Quantifying Thermal Strain of Steel Plate Subjected to Constant Temperature by Distributed Fiber Optic Sensors

Effective strain measurement tools for steel structure at high temperature are limited due to a significant gap in measurement science. This study aims to experimentally and numerically investigate effectiveness and limitation of Rayleigh scattering based, distributed fiber optic sensors (DFOS) without coatings for measuring temperature and strain of a steel plate subjected to a local constant temperature. The DFOS were bonded to the steel plate by an epoxy with different bond lengths to measure coupled strain and temperature effect, while the DFOS near the end of the epoxied segment measured the temperature effect only for temperature discrimination. It was found that the DFOS accurately measured the temperature and strain of the steel plate with different bond lengths of the epoxy, as compared to the thermocouple temperature and thermal-induced strain, respectively. The maximum strain (or temperature) that the DFOS without coatings could measure for the steel plate was less than 1600 $\mu \varepsilon$ (or 150${}^{{}^{\circ}}C$). Moreover, a local finite element model with the calibrated elastic modulus of the epoxy subjected to a uniform temperature field well captured optical fiber strains in the elastic stage. From parametric studies, the effect of the thermal expansion coefficients and elastic moduli of the optical fiber, epoxy, and host material as well as initial defect between the optical fiber and epoxy on the strain transfer coefficient was investigated. The elastic modulus of epoxy within 100 MPa and the rectangular cross-section of epoxy (0.5 mm thick and 4 mm wide) could achieve a strain transfer coefficient of 0.997, while the initial defect had a similar effect on the strain transfer to the protective coating. The normal-distribution epoxy shape was designed for guiding robot assisted intelligent instrumentation and construction in the future.