque is applied. Real-time monitoring enables precise preload verification and adjustment, ensuring uniformity and reducing the risk of failure. This study develops and validates Fibre Bragg Grating (FBG) embedded stud bolts for real-time preload measurement. Four M18 carbon steel grade 8.8 bolts were tested under incremental torque from 10 to 80 Nm at room temperature, with wavelength shifts converted using an optical interrogation system. The results show a near-linear torque–wavelength relationship, with measured sensitivities of 0.0100, 0.0123, 0.0120, and 0.0080 nm/N·m for Bolts B1, B2, B3, and B4, respectively, and R² values of 0.946, 0.974, 0.939, and 0.974, confirming consistent sensitivity ranking and stable sensor performance. Wavelength shifts ranged from approximately 0.08 to 1.07 nm at 80 Nm. These findings demonstrate that FBG-embedded bolts provide a reliable, non-intrusive solution for continuous preload monitoring, improving load control accuracy and enabling predictive maintenance in high-reliability industries such as oil and gas or aerospace.