Yielding, Necking, Drawing

Shear yielding, i.e. the onset of large-scale intersegmental displacements within anon-oriented thermoplastic polymer, has a distinct effect on the stress-strain curve. In tensile tests a drop in engineering stress is usually observed and the yield point defined as the point of maximum load (Fig. 2.10, curves b and c). In other tests, e.g. in a compression test, there may or may not be a load drop but a sudden decrease of da/de may be noticed. The important phenomenon of yielding has been intensively investigated. Review articles have appeared almost annually in recent years in the general literature [e.g. 114,154-164]. 

The classical continuum mechanical criteria for shear yielding have been discussed in Chapter 3, Section III A in terms of the three-dimensional state of stress. 

At this point molecular interpretations are reviewed in an attempt to assess the role of molecular backbone chains. 

Experiments at different temperatures, strain rates, and hydrostatic pressures have established that shear yielding is a thermally activated process [154—168, 170—173]. According to Eyring s theory of flow (Chapter 3) strain rate can be expressed as 

This gives the following dependence of yield stress Oy on strain rate and temperature  

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Polymer A with GIC = 160 J m-2 is typical for thermoset materials which are expected to be brittle [78]. At the other end of the series, polymer E and Phenoxy with G,c > 1 kJ m 2 are tougher than several wellknown thermoplastics (PMM A, PS, PES). In contrast to the more crosslinked polymers, polymer E and Phenoxy PKHJ show necking after yielding in tensile tests with draw ratios A = 1.7 and A = 2.1, respectively (Table 2.1). 

Thermoplastic polymers subjected to a continuous stress above the yield point experience the phenomenon of cold-drawing. At the yield point, the polymer forms a neck at a particular zone of the specimen. As the polymer is elongated further, so this neck region grows, as illustrated in Figure 7.7. 

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