A theory for yield phenomenon of glassy polymers based on the strain non-uniformity under loading conditions

In this work, the yield phenomenon and its related features have been investigated under the concept of strain inhomogeneity, emerged inside the material during deformation processes. This strain non-uniformity in glassy polymers is either a direct consequence of the local microstructural density fluctuations existing in such materials or is the result of the manner by which the free volume is frozen in the glassy state. Assuming a simple strain density distribution function, the rate of plastic deformation can be extracted without any further assumption on a molecular conformational base or any other thermal activated process. The two model parameters required have a physical base related with the magnitude of the free volume and its fluctuation in glassy polymers. Appling this theory on the experimental results for three representative amorphous glassy polymers (PMMA, PS, and PC), all features of yield process, including strain softening effect, are easily described.