Multi-scale interlaminar toughening of fibre-polymer composites

Fibre reinforced polymer (FRP) composites are used in aerospace applications due to the increasing demand for lightweight-yet-strong structures.  However, a long-standing problem with laminated FRP composites is their susceptibility to delamination damage and matrix cracking when subjected to in-plane loading (e.g. bondline failure of joints) and out-of-plane loading (e.g. hail-stones, bird-strike or in-service damage) scenarios. Due to the poor interlaminar fracture toughness and the brittle nature of the polymer matrix, delamination cracks can grow during service loading, further degrading the well-desired structural properties. In improving the delamination resistance, nano-scale or macro-scale reinforcements were used separately to increase the interlaminar fracture toughness of FRP composites. However, natural composites such as bone and nacre contain highly ordered constituents that span multiple length scales. These multi-scale materials invoke unique toughening mechanisms, exhibiting fracture toughness properties that far exceed the total contribution (i.e. synergistic) of their individual constituents.
           
The aim of this PhD project was to investigate the effects of incorporating multi-scale toughening approaches to synergistically enhance the fracture toughness, delamination crack growth resistance and other mechanical properties of FRP composites. Multi-scale toughening was achieved by simultaneously adding reinforcements spanning multiple dimensional scales. Reinforcements that were used in this PhD project included nano-scale carbon and metal oxide-based fillers, micron-scale carbon fillers and macro-scale z-pins.  Strategies to generate synergies (i.e. greater than the expected additive enhancements) on the delamination resistance properties of polymer composites were identified. Systematic investigations were conducted to experimentally measure and numerically predict the fracture toughness properties of the multi-scale reinforced composites with consideration to the volume content, orientation, size, distribution and combination of reinforcements. The degree of the greater-than-expected additive toughening improvement when compared to the individual toughening contribution from each reinforcement were dependent on the types of reinforcements used including the content. Substantial enhancements on the fracture toughness and delamination resistance translated to improvements on the load bearing properties of carbon fibre reinforced composite structures (e.g. laminates and joints). Findings from this PhD thesis helps identify news pathways in enhancing the damage tolerance of fibre-reinforced composites.