The Influence of Material Variability on the Predicted Environmental Behavior of Composite Materials

The stresses and strains induced in a composite material when subjected to mechanical loads are shown to be strongly influenced by prior exposure to both temperature changes (e.g., cooldown from the curing temperature) and moisture absorption (e.g., even normal exposure to ambient conditions after cure). These environmental effects are in turn influenced by normal variabilities in the basic properties of the composite constituents. Using a recently developed finite element numerical model of composite material inelastic hygrothermal response, variations in stresses and strains are predicted as a function of variations in constituent material properties. A typical graphite/epoxy composite is selected for detailed study. This system is of particular interest because of the anisotropic nature of graphite fibers, and the strong temperature and moisture dependence of the epoxy matrix. Specific fiber property variations considered include transverse modulus, major Poisson’s ratio, and in-plane Poisson’s ratio. Results are compared with those obtained for an isotropic fiber composite.