Determining the minimum, critical and maximum fibre content for twisted yarn reinforced plant fibre composites

The effect of fibre volume fraction on aligned PFCs physical and tensile properties has been investigated. There is no correlation between fibre volume fraction and porosity. However, low fibre content PFCs are prone to intra-yarn voids, while high fibre content PFCs are prone to inter-yarn voids. This is due to changing resin flow dynamics with increasing fibre content. The tensile behaviour of PFCs with increasing fibre content is similar to that of conventional FRPs. At low fibre content brittle fracture occurs; increasing fibre content makes the fracture surface serrated and increases the occurrence and length of fibre pull-out. Interestingly, the elastic response for the plant fibres is found to be non-linear and this stress-strain response has been transferred into the composites as well. The effect of fibre content on tensile properties is found to closely follow the rule of mixtures. A void content of up to 4% is found to have minimal effect on the tensile properties of PFCs. The minimum and critical (vf,crit) fibre volume fractions for aligned flax and jute polyester composites are found to be substantially higher than conventional aligned FRPs; vf,crit for jute-polyester and carbon-epoxy is 14.1% and 2.7%, respectively. A simple model has also been developed to approximate the theoretical maximum obtainable fibre volume fraction of PFCs reinforced with staple fibre yarns. The absolute theoretical maximum fibre content is found to be 54.3%, which agrees with experimental values in literature. A high vf,crit (~15%) and low vf,max (~45%) implies that the possible range of employable fibre volume fractions is only 30% for PFCs