Craze initiation experimental observations

A description of crazing with a cohesive surface appears appropriate for the crazes observed in glassy polymers, since the trends reported experimentally are quite well captured. The cohesive surface model distinguishes the three steps of crazing (initiation, thickening, and breakdown) and is flexible enough to incorporate more sophisticated formulations of one of these stages when available. 

GghI (1970). Gent (124) proposed a model in which the hydrostatic tensile stress at an inclusion or local heterogeneity increases the free volume and therefore effectively reduces the Tg of the material. At a sufficiently high stress concentration, the reduction in Tg is sufficient to reduce the local Tg to the test temperature. The reduced yield stress of the material in this mbber-like phase and the hydrostatic tensile stress then leads to cavitation and craze initiation. Implicit in this free-volume approach is that an imposed hydrostatic pressure will tend to prevent the formation of crazes in accordance with experimental observation. The criterion is summarized in the equation for the critical applied stress for initiation,... 

However, experimentally it has been observed that craze growth occurs faster than craze initiation and so it is difficult to explain growth with such a model as it is essentially the same mechanism as initiation. Also, some of the featnres of the craze predicted from such a mechanism, such as a closed cell structure formed... 

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