The time and temperature dependence of fracture toughness for thermoplastics

Abstract

A method for obtaining comparative and intrinsic toughness of thermoplastics involves deriving two apparent material properties, yield stress and a critical value of stress field intensity factor (fracture toughness). These are obtained at various times under load and temperatures. A ductility factor can then be derived and is the ratio of the square of fracture toughness to yield stress. Ductility factors can create a league table of toughness for thermoplastics which correlates reasonably with impact toughness energies. Individual values of fracture toughness and yield stress provide an intrinsic measure of toughness. Consequently, the relative toughness of a range of thermoplastics are explained in these terms. Materials discussed include homopolymers and copolymers of propylene, uPVC, PMMA (and rubber toughened versions) and PES. There is an importance in obtaining geometry independent fracture toughness measurements, otherwise likely artefacts are introduced. Such considerations are discussed for uPVC and polypropylene materials.