A 15-year aging study of CFVE composites: Multiscale insights into interfacial shear, residual stress and fatigue behavior

The improved mechanical performance of polymer-based fiber-reinforced composites has driven the Navy to explore their use in naval applications, particularly under extreme hygrothermal conditions. Since naval ships endure temperature fluctuations and seawater exposure, understanding long-term effects is critical, yet studies beyond a decade of immersion remain scarce. In this work, carbon fiber vinyl ester (CFVE) composites were exposed to seawater for 15 years, then conditioned cyclically at −80°C and 80°C. We employ microscale repush tests to assess residual stresses and complementary single-fiber push-out tests to measure interfacial shear strength, alongside comprehensive micromechanical and optical characterizations. Fractography revealed cracking along tow and stitching circumferences, with matrix protrusion and signs of thermal aging in high-temperature specimens; the −80°C wet samples exhibited clear interfacial opening. Push-out tests indicated increased IFSS after hygrothermal conditioning—a strengthening effect not previously reported—while repush tests showed elevated residual stresses in 80°C-conditioned coupons. Although high scatter in fatigue data precluded definitive statistics, the greatest cycles to failure consistently occurred in + 80°C samples, suggesting macroscale interface strengthening. These findings offer valuable insights into the long-term durability of CFVE composites for naval structural applications.