Stress and crazing

Fig. 7. Craze stress and craze fibril extension ratio vs. chain entanglement contour length in thin films of PTBS, PS, PSAN and PPO. Data from Ref.

Different attempts have been made to correlate craze stress with the yield stress or the fracture stress after yielding. For PMMA a good agreement has b n reported whilst in PVC considerable differences have been found Quantitatively yield stress and craze stress have been found to be of the same magnitude. 

B Zero stress limit of activation energy for plastic deformation C SAXS constant product of flow stress and craze fibril diameter C Pre-exponential factor for craze strain rate, (=Qb )... 

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

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.

Both yield stress and crazing stress vary with temperature, Haward et aL... 

Figure 3.9. Rubber particle straddling craze perpendicular to stress is subjected to triaxial stresses and because of its high bulk modulus becomes load bearing. (After Bucknall )...

Internal stresses occur because when the melt is sheared as it enters the mould cavity the molecules tend to be distorted from the favoured coiled state. If such molecules are allowed to freeze before they can re-coil ( relax ) then they will set up a stress in the mass of the polymer as they attempt to regain the coiled form. Stressed mouldings will be more brittle than unstressed mouldings and are liable to crack and craze, particularly in media such as white spirit. They also show a characteristic pattern when viewed through crossed Polaroids. It is because compression mouldings exhibit less frozen-in stresses that they are preferred for comparative testing. 

Crazing. This develops in such amorphous plastics as acrylics, PVCs, PS, and PCs as creep deformation enters the rupture phase. Crazes start sooner under high stress levels. Crazing occurs in crystalline plastics, but in those its onset is not readily visible. It also occurs in most fiber-reinforced plastics, at the time-dependent knee in the stress-strain curve. 

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. 

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

Good stress-crack and craze resistance, brittle Good processibility, no stress whitening Higher-heat styrenic, brittle... 

The rapid cooling of certain plastic products can result in frozen in stresses and strains (particularly with injection molding). The stresses may decay with time in a viscoelastic manner. However, they will act like any other sustained stress to aggravate cracking or crazing in the presence of aggressive media and hostile environments like UV radiation. 

VI. Stress Cracking and Crazing in Fluids Summary Problems References... 

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