Imparting strength and toughness to brittle composites

Abstract

HIGH strength and high toughness are usually mutually exclusive in composites which have brittle filaments in a brittle matrix. The high tensile strength characteristic of strong interfacial filament–matrix bonding can, however, be combined with the high fracture toughness of weak interfacial bonding if the filaments are arranged to have alternate sections of high and low shear stress (and low and high toughness). Such weak and strong areas can be achieved by appropriate intermittent coating of the fibres. The strong regions ensure that the filament strength is picked up. Randomly positioned weak areas effectively blunt running cracks by the Cook–Gordon mechanism¹, which in turn produces long pull-out lengths with an associated large contribution to toughness. Boron–epoxy composites of volume fraction 0.20–0.25 have been made in this way; they have fracture toughnesses of over 200 kJ m⁻², and they retain rule of mixtures tensile strengths (∼650 MN m⁻²). At the volume fractions used, that apparently represents K_IC values greater than 100 MN m^−3/2.