A step toward molecular‐level understanding of the toughness of glassy plastics

Abstract

AbstractRigid, glassy plastics differ over a wide range in the amount of energy they can absorb before fracturing under the influence of an applied stress. It is here proposed that molecular structure influences energy absorption more significantly by determining the onset of fracture than by its effect on the energy‐dissipating processes themselves. Emphasis has been placed, therefore, on finding a relationship between molecular structure and the onset of fracture. Since many relationships between molecular structure and bulk mechanical response have already been established, it should be a useful step to relate the onset of fracture to bulk mechanical responses. The hypothesis is developed that the appropriate bulk property is the stress‐deformation function of the material. The observed stress‐deformation function is of such a nature that a highly non‐homogeneous deformation is produced at any point of stress concentration. Typically, this is a narrow region in which the material is highly deformed while the deformation in adjacent material is still slight. Fracture initiation is postulated to occur in such a region of high deformation. Differences among polymers in the shape of the stress‐deformation function are demonstrated experimentally. These differences correlate with tough‐brittle behavior. An explanation is proposed in terms of the probability of fracture initiation.