9. Characterization of Thermo-Oxidation in Laminated and Textile Composites

An understanding of the effects of thermo-oxidation in high-temperature PMCs for structural components subjected to arbitrary service environments is critical to life-performance predictions. Durability and degradation mechanisms in composites are fundamentally influenced by the fiber, matrix, and interphase regions that constitute the composite domain. The thermo-oxidative behavior of the composite is significantly different from that of the constituents as the composite microstructure, including the fiber/matrix interphases/interfaces, architecture, and ply layup introduce anisotropy in the diffusion and oxidation behavior. In this work, light microscopy and scanning electron microscopy techniques are used to characterize the oxidative process in laminated and textile carbon-fiber-reinforced polyimide composites. The observed anisotropy in composite oxidation is explained by carefully monitoring the development and growth of damage through the use of fluorescence imaging using dye impregnation. It is shown that alternative pathways for transport of oxygen into the interior of the composite are fiber–matrix debonds and matrix cracks that propagate with the oxidation front. It has been further determined through closer examination of the oxidation front and the crack front for discrete regions of the various composite specimens, that the oxidation front consistently precedes the crack front. This mechanism for accelerated oxidation is an excellent example of the intrinsic coupling of chemical oxidative aging and damage, which needs to be properly represented in predictive models.