Abnormal Fracture Behavior of Glassy Polymers Stretched at Very Low Tensile Rate

Abstract

Tensile fracture properties of glassy poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC) were observed by constant-speed stretching at a wide range of stretching rates, including very low stretching rates. When the stretching rate was decreased to very low value, despite the decrease in stress with decreasing stretching rate, both PMMA and PVC specimens no longer showed ductile deformation accompanied by necking and fractured at low strains around which the yield point appeared. This abnormal "embrittlement" observed at very low strain rates was not due to simple physical aging during stretching. Furthermore, fracture near the yield point as observed at very low strain rates was frequently observed even in the strain rate range where ductile deformation was also achieved. The tendency for fracture to occur around the yield point, which was commonly observed in both PMMA and PVC, is likely a general nature of glassy polymers. Since the yield point is a manifestation of the flow in glassy polymers due to nonlinear relaxation, the nonlinear relaxation mechanism which enables glassy polymers to deform largely probably contains a mechanism that generates nuclei for fracture.