Monitoring the gelation and effective chemical shrinkage of composite curing process with a novel FBG approach

Chemical shrinkage and gelation are critical issues in curing process of composite structures since they have significant influence on generation of distortion or residual stress. In this paper, a new in-situ method to capture the gel point and to measure the effective transverse chemical shrinkage was presented. A tailed FBG (Fiber Bragg Grate) set consists of several FBG sensors with various tail lengths was embedded perpendicular to the fiber direction to monitor the curing process. Gel point was determined with the aid of profile turning point of the FBG set and the results matched well with DMA test. The effective transverse chemical shrinkage which occurs between gelation and vitrification was obtained and compared with the one determined by TMA or bi-material strip test. Both results were used to predict the spring-in of C-specimen manufactured on CFRP tube. It turns out that accurate prediction of spring-in can be obtained with the effective transverse chemical shrinkage determined by tailed FBG set. The in-situ monitoring method with tailed FBG set is an alternative and promising technology to capture the gel point and to reveal the mechanism of curing distortion.