Tensile deformation and failure processes of amine-cured epoxies

Abstract

Evidence is reviewed for the occurrence of microscopic flow under tensile loads in a variety of amine-cured epoxies. The nature of the deformation and failure processes involved in these flow processes are discussed. The slow-crack growth fracture topographies of these epoxies, fractured as a function of temperature and strain-rate, are reviewed, and consist of a rough initiation region, that can contain microvoids and/or fractured fibrils, surrounded by a smooth temperature and strain-rate dependent region. These topographical features are explained by initial course craze formation followed by crack propagation through the craze midrib. The crack then imposes a higher stress field on the craze tip which produces a small plastic zone that results in a smooth fracture topography. Fracture topographies also indicate that shear band propagation can occur in the fracture initiation process. The ductile mechanical response of many of these epoxies together with direct experimental observations from transmission electron microscopy and birefringence studies produce further evidence that flow can occur in these glasses. Both plastic, homogeneous and inhomogeneous deformations can occur. The inhomogeneous deformations can evolve into macroscopic shear bands. The ability of these crosslinked glasses to undergo microscopic flow is discussed in terms of (i) our understanding of their chemical and physical structure and (ii) covalent bond scission.