Yield as an Activated Rate Process

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 

the analysis of Section 11.3.4 reveals the linear relationship between yield stress and log strain rate, corresponding to the observations of Bauwens and co-workers [52]. 

In an earlier paper, Lazurkin [24] rejected a previous proposal by Hookway [22] and Horsley and Nancarrow [57] that the molecular flow occurs because the applied stress reduced the melting point of the crystals. He remarked that similar behaviour is observed for both crystalline and non-crystalline polymers, the dependence of the yield stress on strain rate following the logarithmic form in both cases. 

Haward and Thackray [55] have compared the Eyring activation volumes obtained from yield stress data with the volume of the statistical random link . The latter was obtained from solution studies, by assuming that the real chain can be represented by an equivalent chain with freely jointed links of a particular length. Table 12.1 is based on data collated by Haward and Thackray and shows that the activation volumes are very large in molecular terms and range from about two to 10 times that of the statistical random link. The result 

Polymer Volume of statistical link in solution (nm ) Eyring flow volume V(nm ) 

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Yield as an activated rate process the Eyring equation... 

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