Relaxation Modeling of Unidirectional Carbon Fiber Reinforced Polymer Composites Before and After UV-C Exposure

Carbon fiber-reinforced polymers (CFRPs) are widely used in aerospace for their lightweight and high-performance characteristics. This study examines the long-term viscoelastic behavior of CFRP after UV-C exposure, simulating low Earth orbit conditions. The viscoelastic properties of the polymer were evaluated using dynamic mechanical analysis and the time-temperature superposition principle on both unexposed and UV-C-exposed samples. After UV-C exposure, the polymer’s instantaneous modulus decreased by about 15%. Over a 32-year period, the modulus of the unexposed resin is expected to degrade to approximately 25% of its initial value, while the exposed resin drops to around 15%. These experimental results were incorporated into finite element method models of a unidirectional CFRP representative volume element. The simulations showed that UV-C exposure caused only a slight reduction in the CFRP’s axial relaxation coefficient along the fiber’s axis, with no significant time-dependent degradation, as the fiber dominates this behavior. In contrast, the axial relaxation coefficient perpendicular to the fiber’s axis, as well as the off-diagonal and shear relaxation coefficients, showed more notable changes, with an approximate 10% reduction in their initial values after UV-C exposure. Over 32 years, degradation became much more severe, with differences between the pre- and post-exposure coefficient values reaching up to nearly 60%.