Strain–temperature decoupling of fiber Bragg gratings based on residual stress in CFRP composites

This study proposes a decoupling method for FBG strain and temperature measurements based on the residual stress in CFRP. An FBG is embedded vertically between unidirectional carbon-fiber plies, and the mismatch in thermal expansion coefficients during cooling induces residual stress that causes birefringence in the grating. As a result, the two polarization-dependent reflection peaks exhibit different temperature sensitivities, enabling simultaneous decoupling of strain and temperature from a single FBG. Compared with the conventional dual-FBG decoupling approach, the proposed method yields a maximum strain measurement error of 3.63 µε with a root-mean-square error of 1.91 µε. The maximum deviation between the measured temperature and the calibration values obtained from a platinum resistance thermometer is 0.92 °C, with a root-mean-square error of 0.52 °C. Within a working range of −30 °C to 120 °C and strain levels below 3200 µε, the average temperature RMES and strain RMES from three experiments are 2.02 °C and 42.37 µε, respectively. These results demonstrate that high-accuracy strain–temperature decoupling inside CFRP can be achieved using a single embedded FBG.