10. Modeling Thermo-Oxidative Aging and Degradation of Composites

Long-term durability and use-life of polymeric matrix composites operating at elevated temperatures are limited by their thermo-oxidative stability. Although weight loss testing is traditionally performed to characterize the oxidative degradation of composite systems, the results of such tests are neither translatable to other composites architectures with the same constituents nor scalable to longer exposure times or higher temperatures. A comprehensive modeling framework for understanding the morphological changes in the composites and degradation of the mechanical performance is described in this chapter. A thermo-chemo-mechanics model that defines and utilizes an oxidation state parameter for each constituent is formulated. The effect of oxygen diffusion in the fiber and fiber–matrix interphase on the oxidation of the composite is simula-ted. The role of damage in accelerating the oxidation growth along the fiber direction leading to high orthotropy in lamina oxidation is also addressed. The stiffness changes due to oxidation as well as the strains induced due to shrinkage are explicitly modeled leading to a detailed simulation of oxidation growth around discrete cracks. Oxidation growth in laminated composites is predicted using microscale and homogenization techniques. The model is applied to study the long-term thermal oxidation of polyimide composites.