Temperature and strain monitoring of the curing process of carbon fiber composites based on fiber optic FBG-FP

With the development of the aviation industry, the demand for lightweight and high-strength aircraft has become increasingly significant, leading to the growing use of Carbon Fiber Reinforced Plastic (CFRP) in aircraft, with its usage ratio being a key indicator for evaluating the advancement of commercial aircraft. Temperature and strain can assess the bonding strength and performance of composite materials during the curing process, determine the occurrence of the gel point, provide data support for the improvement of material curing processes, and predict damages such as delamination and debonding. Therefore, accurately monitoring the temperature and strain of CFRP is of great importance. In practical applications of optical fibre sensors, temperature changes and strain variations occur simultaneously, and both temperature and strain will alter the central wavelength of the fibre Bragg grating (FBG). To achieve temperature-strain decoupling, this paper adds a Fabry-Pérot cavity (FP) near the Bragg grating to form an FBG-FP sensor. Due to the low sensitivity of the FP cavity length to temperature and its linear response to strain, the FBG-FP sensor can effectively measure both temperature and strain simultaneously, achieving decoupling. By embedding this FBG-FP sensor within carbon fibre reinforced polymer, the temperature and strain during the curing process of the composite can be monitored. By real-time spectral acquisition, the strain value can be computed from the cavity length using fast Fourier transform (FFT) and maximum mean square error (MSE) methods, and the temperature fluctuation value can be calculated from the wavelength change, resulting in a temperature measurement error of less than 2°C and a strain error of less than 50. Additionally, the FBG-FP sensor embedded in CFRP can be utilised for the structural health monitoring of CFRP throughout its entire lifecycle.