Novel carbon-fibre powder-epoxy composites: interface phenomena and interlaminar fracture behaviour

Carbon fibres with three different sizing agents were used to manufacture unidirectional composites based on a powder epoxy resin. Powder epoxy processing was investigated as a route for fast, cost-effective manufacturing of out-of-autoclave composites compared to more time-consuming vacuum infusion technologies. In this work, a heat-activated epoxy powder was used as a resin system in low-cost vacuum-bag-only prepregs for thick composite parts that are required in the renewable energy industry (e.g. wind turbine blade roots). The importance of interfacial bonding between fibres and the matrix is shown and the impact on the ultimate mechanical performance of the manufactured composites demonstrated. The surface characteristics of the sizing on the carbon fibres were investigated using atomic force microscopy (AFM) and Raman spectroscopy. Results showed that the amount of sizing on the fibres' surfaces was inextricably linked with surface roughness and coverage. This in turn influenced the mechanical and chemical interlocking phenomena occurring at the fibre/matrix interface. The composites’ mechanical performance was evaluated using tensile, flexural and interlaminar fracture toughness tests. Fractographic analysis using optical and scanning electron microscopy (SEM) was likewise employed to analyse the fracture surfaces of the tested/failed composites. Interlaminar fracture toughness testing (DCB Mode-I) revealed that the interfacial adhesion differences could alter the fracture resistance of the composites, hence emphasizing the importance of the interfacial bonding strength between the polymer matrix and the carbon fibres.