Phenomena of nonlinear viscoelasticity

We shall now conader the nonlinear viscoelasticity in the general situation. Experimentally a variety of interesting phenomena have been found in the nonlinear region. Leaving detailed description of them to the literature of rheology,we shall limit ourselves here to a few typical aspects of nonlinear properties. 

Shear flow. The general dependent shear rate K(t)  

By symmetry, the stress components and Oy again vanish identically, so that the relevant stress components are the shear stress a y the normal stresses Ni = a - Oyy, and N2=Oyy - a , which are nonlinear 

An example of 1(1 ) is shown in Fig. 7.18. Both quantities decreases significantly with increasing shear rate. For large shear rate, Ni becomes much larger than o y, and this causes many interesting phenomena known collectively as the Weissenberg effect.  

It is seen that for large k the shear stress reaches a maximum before reaching the steady state value. This phenomena is called the stress overshoot. Sometimes the overshoot is observed for the first normal stress difference at higher shear rate. 

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The chapters remain the same as in the second edition. As before, the emphasis is still on linear viscoelasticity of amorphous polymers. Although the phenomena of nonlinear viscoelasticity are given more attention, this subject and the properties of crystalline and glassy polymers are treated somewhat superficially and the reader is referred to other treatises and reviews, with the admonition that current developments may lead to a clearer understanding within a few years. 

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