Physical basis of distortional and dilational plastic flow in glassy polymers

A molecular theory for distortional plasticity is described based on thermally activated production of molecular kink pairs which incrementally align short molecular segments and thereby produce strain. The process of crazing is described as a micromechanical problem of elastic-plastic expansion of initially stable micropores produced by a thermally activated mechanism under stress to form a craze nucleus, followed by lateral extension of the craze nucleus by a time-dependent addition of craze matter at points of the craze boundary where the local conditions of craze matter conversion can be satisfied. The analysis of the condition of craze nucleation provides a natural framework for the derivation of the dilational yield condition of Sternstein and Ongchin by crazing. The process of fracture by propagation of cracks pushing plastic craze wedges in front of them is also briefly described qualitatively.