AbstractThe tapered cantilever cleavage and the single‐edge‐notch tension fracture toughness tests have been shown to give similar results for several cured epoxy resin systems. Methods for the calculation of fracture energy and critical stress intensity factors are described and discussed. These tests have been used to study the influence of rubber modifiers (“Hycar” CTBN and “Blendex” 311) on the toughness of a conventional epoxy resin (“Epikote” 828) reacted with anhydride curing agents. The modifiers used increase fracture energies and stress intensity factors by factors of approximately 5 and