SHORT-FIBER POLYMER COMPOSITES - A FRACTURE-BASED THEORY OF FIBER 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 predict s curved stress-strain plots for aligned short fibre composites, yet t he evidence for this is unconvincing, and there is much new evidence t hat these stress-strain curves are straight. The time has therefore co me to abandon this approach and take into account, instead, the appare nt brittleness and sudden failure of aligned fibre reinforced polymers . This paper presents the evidence, and introduces the new approach. T his involves microcrack development in composites from stress concentr ations at the fibre ends. Since such failure initiation can occur simu ltaneously at many sites, the stress required to cause abrupt failure across the whole cross section can be estimated by a simple force bala nce. This analysis gives the familiar expressions used for short fibre composites, with one important difference. For carbon reinforced poly mers, the polymer has to reach its breaking strength before failure, s o that there is no minimum volume fraction for reinforcement with thes e composites. With glass, on the other hand, which has a higher breaki ng strain than most thermosets used for composites, the matrix appears unable to exert its full strength. Thus low fibre volume fraction gla ss fibre composites can be weaker than the matrix, and a minimum volum e fraction for reinforcement exists.