Yielding and Instability in Polymers

As we observed at the end of the Chapter 11, the non-linear behaviour of polymers, as represented by the Eyring model, gives rise to a phenomenon resembling yield. The observed maximum stress can be treated as a yield stress, though Equation (11.41) shows that this yield stress depends on the rate of strain. Wineman and Waldron [1] have pointed out that there appear to be two approaches to the modelling of yield in polymers the use of non-linear viscoelasticity and the direct application of metal plasticity. The use of the Eyring model is one example of the former approach. Relatively simple theories of plasticity, where there is no rate dependence, are available from the metals field. These theories, that embody the classical concepts of plasticity, may still be applied usefully to polymers, for instance in cases where changes in strain rate are small. 

Our first task in this chapter is to discuss the relevance of classical ideas of plasticity to the yielding of polymers. Although the yield behaviour is temperature and strain rate dependent, it will be shown that provided that the test conditions are chosen suitably, yield stresses can be measured which satisfy conventional yield criteria. The temperature and time 

Mechanical Properties of Solid Polymers, Third Edition. I. M. Ward and J. Sweeney. 2013 John Wiley Sons, Ltd. Published 2013 by John Wiley Sons, Ltd. 

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