Eyring rate process theory yield stresses

Phenomenologically, the rate and temperature dependence of the yield stress of semi-crystalline poljuners can be described by the Eyring activated state model, as discussed earlier, with either one (100,101) or two (46,102) activated processes. However, developing a theory for the yield of semi-crystalline polymers is clearly complicated by the presence of two distinct phases. It is imclear at present whether... 

The form of this equation is one of the few suggested relations between the variables which has some basis in theory and depends, identically with the time delay before yielding already discussed, on Eyring s theory of rate processes. When strain is plotted against log (time) equations of this type give fairly fiat curves at low stresses and... 

We have already seen in Section 11.3.4 that yield can be modelled using the Eyring process. Many workers [19,49-56] have considered that the applied stress induces molecular flow much along the lines of the Eyring viscosity theory where internal viscosity decreases with increasing stress. The basic equation for the plastic strain rate has been given as Equation... 

A final consideration is that the Argon theory essentially regards yield as nucleation controlled, analogous to the stress-activated movement of dislocations in a crystal produced by the applied stress, aided by thermal fluctuations. The application of the Eyring theory, on the other hand, implies that yield is not concerned with the initiation of the deformation process, but only that the application of stress changes the rate of deformation until it equals imposed rate of change of strain. The Eyring approach is consistent with view that the deformation mechanisms are essentially present at zero stress, and are identical to those observed in linear viscoelastic measurements (site model analyses in Section 7.3.1). Here, a very low stress is applied merely to enable detection of the thermally activated process, without modification of the polymer structure. 

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