The influence of intrinsic strain softening on strain localization in polycarbonate : modeling and experimental validation

Intrinsic strain softening appears to be the main cause for the occurrence of plastic localization phenomena in deformation of glassy polymers. This is supported by the homogeneous plastic deformation behavior that is observed in Polycarbonate samples that have been mechanically pre-treated to remove (saturate) the strain softening effect. In this study some experimental results are presented and a numerical analysis is performed simulating the effect of mechanical conditioning by cyclic torsion on the subsequent deformation of Polycarbonate. To facilitate the numerical analysis of the 'mechanical rejuvenation' effect, a previously developed model, the 'compressible Leonov model', is extended to describe the phenomenological aspects of the large strain mechanical behavior of glassy polymers. Th! e model covers common observable features, like strain rate, temperature and pressure dependent yield and the subsequent strain softening and strain hardening phenomena. The model, as presented in this study, is purely 'single mode' (i.e. only one relaxation time is involved), and therefore it is not possible to capture the non-linear viscoelastic pre-yield behavior accurately. The attention is particularly focussed on the large strain phenomena. From the simulations it becomes clear that the pre-conditioning treatment removes the intrinsic softening effect, which leads to a more stable mode of deformation.