Necking, cold-drawing

Fig. 3. Engineering stress-strain curves for a material showing, with increasing strain, strain localization necking), cold drawing, and strain hardening. Also shown are schematics of the sample geometry at each stage.

Bulk PS deforms with the formation of fibrillated crazes (a) as well as thick fibrils (b). Fibrils thinner than a critical value (here 225 nm, depending on temperature) deform with necking, cold drawing, and homogeneous stretching. In the necking zones, the fiber diameter decreased from about 225 nm to nearly 60-80 nm [19, 20]. 

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. 

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. 

Equation (14.7) corresponds to the slope of the tangent to the curve cr, vs. e drawn from the point e = -1 or X, = 0. Figure 14.7 shows the true stress versus nominal strain curves for polymer samples A and B. Curves B] and B2 are compatible with curve B of Figure 14.6. The so-called Considere construction, Eq. (14.7), is satisfied with the tangent to the curves drawn from E = — 1. The tangential point corresponds to the maximum observed in the curve vs. and therefore with the maximum load that the specimen can support. In practice, the Considere construction is used as a criterion to decide when a polymer will form an unstable neck or form a neck accompanied by cold drawing. 

For curve A it is possible to draw only one tangent from e = — 1. This implies that once the neck has started in the sample it will continue becoming thinner and thinner until fracture is reached. In the case of curves B and B2 there is the possibility of drawing two tangents that fit Eq. (14.7). The second tangent in each case corresponds to the appearance of a minimum in the curve of nominal stress versus nominal strain, which is necessary for the neck to be stable. Therefore it can be stated that the formation of a stable neck, and thus the existenced of cold drawing, will take place when the condition established by Eq. (14.7) is met at two points of the curve of Of vs. E . 

Orientation of semicrystalline polymers below the melting point is often referred to as "cold drawing." Although some stress crystallization does occur, the process primarily involves the transformation of existing crystalline structures. A widely accepted model of the deformation mechanism is that provided by Peterlin (Figure 5) (41). Prior to necking, the crystal lamellae which... 

Figure 11.8 XRD patterns of injection-molded p-PR (a) Before deformation, (b) yielded at 8% strain but without obvious necking, and (c) after cold drawing at 110% strain. (From Reference 26 with permission from Elsevier.)...

Mechanical Properties and Deformation and Failure Processes. The DGEBA-T403 epoxies are ductile in tension at 23 C and exhibit a macroscopic yield point, necking and cold drawing. 

Low-density polyethylene (LDPE) strapping, cut from 0.42 mm thick film, is used to hold four packs of drink cans together. If pulled slowly with the hands, parts of the strapping undergo tensile necking followed by cold drawing of the thin region (Fig. 1.9). Mark parallel lines at 5 mm intervals... 

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