Yielding of glassy polymers: Volume effects

Abstract

A yield criterion is constructed for glassy polymers from results of constant strain‐rate tests in different stress fields. A significant effect of hydrostatic pressure (or volume change) on the yield stress of polystyrene was found. Direct dilatometric measurements were made on polystyrene, poly(methyl methacrylate), polycarbonate, and poly(vinyl formal) samples subjected to uniaxial compression. Both a volume contraction proportional to stress (Poisson's ratio effect) and a volume expansion, which depends on the extent and the history of plastic yielding, were observed. These results are discussed qualitatively in terms of structural models for the glassy state of amorphous polymers.