Modeling of Hygrothermal and Aging Effects in Textile Composites

It is now well known that polymer based composites exhibit time dependent properties, and sub-T g aging of these materials results in a combination of physical and chemical aging phenomena. Therefore, any durability study on polymer composites for elevated use-temperature applications should address both the physical and chemical aspects of aging. In general, aging cause polymers to become stiffer and more brittle with age, increasing the likelihood for more rapid progression of various damage states. In addition to aging, other environmental effects also play an important role in determining the life of a composite material. For example, a fiber reinforced composite material with a polymer matrix will typically absorb moisture in a humid environment and at elevated temperatures. Combined exposure to heat and moisture affects a polymer matrix composite (PMC) in a variety of ways. Hygrothermal swelling causes a change in the residual stresses within the composite that could lead to micro-crack formation. These micro-cracks in turn provide fast diffusion paths and thus alter the moisture absorption characteristics of the laminate. In this paper, theory of irreversible thermodynamics is applied within the framework of continuum mechanics to derive governing equations for diffusion and aging in a PMC from first principles. For model verification, the model predictions are compared with experimental data for a 5-harness satin weave graphite/epoxy [0/90/0/90] s laminate with distributed matrix micro-cracks