Correlation between phase morphology and interlaminar fracture toughness of carbon fibre reinforced epoxy composites modified by multiscale hybridisation

Matrix hybridisation, achieved by incorporating multiple tougheners, is a promising strategy to improve the toughness of brittle epoxy by enabling diverse mechanisms to dissipate crack energy at multi-length scales. This study, however, highlights that fracture toughness gains in thermoplastic (TP) and carbon nanotube (CNT) modified epoxy matrices may not directly translate to the resulting carbon fibre-reinforced polymer (CFRP) composites. The investigation clarifies the complex interaction of resin functionality (bi- and tetra-functional), multiscale tougheners and fibre reinforcements in influencing phase morphology and the resulting Mode-I fracture toughness of CFRP composites. It is revealed that for the bi-functional epoxy-based CFRP composites, TP modification causes non-uniform morphology due to the barrier effect of fibres during phase separation, while CNTs partially offset this by promoting more uniform phase distribution. Nevertheless, fracture toughness improvement is statistically insignificant when introducing CNTs into TP/epoxy CFRP composites, due to limited crack propagation through tough TP-rich phases. In contrast, incorporating CNT and TP in tetra-functional CFRP composites yields a hierarchical array of crack energy dissipation mechanisms at various length scales, resulting in a more pronounced toughness increase. The strategies for controlling phase separation dynamics to further enhance the toughness of CFRP composites modified by CNT/TP hybridisation are also introduced.