Necking phenomenon

Investigation of the Necking Phenomenon in Ph.D. Thesis, The Technical University of... 

Na YH, Tanaka Y, Kawauchi Y, Furukawa H, Sumiyoshi T, Gong JP, Osada Y (2006) Necking phenomenon of double-network gel. Macromolecules 39 4641-4645... 

In most polymers, a marked necking phenomenon occurs very early after the yield point. This is the reason why it is not possible, in the range of large deformations, to determine strains in a large representative volume element (RVE). Consequently none of the dilatometers utilized to date can be used, except in a very restricted strain range. The latter statement concerns dual clip gage extensometers (axial + transversal) and also hquid displacement dilatometers (23). Once plastic instability has... 

The necking phenomenon observed upon stretching a polymer film at a constant temperature is a well-known consequence of a negative feedback loop driven by the interplay between the increase in temperature associated with the sample deformation and its glassification caused by the heat exchange with the environment (55). Oscillatory behavior and period-doubling in the stress resulting from a constant strain rate have been experimentally observed. 

Figure 5.39. Schematic presentation of the necking phenomenon (B) obtained by the drawing of an amorphous poiymer sample.

The apparent decrease in engineering stress with continned deformation past the maximnm point of Figure 6.11 is due to the necking phenomenon. As explained in Section 6.7, the true stress (within the neck) actually increases. 

At HOY speeds, the rate of increase in orientation levels off but the rate of crystallization increases dramatically. Air drag and inertial contributions to the threadline stress become large. Under these conditions, crystallization occurs very rapidly over a small filament length and a phenomenon called neck-draw occurs (68,75,76). The molecular stmcture is stable, fiber tensde strength is adequate for many uses, thermal shrinkage is low, and dye rates are higher than traditional slow speed spun, drawn, and heat-set products (77). 

The concept of a ductile-to-brittle transition temperature in plastics is likewise well known in metals, notched metal products being more prone to brittle failure than unnotched specimens. Of course there are major differences, such as the short time moduli of many plastics compared with those in steel, that may be 30 x 106 psi (207 x 106 kPa). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck. Tliese different engineering characteristics also have important effects on certain aspects of impact resistance. 

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. 

It is important to note that some types of dewar necks are made of plastic materials which are permeable to gases and in particular to He. The permeation phenomenon has a strong dependence on temperature and is negligible at 4K (see e.g. ref. [6]). If the dewar remains for a long time at room temperature in an atmosphere containing He gas, the vacuum space is slowly filled with He which must be pumped before the filling with cryogenic liquids. 

The tensile test is typically destructive that is, the sample is extended until it plasticly deforms or breaks, though this need not be the case if only elastic modulus determinations are desired. As described in the previous section, ductile materials past their yield point undergo plastic deformation and, in doing so, exhibit a reduction in the cross-sectional area in a phenomenon known as necking. 

The phenomenon is illustrated by the following experiment A large test-tube A, Fig. 11, and a two-neeked bottle, B, are fitted as indicated in the diagram the two-necked... 

I introduced phosphorus into a receiver having a stopcock, which had been exhausted, and admitted oxymuriatic acid gas. As soon as the retort was full the phosphorus entered into combustion, throwing forth pale white flames. A white sublimate collected in the top of the retort, and a fluid as limpid as water trickled down the sides of the neck. The gas seemed to be entirely absorbed, for, when the stopcock was opened, a fresh quantity of oxymuriatic acid gas, nearly as much as could have filled the retort, entered. The same phenomenon of inflammation again took place, with similar results. Oxymuriatic acid gas was admitted until the whole of the phosphorus was consumed. 

Today, from an engineering point of view, the name superplasticity is ascribed to a polycrystalline material pulled out to very high tensile elongations prior to failure with necking-free strain. This phenomenon is usually found in many metals, alloys, intermetallics, composites and ceramics (recently in high-temperature superconductor ceramics) when the grain size is small enough, less than 10 pm for metals and less than 1 pm for ceramics. 

Two examples may be used to illustrate the complexity of problems of this kind. When film is made by extrusion followed by casting on chill rolls there can be a tendency for the extruded web to shrink inwards towards the centre of the rolls—the phenomenon known as neck-in . The edge of film concerned becomes thicker than the rest. It has been found that more elastic melts, capable of keeping a tension in the direction of extrusion, are less liable to exhibit this fault. 

A phenomenon often encountered in drawing is necking, which may be described as a discontinuity in the reduction of the diameter of the specimen in the direction of stretching. The name "neck" has been chosen because in fibres the shape of this discontinuity often shows some similarity with the neck of a bottle. 

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. 

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