Hygrothermal Degradation Effects on Mode I Fracture in Adhesively Bonded Carbon–Epoxy Composites

Hygrothermal ageing can affect the mechanical performance of adhesively bonded carbon-fibre reinforced polymer (CFRP) joints under both static and fatigue loading. Understanding how their fracture behaviour evolves with exposure time is important for assessing the durability of bonded composite structures. This study analyses the influence of hygrothermal ageing time on the Mode I static and fatigue behaviour of carbon/epoxy joints bonded with an epoxy adhesive. Specimens were exposed to 60 °C and 70% relative humidity for 1, 2, 4, and 12 weeks and compared with unaged specimens. Double Cantilever Beam (DCB) tests were used to obtain the critical energy release rate GIC, fatigue initiation was analysed with a Weibull regression model, and crack growth rate curves were obtained. The analysis of intermediate exposure periods and the statistical comparison of the static results are the main contributions with respect to previous work. GIC reached its minimum after 2 weeks (−12%) and increased by 19% after 12 weeks, the only statistically significant change. Fatigue initiation resistance was lowest after 1 week and improved for longer exposures, whereas the crack growth rate of aged specimens remained higher than that of the unaged material. These results suggest a possible competition between moisture-induced plasticisation and post-curing of the adhesive, which affects crack initiation and propagation differently.