Ductility potential of brittle epoxies: Thermomechanical behaviour of plastically-deformed fully-cured composite resins

The thermoset matrices that are typically used in structural composites are generally well known for extreme brittleness, sensitivity to defects, and poor performance at complex strain states. These features impede a full material characterisation and an understanding of their behaviour. It is, however, of fundamental importance to separate the scale-dependent and defect-imposed failure from the bulk material performance of epoxies, to enable significant improvements in ductility to be realised. The current paper suggests a new experimental routine for investigating the ductility limits of brittle epoxies and accumulating the large macro-scale volumes of plastically deformed epoxies, necessary to study physical and mechanical properties of cured thermosets following yield. It has been shown that a fully cured, densely cross-linked epoxy can undergo at least 50% of the equivalent plastic strain without loss in stiffness and with no detectable degradation of internal architecture. It has also been shown that the deformed epoxies change their thermo-visco-elastic behaviour. A comparative study of plain and toughened epoxies has shown that the former have higher ductility potential than the systems heavily loaded with thermoplastics. This implies that in order to achieve improvements in thermoset ductility, a revised concept of epoxy modification may be required. (C) 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license