High-temperature strain sensor based on sapphire fiber Bragg grating

Sapphire fiber Bragg grating (SFBG) is a promising high-temperature strain sensor due to its melting point of 2045 degrees C. However, the study on the long-term stability of SFBG under high temperature with an applied strain is still missing. In this paper, we reported for the first time to our knowledge on the critical temperature point of plastic deformation of the SFBG and demonstrated that the SFBG strain sensor can operate stably below 1200 degrees C. At first, we experimentally investigated the topography and the spectral characteristics of the SFBG at different temperatures (i.e., 25 degrees C, 1180 degrees C, and 1600 degrees C) with applied 650 mu epsilon. The reflection peak of the SFBG exhibits a redshift of about 15 nm and broadens gradually within 8 h at 1600 degrees C, and the tensile force value decreases by 0.60 N in this process. After the test, the diameter of the SFBG region decreases from 100 to 88.6 mu m, and the grating period is extended from 1.76 to 1.79 mu m. This indicates that the plastic deformation of the SFBG happened indeed, and it was elongated irreversibly. Moreover, the stability of the Bragg wavelength of the SFBG under high temperature with the applied strain was evaluated. The result demonstrates the SFBG can be used to measure strain reliably below 1200 degrees C. Furthermore, the strain experiments of SFBG at 25 degrees C, 800 degrees C, and 1100 degrees C have been carried out. A linear fitting curve with high fitness (R-2 > 0.99) and a lower strain measurement error (<15 <mu>epsilon) can be obtained. The aforementioned results make SFBG promising for high-temperature strain sensing in many fields, such as, power plants, gas turbines, and aerospace vehicles.