The Phenomenology of Yield

There have been two main contrasting approaches to the establishment of a yield criterion for oriented polymers. On the one hand we have those 

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The resulting, isolated, flow stress contribution is shown in Figure 31 (left). As was discussed in the first part of this chapter, dealing with the phenomenology of yielding, upon plastic deformation of the material all influence of its prior thermomechanical history is erased and the material shows a response independent of prior history. From the resulting minimum in stress 0, 0, we can, therefore, calculate with the help of eqn [5] a first estimate of the rejuvenated reference viscosity 7r,o, since fi and tq are known, Sa= 0, and, finally, the strain rate fi is prescribed. Note that we obtain an estimate of this parameter since its calculation is based on a number of assumptions as listed earlier. At the same time we can also obtain an estimate of the value of the state parameter Sa since the softening drop A[Pg.739]

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