Interaction of oxidation and damage in high temperature polymeric matrix composites

Polymeric matrix composites with long term durability requirements at high temperatures must be designed to resist degradation due to physical aging, chemical changes and thermo-oxidation. This paper describes the interaction between oxidation and damage during high temperature aging of polymeric matrix composites. The oxidation layer growth in neat resins depends upon the relative dominance of the oxygen diffusion rate in oxidized region and the reaction rate in the un-oxidized region. Oxidation in fiber-reinforced composites is seen to be orthotropic with axial direction of fiber being the preferred oxidation growth direction. Transverse oxidation growth correlates with growth rates in neat resins after accounting for the fiber microstructure effects and attributing additional diffusivity to the fibers and fiber-matrix interphase. However, the oxidation growth in areas where discrete cracking is observed is substantially higher. Close coupling is observed between discrete crack growth rates and oxidation layer growth rates in axial direction. Damage evolution and the interaction of damage and oxygen diffusivity are critical factors and must be considered for oxidation growth prediction in composite materials. In this paper, a model-based analysis of oxidation in composites is presented using a systematic methodology that determines the relative effects of the matrix oxidation, role of fiber and interface effects and that of the damage growth. Carbon fiber-reinforced PMR-15 composites are used for both experimental characterization and simulations.