Modeling long-term degradation due to moisture and oxygen in polymeric matrix composites

Exposure to elements, such as moisture, oxygen and nitrogen, in the environment is deleterious to the constituents of a polymeric matrix composite. This environmental degradation is accelerated under high stress conditions and at high temperatures and humidity. Modeling the long-term degradation of a polymeric composite requires consideration of the constituent phase (fiber and matrix) degradation, interphase properties and damage evolution kinetics. The fundamental physical mechanisms controlling the environmental degradation are small molecule (particularly moisture and oxygen) diffusion behavior in the matrix, fiber and fiber–matrix interphase, the damage evolution kinetics and its interaction with the diffusion behavior, and the mechanical and chemical stability (reactivity) of the fiber and matrix phases with the gaseous penetrant. A model that enables long-term simulation of gaseous element diffusion and determine the role of fiber-matrix interphase on the effective diffusivity in the composite is presented in this paper. Three-dimensional finite element methods are used to model representative volume elements and consider anisotropic material behavior in all the phases. Particular attention is paid to the fiber–matrix interphase region and its influence on the moisture diffusion behavior in composites. Numerical simulations and selected correlations with experimental results are presented. Most results pertain to composites with the PMR-15 matrix.