Experimental evidence for yield criteria in polymers

The second maximum, as we have seen previously, is not observed if the true stress-strain curve is plotted instead of the load-extension curve. The first maximum, on the other hand, would exist on the true stress-strain curve. It is called an intrinsic yield point because it relates to the intrinsic behaviour of the material. 

Many studies of the yield behaviour of polymers have bypassed the question of strain rate and temperature and sought to establish a yield eriterion as diseussed in Section 11.2 above. In very general terms sueh studies divide into two eategories (1) those that attempt to define a yield eriterion on the basis of determining yield for different stress states (2) those that eonfine the experimental studies to an examination of the influence of hydrostatie pressure on the yield behaviour. 

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