Mechanical behavior cold drawing

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. 

The majority of wires and metallic filaments are produced by drawing. The extreme cold work to which the metal is subjected modify its microstructure and in most cases introduces a strong fiber texture. Both the microstructure and the texture are known to have a strong influence on the mechanical behavior of the wires and are in many cases exploited to achieve the desired properties. Moreover, the deformation and the material flow are very nonhomogeneous and depend on the form of the die, the friction between wire and die, and the strain hardening capacity of the metal. This gives rise to heterogeneous microstructures and may cause macroscopic structural defects. 

Now it cannot escape the casual reader that the various theories of necking in cold-drawing all describe the comportment of fibers and thin strips. This is not accidental indeed, to the untutored eye the specimens used by Zapas and Crissman were about 15 cm. long, only several centimeters wide, and of negligable thickness. How Is this related to the necking Can a full three dimensional theory based on appropriate principles also predict this instability or Is the geometry of the specimen also very important The mechanical phenomenon is not restricted in this way it occurs in tubes for example. Some work of Spector [23] may be of use here. From a different perspective, it is possible to ask if there are families of time dependent St. Venant-type solutions for this sort of material which display the appropriate behavior. 

Molecular weight also influences the static fracture response as well as the mechanical behavior of UHMWPE at large strains [7]. For example, beyond the polymer yield point, the hardening or cold drawing portion behavior in uniaxial tension is sensitive to the molecular weight. Figure 2.2 illustrates the tme-stress strain curve in uniaxial tension (room temperature, 30 mm/min) for two grades of UHMWPE, in comparison with HDPE. 

Brasses and other Cu-based alloys are mainly supplied as wrought alloys which designate the state of the final material which is obtained by a final shaping operation (rolling, rod, and wire drawing etc.) with a controlled degree of cold work. The work-hardening behavior of the alloys is, thus, exploited to vary the final mechanical properties in comparatively wide limits... 

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