Effects of aramid-fibre toughening on interfacial fracture toughness of epoxy adhesive joint between carbon-fibre face sheet and aluminium substrate

Brittle epoxy adhesive joints, between the carbon-fibre/epoxy face sheets and aluminium substrate, were toughened using randomly-distributed short aramid fibres. In this study, effects of the epoxy adhesive thickness on interfacial fracture toughness of the adhesive joints, with and without short aramid-fibre toughening, were investigated. Short aramid fibres of 6 or 14 mm in length with an area density of 12 g/m2 were inserted between the carbon-fibre face sheet and aluminium substrate during the laminating process. Two and six layers of aluminium foils were inserted at the interface to form the controlled thin and thick adhesive joints, which are around 20 and 70 μm in thickness. The two “composite adhesive joints”, with different volume densities of short aramid fibres, reversed the adhesive-thickness influence on the interfacial toughness in comparison to that of the plain epoxy adhesive joints. However, both “composite adhesive joints” with low and high aramid-fibre densities resulted in significant improvement in the interfacial toughness. Analytical models, together with optical and scanning electron microscopy observations, were used to explain the experimental findings, and relevant toughening mechanisms.