Synergistic delamination toughening of composites using multi-scale carbon reinforcements

Multi-scale toughening is a key strategy employed by biological systems, made of intrinsically brittle constituents, to achieve high damage tolerance. This paper presents an investigation of the synergistic enhancements to the mode I interlaminar fracture toughness of fibre-polymer composite laminates using multi-scale carbon reinforcements. By combining carbon nanofibres (CNFs) dispersed in the matrix and z-pins in the laminate thickness at various contents, an extra mechanism of energy dissipation occurs. This additional mechanism synergistically improves the laminate's resistance to delamination growth under mode I loading. Addition of the nanofibres in the matrix increases the interfacial strength and frictional energy dissipation during z-pin pull-out, thus generating a greater-than-additive toughening effect that would not have existed should either the nanofibres or the z-pins been deployed alone. The results reveal that the magnitude of the synergistic toughening effect was dependent on the volume fraction and combinations of CNFs and z-pins used; where synergy values ranged between 24 and 69% over the expected additive toughness value. A numerical model was developed to successfully predict the crack growth resistance and the synergistic toughening effect with filler content of the multi-scale composites.