Composite cure monitoring with Bragg grating sensors

Residual stress is induced in fiber composite materials during the cure process because the thermal expansion coefficient of the fiber is generally much lower than that of the polymer matrix. The two materials are "locked" together at the cure temperature. Then, as they cool, the matrix attempts to contract more than the fiber leading to tension in the matrix and compression in the fiber. This can lead to the formation of microcracks parallel to the fibers in thick composite plies or yarns. The magnitude of residual stress can be reduced by modifying the cure cycle; however, optimizing the cure cycle requires a complete understanding of the state of curl throughout the composite. This is a complex problem - especially in thick composites. Pilot studies have been performed placing Bragg grating sensors in glass fabric preforms and monitoring the response of the grating during resin infusion and cure. The typical response shows the initial thermal expansion of the Bragg grating, a rapid contraction of the grating as the resin gels, slower contraction during cure, and thermal contraction at the composite thermal expansion coefficient during cool down. This data is then used with micromechanical models of the fiber/matrix interaction during cure to establish material parameters for cure simulation. Once verified, these curl simulation methods will be used to optimize tooling design and cure cycles in composite components.