The effect of sudden strain‐rate change on the yield behavior of bisphenol‐A polycarbonate

We wish to investigate the path dependence of yield processes in glassy polymers and to understand this dependence, if any, in terms of molecular mechanisms. To this end the effect of sudden strain‐rate change on the yield behavior of bisphenol‐A polycarbonate has been investigated. In the experiment, a specimen was initially deformed at a constant elongation rate R1; at an elongation Δlc the rate was suddenly changed to another constant elongation rate R2. R1 was either lower or higher than R2. The yield stress subsequent to the rate change was found to be lower than if the entire deformation was at R1. These results can be satisfactorily explained in terms of Robertson's theory of the yielding of glassy polymers if it is assumed that the “bond flexing” in his theory manifests itself in anelastic deformation.