Void Growth in Glassy Polymers: Effect of Yield Properties on Hydrostatic Expansion

Void growth in plastically deforming glassy polymers is investigated by means of a simple spherical symmetric model. This type of void growth occurs in cavitated polymer-rubber blends and, at a smaller scale, during craze initiation. The study serves to provide approximate values for the stresses required for elastic-viscoplastic void growth under hydrostatic loading conditions. The constitutive model accounts for features such as rate and temperature dependent yield, intrinsic strain softening after yield, and subsequent hardening due to molecular alignment at large deformations. The separate effects of these features on void expansion and the stress distribution are studied. Due to the relatively large strain at yield for most glassy polymers, elastic effects play an important role even at macroscopic yield. Therefore, predictions of the maximum stress are significantly lower than those based on rigid-plastic behaviour, especially for low void volume fractions.