Global and Local Approaches Applied to Mode I Fracture Toughness Measurement of Bi-Material Bonded Joints

Abstract

Nowadays, the application of different materials is quite common to optimise the performance of structural components. In this context, bonding between metals and composites becomes a frequent strategy. However, the appropriate design requires special care owing to the complex stress profile in the adhesive layer, caused by the stiffness mismatch inherent to these solutions. A crucial aspect relies on fracture characterisation under mode I loading of bi-material bonded joints. Therefore, in this work, two different approaches are analysed regarding the attainment of an almost pure mode I of Carbon–Epoxy/Aluminium bonded joints using the asymmetric double cantilever beam test. One method relies on a global approach that matches the bending stiffness of the specimen arms, while the second, the longitudinal strain-based criterion, matches the longitudinal strain distributions of the two adherends at the bondline, making it a local method. The global process was validated against experimental results and subsequently compared to the local methodology. Additionally, several combinations of metallic and fibre-reinforced polymers were analysed numerically using the finite element method with a cohesive zone model to compare results from the two procedures. The consistently low presence of mode II loading across all cases demonstrates that both the local and global approaches accurately capture a predominant mode I loading condition under the investigated scenarios.