Cure Monitoring and Internal Strain Measurement of 3-D Hybrid Braided Composites using Fiber Bragg Grating Sensor

Curing behavior and internal strain of 3-D hybrid braided composites is studied using embedded fiber Bragg grating (FBG) sensors. Thermo-optic coefficients and photo-elastic constant of fabricated FBG sensors are experimentally determined. A seven-layered three-dimensional (3-D) braided preform is fabricated. S2 glass fiber is used as the main braiding yarn, and Kevlar fiber is inserted in the exterior surface layer in the axial direction. Several nylon fibers are braided together in order to replace the FBG sensors after braiding. FBG sensors are embedded along the curved braiding yarn as well as in the machine direction. An FBG-embedded 3-D braided composite is fabricated using the resin-transfer molding (RTM) process. During the cure process, the expansion and contraction caused by the matrix cure as well as temperature changes are monitored. Local dimensional changes are measured quantitatively using an FBG sensor system. The FBG sensor embedded along a curved braiding yarn exhibited a relatively small amount of shrinkage. By using embedded FBG sensors and surface-bonded electric strain gages, internal strains of the 3-D braided composite are measured during compressive and flexural tests, and the results show good agreement with each other.