This work investigates the effect of long-term environmental exposure on the performance of composite-to-metal bonded joints. Specimens are manufactured using a carbon-fiber reinforced polymer (CFRP) co-bonded to a steel member with epoxy adhesive and aged in a salt spray chamber. The mixed-mode fracture behavior of the non-aged and aged specimens is assessed using the mixed-mode bending (MMB) test apparatus. The fracture energy is calculated using the finite elements method and an analytical approach, the strain-based method (SBM). The SBM showed to be a simple and accurate method to obtain the total fracture energy and the fracture mode ratio of the bi-material specimen. Ageing increased the fracture toughness at crack initiation by 27% for specimens loaded at 20% mode II and 7% for specimens loaded at 15% mode II. This can be related to the shear behavior and plasticization of the adhesive material. During crack propagation, the fracture toughness remained similar for specimens loaded at 20% mode II and decreased by 15% in specimens loaded at 15% mode II. Fractography analysis together with chemical characterization showed that the penetration of moisture at the edges of the fracture surface produced adhesive failure in these regions affected by moisture. Moreover, the failure mode at the unaffected regions of the fracture surface shifted from cohesive to a combination of thin-layer cohesive and adhesive failure after ageing. The results contributed to describe the effect of ageing on the fracture behavior of bonded materials.@en

In this work, peel tests were used to assess the adhesion quality between carbon fiber reinforced polymers (CFRP) and carbon steel plates. The tests were performed according to the ASTM standard of floating roller peel tests (D3167–97) with a new specimen layup. The layup and geometry of specimens was defined in order to have the CFRP as the rigid substrate and the steel plate as a flexible substrate. The aim is to assess the adhesion quality of the interface in dry-conditions (short-term) and after ageing (long term). Specimens were inside the salt spray cabinet for 30 days to absorb the moisture. Results show that peel load are not significantly affected by the ageing condition. The specimens tested at dry-conditions and ageing condition showed a cohesive failure within the adhesive layer, which indicates a good adhesion between the CFRP-Steel interfaces. Long term durability is suggested by keeping the specimen for longer duration in order to observe the degraded trend and adhesion quality of the joints.@en