Short Fibre Polymer Composites: a Fracture-Based Theory of Fibre Reinforcement

The interphase between reinforcing fibers and polymers is brittle, and does not behave in the way it was assumed to when the standard theory for composite strength was developed. Futhermore, this theory predicts curved stress-strain plots for aligned short fibre composites, yet the evidence for this is unconvincing, and there is much new evidence that these stress-strain curves are straight. The time has therefore come to abandon this approach and take into account, instead, the apparent brittleness and sudden failure of aligned fibre reinforced polymers. This paper presents the evidence, and introduces the new approach. This involves microcrack development in composites from stress concentrations at the fibre ends. Since such failure initiation can occur simultaneously at many sites, the stress required to cause abrupt failure across the whole cross section can be estimated by a simple force balance. This analysis gives the familiar expressions used for short fibre composites, with one important difference. For carbon reinforced polymers, the polymer has to reach its breaking strength before failure, so that there is no minimum volume fraction for reinforcement with these composites. With glass, on the other hand, which has a higher breaking strain than most thermosets used for composites, the matrix appears unable to exert its full strength. Thus low fibre volume fraction glass fibre composites can be weaker than the matrix, and a minimum volume fraction for reinforcement exists.