Effects of pre-cracking methods on fracture behaviour of an Araldite-F epoxy and its rubber-modified systems

The fracture behaviour of an Araldite-F epoxy and its rubber-modified syste ms was evaluated using compact tension specimens pre-cracked by three metho ds, namely, razor blade pressing, razor blade tapping and fatigue pre-crack ing. The results show that the razor blade tapping method produces a lowest critical stress intensity factor, K-lc, while the razor blade pressing pro duces an abnormally high K-lc, being about five times higher than the forme r for the pure epoxy. Transmission polarized optical microscopy reveals tha t the crack tip produced by razor blade pressing in the pure epoxy specimen was completely surrounded by a plastic deformation zone with compressive r esidual stress, but the crack tip produced by razor blade tapping was free of residual stress and plastic deformation. It was found that the sensitivi ty of the fracture toughness value to the pre-cracking methods decreases af ter the pure epoxy was modified by 10% core-shell rubber or 10% liquid rubb er. (C) 1998 Kluwer Academic Publishers.