Craze strain

It is interesting to remark that like most mechanical parameters the crazing stress exhibits viscoelastic characteristics, decreasing with increasing temperature and with decreasing strain rate (which is an indication that it is better to speak of a crazing strain). We come back to the discussion of crazes in Sect. 13.5.5. 

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. 

Correlation function Elongational strain Strain rate Craze strain Elastic strain Shear strain Interfacial length Extension ratio... 

Total number of crazes, strain rate exponent in non-linear viscosity... 

From Eq. (18) we note that the total axial craze strain rate according to Eq. (6) is simply... 

In this geometry, the craze strain rate becomes... 

An increase in M is also reported to increase the temperature at which a shear to craze transition occurs in both PC and poly(ether sulfone) (PES) and, above the transition temperature, the craze strain, or stress, is greater for the higher molecular weight samples These changes are a result of a greater resistance to disentanglement in the higher M polymers. 

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 )... 

Macroscopic crazing strain rate X Craze matter extension ratio... 

When all the diluent is in solution, i.e., when v Vg, where Vg is the diluent concentration at the solubility limit in the homo-PS, the plasticization of interest is homogeneously spread out over the entire polymer. Then the craze strain rate, given by the basic kinematic rate equation of Argon et al. is... 

At shorter times, the growth process again is controlled by the availability of the environment but now there is no single limiting speed since d 8. The pore area may be approximated by assuming that the craze strain is given by 6 /do, where dg is a measure of craze thickness, and if it is assumed that pores form at a spacing of lo, then we have ... 

The kinematics of the craze-induced plasticity is viewed, as shown in the sketch of Fig. 13.25, as a set of parallel crazes perpendicular to the tensile extension direction at an average spacing of Lq, thickening at a rate of S, the time rate of increase of craze thickness. The resulting average plastic-craze strain rate is... 

Conditions resulting in the creation of free volume, such as crazing (stress whitening) and cavitation relate to the magnitude of the hydrostatic stress. Level of the critical (or crazing) strain, Ecr at such occurrences can be predicted using the relation ... 

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