A microscopy Study of the Transition From Yielding to Crazing in Polypropylene

The deformation mechanisms in polypropylene as a function of strain rate have been studied by scanning electron microscopy. Injection molded, dogbone specimens were tested in tension at a temperature of 50°C and strain rates from 10-4 to 90 s-1. With increasing strain rate a gradual transition from cold drawing and necking to nearly homogeneous deformation was observed. The transition is characterized by a strong elastic recoil after fracture. At ε = 90 s-1, the elastic recovery has its maximum value of 70% of the strain at break (ε = 0.3). The morphology was studied post mortem at small strains outside the neck using permanganically etched samples. The transition in the macroscopic behavior was found to coincide with a change in the deformation mechanism. Whereas yielding is the dominant mechanism at low strain rates, homogeneous voiding is found at ε = 90 s-1. A correlation is suggested between the failure of chain slip within the lamellae and the occurrence of voiding, leading to distinct voiding patterns in the spherulites.