Glassy polymers crazing

Athermal tensile yield stress of a glassy polymer Craze half length... 

R. Schirrer Damage mechanisms in amorphous glassy polymers Crazing, pages 488-499. In J. Lemaitre [92], 2001. 

Early studies of the failure process in glassy polymers, crazing, placed emphasis on the idea that one could separate stress effects frail strain effects by taking advantage of the face that even in the glassy state the mecheuiical response was strongly time dependent. Further studies in the area of variable rate impact... 

Although crazes are typical of amorphous glassy polymers, crazes or crazelike features have also been observed in many other amorphous polymers (see below), semicrystalline polymers (eg, in PE, PP, PA), and in rubber-modified high impact polymers (eg, HIPS, ABS). In the literature they were often designated as microcracks, deformation zones, or jdeld zones (28). 

Models of chain pull-out in glassy polymers without crazing... 

When the stress that can be bom at the interface between two glassy polymers increases to the point that a craze can form then the toughness increases considerably as energy is now dissipated in forming and extending the craze structure. The most used model that describes the micro-mechanics of crazing failure was proposed by Brown [8] in a fairly simple and approximate form. This model has since been improved and extended by a number of authors. As the original form of the model is simple and physically intuitive it will be described first and then the improvements will be discussed. 

Usually, the molecular strands are coiled in the glassy polymer. They become stretched when a crack arrives and starts to build up the deformation zone. Presumably, strain softened polymer molecules from the bulk material are drawn into the deformation zone. This microscopic surface drawing mechanism may be considered to be analogous to that observed in lateral craze growth or in necking of thermoplastics. Chan, Donald and Kramer [87] observed by transmission electron microscopy how polymer chains were drawn into the fibrils at the craze-matrix-interface in PS films [92]. One explanation, the hypothesis of devitrification by Gent and Thomas [89] was set forth as early as 1972. 

Water molecules combine the tendency to cluster, craze and plasticize the epoxy matrices with the characteristic of easily diffusion in the polymer1 10). The morphology of the thermoset may be adversaly influenced by the presence of the sorbed moisture. The diffusion of the water in glassy polymers able to link the penetrant molecules is, therefore, characterized by various mechanisms of sorption which may be isolated giving useful information on the polymer fine structure. 

Kambour, R. P. and Gruner, C. L., Effects of polar group incorporation on crazing of glassy polymers styrene-acrylonitrile copolymer and a bisphenol polycarbonate, J. Polym. Sci., Polym. Phys. Ed., 16, 703 (1978). 

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