Rayleigh scattering based, thermal-induced displacement measurement along a steel plate at high temperature

This study aims to quantify Rayleigh scattering based measurement accuracy of distributed fiber optic sensors under a heating-holding load protocol and characterize the effect of polymer coating on the sensors as the polymer softens and melts away at elevated temperatures. Two segments of a coated single-mode optical fiber were loosely attached and firmly bonded to a steel plate, respectively. When locally heated up to 405 $°\text{C}$ in a furnace, the two segments were used to measure temperature alone and thermal-induced strain. The axial displacement associated with the strain measurement was compared with that of a dial gauge. A temperature increment of 13 ​°C (≪ 20 ​°C) is recommended to ensure successful correlation analysis of Rayleigh scattering signals. The polymer was found to start softening at 155 $°\text{C}$ and melting at 267 ​°C. As temperature sensors, optical fibers with an insulation sheath can accurately measure temperature untill 155 ​°C without and till 267 ​°C with thermo-mechanical analysis of sheath-fiber bonding behavior. As strain sensors, optical fibers with temperature compensation are accurate for strain measurement up to 155 ​°C, above which strain transfer analysis is required due to softening of the fiber coating. Under a large temperature gradient covering 155–267 °C, the fibers attached on steel plates with epoxy function like an extensometer as a center portion of the coating with the high-end temperature is melted as verified by microscopic analysis.