Cold drawing, yield stresses

We will use readily available plastic films to demonstrate stress-strain behavior. Students should be able to relate the physical behavior of thin films to the concepts of orientation and crystallinity. They should be able to explain terms such as cold drawing, yielding, and machine and transverse directions. 

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. 

Let s start by looking at a simple polymer, polyethylene, that has a lot going on in its stress/strain plots (Figure 13-38). Flexible, semi-crystalline polymers such as this (where the T of the amorphous domains is below room temperature) usually display a considerable amount of yielding or cold-drawing, as long as they are not stretched too quickly. For small deformations, Hookean elastic-type behavior (more or less) is observed, but beyond what is called the yield point irreversible deformation occurs. 

Fig. 11-20. Tensile stress-strain curve and lest specimen appearance for a polymer which yields and cold draws.

Stress-strain experiments have traditionally been the most widely used mechanical test but probably the least understood in terms of interpretation. In stress-strain tests the specimen is deformed (pulled) at a constant rate, and the stress required for this deformation is measured simultaneously (Figure 13.1). As we shall see in subsequent discussions, polymers exhibit a wide variation of behavior in stress-strain tests, ranging from hard and brittle to ductile, including yield and cold drawing. The utility of stress-strain tests for design with polymeric materials can be greatly enhanced if tests are carried out over a wide range of temperatures and strain rates. 

When an initially isotropic polymer is drawn or extruded to a high deformation ratio under suitable conditions, it develops appreciable anisotropy which is apparent in mechanical tests at all stresses up to and beyond the yield stress. A typical example of the anisotropy of yield observed is shown in Fig. 2 in which the tensile yield stress of oriented polyethylene terephthalate (PET) sheets is shown as a fiinction of the an e 6 between the tensile axis (TA) and the initial draw direction (IDD). This large anisotropy is somewhat similar to that observed in cold-rolled metal sheets, for which theories of anisotropic plasticity were suggestedby Hill, Yoshimura and others. A modification of the theory... 

Stress-strain curves for the transitional Copolymer D/PPO and incompatible PpClS/PPO blends given in Figs. 3 and 4, respectively, again indicate embrittlement at 60 to 80% PPO. Unlike unblended PPO and the high PPO content compatible blends that yield and cold draw, the high PPO content two-phase blends do not appear to initiate a stable neck region and failure occurs shortly after the yield point. 

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