Fracture toughness of Kevlar-epoxy composites with controlled interfacial bonding

The fracture toughness of Kevlar-epoxy resin composites with intermittent fibre bonding of a silicone vacuum fluid (SVF-200) and a polyurethane varnish (Estapol 7008) have been studied over the temperature range -60 to 40° C and strain rates 0.03 to 5000 min^-1. Whilst both coating materials give similar tensile strengths their effects on toughness are very different. As far as toughening is concerned Estapol 7008 is more effective than SVF-200. The toughening effect increases with increasing intermittent lengths of the Estapol-7008 coating, i.e. coating parameter C, increasing temperature and decreasing strain rate. At low strain rates and high temperatures, for C=1, the toughness increase is some 200 to 300% compared to the uncoated composites. Some initial work has also been conducted for hygrothermally aged uncoated and coated fibre composites. The SVF-200 coated composites do not show any toughness degradation compared to the dry control samples. However, both the uncoated and Estapol-7008 coated composites suffer some toughness loss. Even so, the toughness of the fully coated aged specimens is as good as the uncoated dry controls. A fracture analysis is presented which gives reasonable agreement between predicted fracture toughness values and experimental measurements. It is shown that fibre pull-out toughness and fibre fracture work are the main contributors to the total fracture toughness of these fibre composites; their relative significance being dependent on the type of coating material, the temperature and strain rate of testing.