Influence of thermal stress on temperature sensitivity of FBG-GFRP bars

Abstract

The influence of thermal stress on the temperature sensitivity of fiber Bragg grating-glass fiber reinforce polymer (FBG-GFRP) bars is studied by three methods, namely, direct experimental calibration method, stress analysis (finite element analysis) method and the method of apparent temperature sensitivity coefficient. In comparison with the other two methods, fewer parameters are required and the calculation is simple in the method of apparent temperature sensitivity coefficient, while the analytical error is limited within 2%. It is concluded that the results of the method of apparent temperature sensitivity coefficient could be good reference for engineering applications.At present, fiber Bragg grating (FBG) sensor is the first choice for structural health monitoring due to its advantages, such as anti-electromagnetic interference, small size, quasi-distributed sensing, corrosion-proof and absolute measurement, etc [1][2][3] . Smart glass fiber reinforce polymer (GFRP) bar was developed by Kalamkarov et al [4] through embedding FBG sensors in a GFRP bar to combine the reinforcement function and the sensing function together in concrete structures. When the temperature fluctuation occurs, the thermal stress is induced by thermal mismatch due to the difference of the thermal expansion coefficients between the optical fiber and the matrix material. Therefore, FBG is highly sensitive not only to strain (as designed), but also to temperature fluctuation. Numerous studies have been carried out to separate these two effects and/or to find a way for temperature compensation [5][6][7][8] , but the influence of thermal stress on temperature sensitivity of FBG is rarely studied.In this paper, stresses on all directions in FBG embedded in the matrix material are analyzed. With the finite element software we can calculate stresses in FBG, and emulation data is obtained to quest the temperature sensitivity coefficient of smart GFRP bar. Moreover, the approximate formula is derived for the simplification of calculation and the convenience of application.In working environment, due to the mismatch of the thermal expansion coefficient, FBG is under stresses from the matrix material in both axial and radial directions, resulting in additional drift from the central wavelength of FBG. Generally, the influence of temperature on the central wavelength of FBG could be divided into three cases: wavelength drift caused by thermo-optic effect and thermal expansion B1 , wavelength drift caused by axial stress B2 , and wavelength drift caused by radial stress B3 . According to Ref.[9], the total wavelength drift can be regarded as the superposition of these three cases, that is: B = B1 + B2 + B3 .When temperature varies, the wavelength drift formula without considering the waveguide effect [9] is given by , 2 B 12 12 11 2 eff B n 1 B zz rr zz p p p n Twhere n is the thermo-optic coefficient of FBG, B is the central wavelength of FBG, p ij is the elastic-optic coefficient of FBG, n eff is the effective refractive index of FBG, and rr an...