Fundamental processes of craze growth and fracture

Abstract

Recent advances in quantitative microscopy and low-angle electron diffraction methods have made it possible to probe the fundamental processes of craze fibril formation and craze fibril breakdown. Both the scale of fibrillation within the craze and the magnitude of the crazing stress may be successfully described by a variant of the Taylor meniscus instability process. Within this framework, the key parameter in governing craze growth is the craze surface energy Γ. In turn Γ reflects the mechanism by which entangled strands are lost (through either chain scission or chain disentanglement) in producing the surfaces of the craze fibrils. A new model, which describes the temperature, strain rate and molecular weight dependence of the crazing stress is presented. This approach provides a clear rationale for the hitherto confusing data on crazing to shear deformation transitions in a wide variety of polymers. Moreover, the modification of the polymer network during craze formation has important implications for craze breakdown. In particular, at low temperatures where chain scission is the dominant process, the molecular weight of the polymer in the fibrils is markedly reduced. A molecular description of craze fibril breakdown based on microscopic measurements of the scale of the fibrillation in the craze and the statistics of craze fibril breakdown is proposed. Satisfactory agreement between the predictions of this model and the experimental data for a variety of glassy polymers is found.