Neck behavior under tensile stress

On the other hand, neck behavior under tensile stress is described by elastic theory at small elongations as... 

FFs that are parameterized for high-pressure conditions can still lead to behavior that differs from that observed in experiments. For instance, it is common practice to treat the interatomic interactions with Lennard-Jones (LJ) potentials. Although this method is convenient from a computational standpoint, it is known that LJ potentials do not reproduce experimentally observed behavior such as necking, where a material attempts to minimize surface area and will break under large tensile stresses. Many other examples exist where particular types of FFs cannot reproduce properties of materials, and once again, we emphasize that one should ensure that the FF used in the simulation is sufficiently accurate. 

From tests conducted on polymer glasses under both tension and compression (19-23) the true stress versus true strain rate response can be determined and typical plots are shown in Figure 5 (19). As can be seen, qualitatively the true stress-true strain behaviors are similar for both deformation modes. Importantly they both show a drop in true stress at the yield point. The observation of a drop in true stress without the localization (necking) seen in tensile tests confirms that the material is undergoing intrinsic strain softening. Importantly this shows that localization is not a necessary requirement for jdeld and fiudlier that... 

Of course, metals may experience plastic deformation under the influence of applied compressive, shear, and torsional loads. The resulting stress-strain behavior into the plastic region is similar to the tensile counterpart (Figure 6.10a yielding and the associated curvature). However, for compression, there is no maximum because necking does not occur furthermore, the mode of fracture is different from that for tension. 

---