6. Moisture-Induced Degradation

Abstract

Composites provide advantages over conventional structural upgrade systems by offering up to 50% first-cost savings and lower life-cycle costs, often with additional benefits such as easier installation and improved safety. Fiber-reinforced polymer (FRP) composites are finding increasing applications as primary structural components in aerospace and automotive applications, bridges, building repair, and the oil and gas pipeline industry. These composites are typically exposed to a variety of aggressive environments, such as extreme temperature cycles, ultraviolet (UV) radiation, moisture, alkaline/salt environments, etc. However, no capability currently exists for reliably projecting the future state and conditions of composites used in various environments. The accurate determination of diffusivity and moisture uptake in a polymer composite is a key step in the accurate prediction of moisture-induced degradation. With this in mind, the chapter is subdivided into three sections: (1) the combined influence of damage and stress on moisture diffusion within the (bulk) polymer matrix in a polymer composite, (2) the combined influence of strain gradient, relative humidity, and temperature on moisture diffusion at the fiber–matrix and/or interlaminar interface, and (3) a simple mechanism-based model to predict strength degradation in a composite due to moisture ingress. The discussions presented in this chapter are primarily directed toward thermoset resins, such as epoxy.