Dual-FBG arrays hybrid measurement technology for mechanical strain, temperature, and thermal strain on composite materials

The response measurement of spacecraft during reentry is very important for designing the base composite structure. This paper proposes a hybrid measurement method to measure the mechanical strain, the temperature, and the thermal strain using dual fiber Bragg grating (FBG) arrays. The decoupling of temperature and stain is illustrated by a three-step process such that triple physical variables are measured simultaneously. For the base composite structure, four heating scenarios with thermal steady and non-steady states encountered during the lifting re-entry were designed to illustrate the feasibility of the proposed method. Under the thermal non-steady state, the average absolute error of the temperature measured by the dual-FBG and the thermocouple did not exceed 2.799 K. Meanwhile, the dual FBG-based thermal strain was compared with the thermally measured strain with an average relative error of no more than 2.913%. A finite element model at different temperatures was developed to calculate mechanical responses and was compared with the results obtained by FBG. The results show significant agreement between the measurement and the simulation, with a maximum error of 3.61%.