Optical fiber sensors will be developed for applications in composite materials that are processed at high-temperatures (>1000oC). Specifically, novel optical fibers will be used for two types of in-situ measurements: temperature and stain. Temperature sensors will be synthesized with rare-earth dopants that will permit spatially resolved temperature measurements in the range of 500- 1200oC. Strain sensors based on Fabry-Perot gratings and using optical time domain reflectometry will both be investigated. The temperature and strain sensors that are developed will be incorporated in fiber-reinforced, ceramic and intermetallic matrix composites that will be produced by chemical vapor infiltration (CVI). Because these materials already contain fibers, the incorporation of additional optical fiber sensors will not significantly perturb the microstructure. A variety of advanced, thermal gradient CVI processes are currently being developed, and the optical fiber sensors that will be developed as part of this program will make it possible to monitor temperature gradients during processing. A relatively simple thermal gradient CVI system will be used to make in- situ optical fiber temperature measurements during composite fabrication. The microstructure of these materials will also be characterized quantitatively, and a numerical model of the infiltration process will be developed to analyze both the temperature gradients and the microstructure evolution. Comparisons between the modeling calculations and the experimental measurements will provide insight into the infiltration process, and demonstrate the potential for using optical fiber temperature sensors. The strain sensors will provide measurements during processing, and more importantly, they will make it possible to measure strain when the composite is in use. These in-situ strain measurements offer a novel method to study the stress-strain behavior of the material. In brittle matrix composites, the matrix phase typically begins to microcrack long before the material fails, thus these in-situ strain measurements are particularly important because it will be possible to monitor the degradation of these materials before macroscopic failure actually occurs.
