Multi-scale toughening of fibre composites using carbon nanofibres and z-pins

Improving the interlaminar fracture toughness of fibre-reinforced composites based on thermosetting polymeric matrices is of significant interest to a broad range of applications. In the present work we report a multi-scale approach to synergistically toughen composites by combining nano- and macro-scale reinforcements inspired by natural composite materials. Carbon reinforcements with two different length scales are used: nano-scale carbon nanofibres (∼100 nm diameter) and macro-scale carbon z-pins (∼280 μm diameter) to reinforce continuous carbon-fibre composites in the through-thickness direction. The resultant composite, featuring three-dimensional reinforcement architecture, possesses triple toughening mechanisms at three different scales, thus yielding a synergistic effect. At the nano-scale, the carbon nanofibres alone promote high mode I delamination resistance (∼70% increase in interlaminar fracture energy) by multiple intrinsic and extrinsic toughening processes around the crack tip. The macro-size carbon z-pins, together with the crossover continuous fibres, promote a strong extrinsic toughening mechanism (∼200% increase in the interlaminar fracture energy) behind the crack tip and over a larger length-scale via both the z-pins and crossover fibres bridging the crack faces. When used concurrently, the nanofillers and z-pins promote a higher toughness under quasi-static loading (∼400% increase in fracture energy) than when used separately due to a multiplicative effect from the interplay between intrinsic and extrinsic toughening processes operative ahead of, and behind, the crack tip. Under mode I interlaminar cyclic-fatigue loading, the multi-scale laminates show a strong improvement in resistance against fatigue delamination growth. Similar to the synergistic increase in fracture energy, a greater increase in the delamination fatigue resistance occurs when both are active together. However, the results indicate that the synergistic effect of the multi-scale toughening is statistically significant under quasi-static loading but not under fatigue loading. A very small reduction (∼2%) in the tensile strength is observed for the multi-scale reinforced laminates.