Volume Change during Deformation and Pressure Dependence of Yield Stress

Abstract

Polymers are distinguished from metals by the considerable pressure dependence of their yield stresses, and one might expect this to be associated with a plastic volume change during yielding. We have measured the macroscopic volume changes during uniaxial compression of PMMA and PC, using a mercury-filled dilatometer. For PMMA a plastic volume change cannot be defined unambiguously because the volume-stress curve is reversible upon unloading to the yield point. For PC the plastic volume change is negative. Hence, if the pressure dependence of yielding is associated with this volume change, one would predict a decrease of the yield stress with increasing pressure. Contrary to this, we find that pressure increases the yield stress of PC. Hence, the plastic volume change is not directly related to the pressure dependence of the yield stress.