Fiber Bragg gratings have the advantages of small size and light weight, therefore can be embedded into composites to monitor the forming temperature. However, annealing and calibration are always required due to the high temperature demanded for thermoplastic composite forming, which will cost a lot of time and energy. An encapsulated fiber Bragg grating was successfully proposed to isolate strains and overcome the challenge of high forming temperature up to 332°C and high forming pressure of 2 MPa, though pre-annealing needs to be applied each time before actual measurement. In this study, by applying a polynomial numerical fitting algorithm obtained from reference fiber Bragg gratings in the same sensor batch, the nonlinear temperature response and irreversible Bragg wavelength shift under high temperature can be precisely estimated. Forming temperature monitoring of glass fiber/polyetherimide composite was successfully achieved using the unannealed FBGs. The temperature curve was reconstructed with an average difference of 0.8°C compared to the reference thermocouple. These results demonstrate the application of unannealed FBGs in in-situ composite forming monitoring of high temperatures.
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