Failure mechanisms in polyolefines: The role of crazing, shear yielding and the entanglement network

Macroscopic deformation and failure modes of polyolefines are reviewed in terms of deformation and failure models based on the craze initiation and propagation model of Kramer-Berger and the craze-crack transition model of Kramer-Brown. Although these models were formulated for amorphous polymers they are also valid for semi-crystalline polymers. The important role of the underlying molecular entanglement network in this approach is reflected by the strain hardening behaviour which is shown to be a robust measure for predicting slow crack growth performance. The polymer network response explains the experimentally observed presence of two Brittle-Ductile transitions, one at low temperature or high strain rates, linked with chain scission which dominates crazing, the other at elevated temperatures or low strain rates which involves disentanglement crazing. The relation between these two Brittle-Ductile transitions and the major transition temperatures for molecular mobility such as the glass transition and the crystal α relaxation temperature are discussed. Valid strategies for increasing the crack propagation resistance in polyolefines are reviewed. Finally an outlook for further research to complement the present knowledge base is formulated.