Functional composites with damage control and repair

Fibre Reinforced Polymer (FRP) composite structures are subjected during service to low energy impacts or unexpected loads, leading to damage. Microcracks are generally first formed in the matrix and can reach up to several hundred microns in thickness. In many living systems, an initial self-sealing phase followed by self-healing of the original tissue leads to highly effective repair of minor damage events. A bio-inspired strategy for the efficient healing of microcracks in FRPs is hence to: (i) autonomously close cracks whose thickness is above a certain threshold to ensure better crack faces registry; (ii) use a healing matrix with structural properties close to that of a conventional FRP matrix, that is able to expand and fill the cracked regions repeatedly after multiple damage events. This strategy was investigated in the present work through (i) the use of Shape Memory Alloy wires (SMAs) as stitches in FRPs and (ii) a healing matrix based on thermoset-thermoplastic phase-separated blends. Blends composed of epoxy resin and different thermoplastics (PCL, PLA and PMMA) were evaluated for their potential as healing matrix, based on their room temperature toughness, stiffness and their capacity to heal when subjected to a moderate heating cycle. Three types of blend morphology resulted from polymerisation-induced phase separation during cure, depending on the thermoplastic content, including an interconnected particulate epoxy phase and a co-continuous thermoplastic phase at high thermoplastic contents. An optimal composition was found for epoxy-25vol%PCL blends. The PCL phase expands by 14% in volume upon melting at 150 °C, therefore enabling filling of small cracks. When further integrated to FRPs (with an adapted vacuum infusion moulding process), this healing matrix led to composites with similar stiffness and strength to that of pure epoxy composites, but also to full recovery of compression after impact strength for low damage extent (impacts of 8.5 J). To provide healing for larger damage extent, NiTiCu SMA wires, with a diameter of 150µm, were stitched in the dry fabric before processing. After damage and upon heating at 150 °C, SMA stitches, that have been stretched and partly debonded upon crack propagation, efficiently closed the cracks. This procedure demonstrated the capacity of the wires to close 200 µm thick cracks in the FRPs and led to fully heal impact damage events up to 17 J, corresponding to the main concern of maintenance activities in the composite industry (e.g. tools dropped from 1-2 m height). Finally, as an alternative to blends, PCL electrospun nanofibrous veils were interleaved into fabrics, infused with epoxy and cured to reach a microstructure combining both phase-separated domains as well as intact nanofibre regions. Interlaminar crack propagation demonstrated up to 48% toughness increase as compared to reference specimens. However, healing was prevented due to reduced flow of PCL in the fine channels resulting from phase separation, showing the limitation of this approach as compared to the use of blends and stitches. Phase-separated epoxy-PCL composites (with or without stitched SMA wires), considering their manufacturing feasibility through conventional industrial processes, their acceptable mechanical properties and their ability to fully heal low-velocity impact damage, demonstrated their relevance for composite structures that are subjected to moderate loads and not easily accessible to repair.