Void growth in glassy polymers

This paper deals with a study of voids in amorphous glassy polymers that exhibit elastic-viscoplasticity with rate dependent yield, intrinsic softening and progressive strain hardening at large strains. The study is motivated by the plastic deformation in voided polymer-rubber blends caused by cavitation of the rubber particles, and thus attempts to contribute to the understanding of the toughening mechanisms in blends. Axisymmetric cell analyses are presented to study the plastic deformation around initially spherical voids and their resulting growth in terms of size and shape up to large overall strains. This void growth is demonstrated to inherit particular properties from the typical features of plasticity in glassy polymers, viz. small strain softening and large strain hardening. The role of strain localization into shear bands and their subsequent propagation in controlling void growth is highlighted. Furthermore, an approximate constitutive model is presented for the description of the macroscopic overall behaviour of porous glassy polymers. This model includes a modification of existing porous plasticity models to account for elasticity effects on the initiation of overall plasticity, which are important in polymers because of their relatively high yield strain. Its predictions are compared with the results from the numerical cell analyses.