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 50degreesC 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 (epsilon) over dot = 90 s(-1), the elastic recovery has its maximum value of 70% of the strain at break (epsilon = 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 (epsilon) over dot = 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.