Strain-hardening phenomenon

After the nominal stress, cross section), and nominal strain, en = dL/L0. 

The strain hardening phenomenon also means that any extensional reactivation of CSBs tends to occur by dilatational opening of the margins to form potential fluid flow conduits and sites for the... 

At higher spinning speeds, the initial drop in the apparent viscosity is higher and faster, followed by even faster rise after the stress-induced crystallization takes place. The freeze point is achieved soon after this strain-hardening phenomenon takes place. In contrast, no such changes in apparent viscosity are seen at low spinning speeds. 

The strain-hardening phenomenon is explained on the basis of dislocation-dislocation strain field interactions similar to those discussed in Section 7.3. The dislocation density in a metal increases with deformation or cold work because of dislocation multiplication or the formation of new dislocations, as noted previously. Consequently, the average distance of separation between dislocations decreases—the dislocations are positioned closer together. On the average, dislocation-dislocation strain interactions are repulsive. The net result is that the motion of a dislocation is hindered by the presence of other dislocations. As the dislocation density increases, this resistance to dislocation motion by other dislocations becomes more pronounced. Thus, the imposed stress necessary to deform a metal increases with increasing cold work. 

The presence of grain boundaries also affect slip in the material even after plastic deformation has commenced. A phenomenon known as strain hardening, or work hardening or cold working, is the result of constrained dislocation mobility and increased... 

Strain hardening is an abrupt, positive deviation of from the Trouton rule as e increases with time. In the view of Hwang and Kokini (1991), with reference to polysaccharides, this phenomenon is due to branch points acting as hooks, thereby increasing the resistance to flow. 

A number of materials, however, show stress-strain curves of the shape sketched in Fig. 24.10. After the normal convex first part, the stress-strain curve shows an inversion point, after which the stress increases rapidly with strain. This phenomenon is sometimes called "strain hardening". In this case, a straight line through the origin can intersect the stress-strain curve at two points A and B. This means that only the intersection points A and B are possible conditions. The intermediate intersection point C is unstable. So in this case two parts of the specimen, e.g. a fibre, with different draw ratios and, hence, different cross sections can coexist. If the fibre is stretched, part of the material with a cross section of point A is converted into material with a cross section of point B. In contrast, in Chap. 13 the Considere plot is defined as the tangent line on the true stress-strain curve. 

A further temperature rise leads to necking, with the possibility of cold drawing the latter phenomenon is dependent on the stability of the neck, and is governed by the level of adiabatic heating and strain hardening. In this case the extensions can be very large. 

The phenomenon of strain hardening in polymers is a consequence of orientation of molecular chains in the stretch direction. If the necked material is a semicrystalline polymer, like polyethylene or a crystallizable polyester or nylon, the crystallite structure will change during yielding. Initial spherulitic or row nucleated structures will be disrupted by sliding of crystallites and lamellae, to yield morphologies like that shown in Fig. 11-7. 

The condition for stability is thus that the stress in the material increases with the strain, a phenomenon called strain hardening, and that the relative increase is more than proportional to the increase in the uniaxial Hencky strain. Of course, when elongation goes on, the thread will finally break, but at a far larger strain value than would occur in the absence of strain hardening. 

Furthermore, the strain hardening exponent, fig. 8.42, dropped suddenly at 250 K as a result of elongation produced during the p to a transformation. The strain rate sensitivity, fig. 8.42, exhibited a distinct minimum centered at about 350 K. Whether this is associated with the p to a transformation or is related to another phenomenon is not known. 

---