Comparisons of stress-strain and fracture behaviour of thermosetting resins have shown viscoelastic non-linearity to be compatible with the complex fracture behaviour of thermosets. Fracture on a load controlled test machine has shown additional evidence that non-linearity is present in the material. This phenomenon is linked to previous studies of the fracture surface that have shown evidence of viscoelasticity in the formation of the surface texture. The proposed mechanism is consistent with a crack tip blunting model of stick-slip behaviour. Parallels are shown to exist between known thermoset fracture behaviour and phenomena observed in composite delamination studies.