Craze initiation pressure

CEPP Chemical Emergency Preparedness Pro- CIP craze initiation pressure... 

The effect of temperature is complex. It has been shown that the higher the test environment temperature, the faster the ESC onset. The transition to brittle behavior is accelerated to shorter times by increasing temperature, cyclic loading, dilational stress, and stress concentrations. Localized concentration of the stress due to local geometrical features as notches, voids, and inclusions will increase the stress and modify the nature of the stress field. Craze initiation is accelerated by stress fields with high dilational stress and retarded under hydrostatic pressure (Wright 1996). 

Crazing requires the presence of dUatational component in the stress tensor and may be inhibited by hydrostatic pressure. On the other hand, it is enhanced by the presence of triaxial tensile stress (Kinloch and Young 1983). Unfortunately, such a stress state exists ahead of large flaws or notches in relative thick specimens (plane-strain conditions). Therefore, the presence of sharp cracks, notches, or defects in thick specimens wUl favor craze initiation leading to brittle fracture, which is opposite to a bulk shear yielding mechanism that leads usually to ductile behavior. 

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,... 

Localized concentration of the stress due to local geometrical features as notches, voids, and inclusions will increase the stress and modify the nature of the stress field. Craze initiation is accelerated by stress fields with high dilational stress and retarded under hydrostatic pressure. 

Crazes usually form under tensile stress when a critical strain is surpassed they do not occur under compressive stress applying hydrostatic pressure during tensile deformation can even inhibit their development. Crazes always nucleate preferentially at points of triaxial stress concentration. It is the dilatational strain which initiates crazes and cracks. 

If we consider a craze grown from some small initial flaw which has propagated along the surface, a profile of the form shown in Fig. 29 would be expected. The pressure profile within the craze must satisfy Eq. (60) and, in general, terms of the form ... 

Fig. 18. Craze density as a function of initiation time for polystyrene subjected to a combination of deviatoric stresses (s) and negative pressures (tr). After Argon and Hannoosh (111), with permission of Taylor and Francis Ltd., http //www.tandfcp.uk/journals.

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