Influence of the loading rate on the fracture resistance of isotactic polypropylene and impact modified isotactic polypropylene

The fracture behaviour of an isotactic polypropylene and an impact modified isotactic polypropylene containing 15 wt% of ethylene-propylene rubber has been investigated at test speeds of between 0.1 mm/s and 14 m/s, using com pact-tension specimens. The homopolymer displayed a ductile-brittle transit ion as the test speed was increased, which was associated with a transition from shear deformation to crazing. Deformation of the modified polypropyle ne was characterized by stable crack propagation over the whole range of te st speeds, stress-whitening at the crack tip and, at lower speeds, by the f ormation of extensive shear lips on the fracture surfaces. An observed incr ease in the size of the crack tip damage zone with test speed suggested a t ransition from plane stress to plane strain conditions. This is discussed i n terms of a simple model, which takes into account the variation of the sa mple stress state as the test speed increases. (C) 2000 Elsevier Science Lt d. All rights reserved.