Pitfalls in the evaluation of impact fracture of polymer-based materials

The fracture behaviour of a number of short- and long-GF/polypropylene-based materials, long-GF reinforced nylon-6,6 and CF/epoxy composites was assessed using an instrumented impact tester. A meaningful assessment of the mechanical response of the specimens to the instrumented loading was shown to depend on the stability of the specimen support system and hence specimen resonance. The fracture properties (K_c and G_c) were in general independent of specimen dimensions and moulding flaws in the case of unfilled polypropylene. Fibre-containing systems demonstrated significant variation in the properties as a function of fibre distribution. Preparation of the test-pieces had a marked effect, particularly for specimens containing a high concentration of long fibres. An equivalent notch depth for specimens containing a machined notch and dispersed fibres is defined.