Fracture toughness of Kevlar-epoxy composites with controlled interfacial bonding

The fracture toughness of Kevlar-epoxy resin composites with intermittent fiber bonding of a silicone vacuum fluid (SVF-200) and a polyurethane varnish (Estapol 7008) have been studied over the temperature range -60 to 40 degree C and strain rates 0.03 to 5000 min super(-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. A fracture analysis is presented which gives reasonable agreement between predicted fracture toughness values and experimental measurement. It is shown that fiber pull-out toughness and fiber fracture work are the main contributors to the total fracture toughness of these fiber composites; their relative significance being dependent on the type of coating material, the temperature and strain rate of testing.