Interfacial Toughening Strategies for Impact and Fatigue Tolerant Structural Biocomposites

Biocomposites reinforced with continuous natural plant fibres such as flax can replace conventional composites in various fields. For instance, flax fibres have higher density-normalised stiffness than glass fibres, higher damping than synthetic fibres, environmental merits, and good end-of-life options. Although the industry regulations on energy consumption and circularity have continuously accelerated the production volume of flax fibre reinforced structural composites, their share in the plastics and composites market is not yet optimum. One of the main issues is their long-term durability under dynamic loading conditions, which is critical for their main application fields, such as boats, sports and automotive. Regardless of the type of polymer matrices and their toughness, flax fibre composites subjected to impact and fatigue loading present brittle behaviour.

In this thesis, various interfacial toughening strategies and their effects on low- velocity impact resistance and fatigue performance of flax fibre composites are elucidated. The aim was to promote energy dissipation through interfacial sliding between fibre-matrix while providing sufficient interfacial adhesion for effective load transfer between fibre and matrix. The three main strategies were: (i) to deposit functionalised multi-layer graphene oxide crystals on the fibre to enable synergy between interfacial adhesion and sliding under dynamic loads, (ii) to coat fibres with a biobased thermoplastic coating to create a ductile phase between flax-epoxy, and (iii) to benefit from ductility of non-dried fibres through moisture insensitive in-situ polymerisation of the poly (methyl methacrylate) (PMMA) thermoplastic resin.

The interfacial toughening results showed the possibility of creating synergy between properties such as stiffness and toughness for flax-PMMA and flax-epoxy composites with 40−100% better impact perforation energy, suppressed fibre failure, and 17−20% better fatigue performance. The scientific impact of this thesis was to elaborate on dynamic failure modes and means to tailor natural plant fibre composites as durable structural materials for sports and automotive applications.