Effect of Saline and Hygrothermal Exposure on the Mode I Fatigue Behavior of CFRP Adhesive Joints

This work investigates the Mode I fracture behavior of adhesive joints manufactured from unidirectional carbon fiber-reinforced epoxy composites (CFRP) under static and fatigue loading. Specimens were exposed to two degradation environments: hygrothermal conditions (60 °C, 70% RH) and saline conditions (35 ± 2 °C, 89% RH), for 1 and 12 weeks, and compared with non-exposed material. Double Cantilever Beam (DCB) tests were conducted to evaluate the influence of aging on fracture toughness. Thermal (Differential Scanning Calorimetry, DSC) and spectroscopic (Fourier Transform Infrared Spectroscopy, FTIR) analyses were performed to identify degradation mechanisms. DSC results showed no significant variation in glass transition temperature (Tg) under saline exposure, whereas hygrothermal aging increased Tg, indicating post-curing effects. FTIR analysis revealed moisture uptake and oxidation under saline conditions, while hygrothermal exposure mainly led to structural rearrangement. Critical energy release rate (GIC) values were used to define fatigue test conditions, enabling the construction of fatigue initiation (ΔG–N) and crack propagation (G–da/dN) curves. A Weibull-based model was applied to describe fatigue initiation behavior. Results show that saline exposure promotes progressive degradation, whereas hygrothermal conditions may enhance performance due to post-curing effects.