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 (Formula presented.). Over a 32-year period, the modulus of the unexposed resin is expected to degrade to approximately (Formula presented.) of its initial value, while the exposed resin drops to around (Formula presented.). 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 (Formula presented.) 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 (Formula presented.).