Polystyrene crazing stress

It follows that for a constant refractive index the craze strain is also constant. Our observations on polystyrene indicate that neither the craze refractive index nor the ratios kB/kc and kB/kh are constant along the length of the craze (Figure 6). The values of A calculated from Equation 4 are shown in Figure 7. That the craze strain is not constant does not preclude the possibility that the craze stress is still constant, as might be the case for an ideal plastic material. However, the experiments on craze stress-strain properties by Kambour (10) and Hoare and Hull (11) indicate that this is not the case. 

Figure 14.16 Dependence of crazing stress on temperature at different strain rates for polystyrene. Strain rates ( ) 0.00067s ( ) 0.026 s (A) 0.267 s. (From Ref. 25.)...

Figure 14.17 Relationship between yield stress and crazing stress with temperature, for polystyrene. (From Ref. 26.)...

Similar stress-strain curves have been obtained for polystyrene crazes. However, these results do not necessarily reveal the real mechanical behavior of the craze. The removal of the solvent from samples will cause shrinkage and have a significant plasticizing effect on the craze fibrils. This has to... 

A quantitative analysis of craze shape and mass thickness contrast within the craze allowed Lauterwasser and Kramer [382] to derive the stress profile existing along a polystyrene craze. Kramer and his coworkers have extended this study to many other polymers, relating the mean density of craze material to entanglement density in the polymer glass and to toughness [395] without a basic change of preparation technique. 

Kozlov, G. V, Mikitzev, A. K. (1987). The Dependence of Crazing Stress on Testing Temperature at Polystyrene Impact Fracture Izvestiya VUZov Severo-Kavkazsk Region estestv nauk, 3, 66-69. 

Fig. 9.9. Newly formed craze in thin slice cut from uncrazed bulk polystyrene craze growing from left to right in a direction perpendicular to that of the uniaxial tensile stress (Courtesy D. Hull 11061).

Fig. 9.13. Breaking stress and crazing stress as a function of molecular weight of polystyrene at 25 °C (after (1461). M entanglement molecular weight.

There are other conditions that result from the frozen-in stresses. In materials such as crystal polystyrene, which have low elongation to fracture and are in the glassy state at room temperature, a frequent result is crazing it is the appearance of many fine microcracks across the material in a direction perpendicular to the stress direction. This result may not appear immediately and may occur by exposure to either a mildly solvent liquid or vapor. Styrene products dipped in kerosene will craze quickly in stressed areas. 

The other effect of having a stretched area is a reduction in resistance to stress cracking. Crazing is a possibility in such areas such as in polystyrenes, and environmental stress cracking caused by solvent substances will occur in the stretched areas. This is a particularly important consideration in vacuum formed products used for packaging food that frequently has some solvent action on the plastics. 

Experimental Evidence. Morphology. Figure 3 (33) shows in phase contrast microscopy the development of crack or craze patterns around rubber particles in a toughened polystyrene. The lack of dependence of crack inclination on direction of stress is especially marked in this micrograph, and can be explained only by reference to dynamic branching rather than to crack or craze nucleation by stress raisers. Schmitt and Keskkula refer to the lines as craze cracks and cracks. ... 

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