Plastic flow instability

The essence of Krafft s model involves the relationship between the onset of crack growth with that of plastic flow instability in these tensile ligaments ahead of the crack tip. Assuming that the strains within the crack tip plastic zone are constrained by the surrounding elastic material, the strain inside the plastic zone would follow a singularity as dictated by the surrounding elastic stress-strain field namely,... 

He proposed that crack growth instability, or fracture, would correspond to the onset of plastic flow instability (or necking) in the tensile ligament(s) at the crack tip or when e at r = dj. Thus,... 

In this chapter the subject of primary interest is the plastic-flow instabilities occurring in extensional deformation of fibers or bars of solid polymers with an inelastic constitutive response as discussed in Chapters 7-9. There, localization in shear was featured prominently both in unit plastic events in the form of shear transformations and also in the form of more homogenized processes resulting in macro shear bands. The discussion here concentrates on complementary instabilities occurring in extensional plastic fiow of fibers or bars of solid polymers from a... 

If we load a material in compression, the force-displacement curve is simply the reverse of that for tension at small strains, but it becomes different at larger strains. As the specimen squashes down, becoming shorter and fatter to conserve volume, the load needed to keep it flowing rises (Fig. 8.6). No instability such as necking appears, and the specimen can be squashed almost indefinitely, this process only being limited eventually by severe cracking in the specimen or the plastic flow of the compression plates. 

Dennison M. T., "Flow instability in polymer melts a review," Plastics... 

As with UV light, heat tends to oxidize polymers. The symptoms are embrittlement, melt flow instability, loss of tensile properties and discolouration. The mechanism of stabilization is therefore to prevent oxidatimi or to mitigate its effects. Plastics, particularly thermoplastics, also require stabilization protection against degradation from heat during processing or in use. 

The Var der Waals energy versus the distance between CNT in array for square and triangle lattices is presented in Fig. 2. The square lattice is instable with respect to spontaneous transition to triangle lattice with the minimal distance between tubes a 0.6 nm due to negative value of Ci2 module. For a > 0.7 nm the triangle lattice becomes also instable and the plastic flow of array could arise without any resistance. 

Deformation instabilities in extensional plastic flow of polymers... 

For uniform and stable extrusion it is important to check periodically the drive system, the take-up device, and other equipment, and compare it to its original performance. If variations are excessive, all kinds of problems will develop in the extruded product. An elaborate process-control system can help, but it is best to improve stability in all facets of the extrusion line. Some examples of instabilities and problem areas include 1) non-uniform plastics flow in the hopper 2) troublesome bridging, with excessive barrel heat that melts the solidified plastic in the hopper and feed section and stops the plastic flow 3) variations in barrel heat, screw heat, screw speed, the screw power drive, die heat, die head pressure, and the take-up device 4) insufficient melting or mixing capacity 5) insufficient pressure-generating capacity 6) wear or damage of the screw or barrel 7) melt fracture/sharkskin (see Chapter 2), and so on. 

Surface defects in the plastic extrudate can occur due to flow instabilities or degradation of the plastic during the extrusion process. Some common defects and possible causes are hsted in Table B.2. 

The elastic-plastic tensile instability point in mild steel has received much attention and many explanations. Some polymers, such as polycarbonate, exhibit a similar phenomenon. Both steel and polycarbonate not only show an upper and lower yield point but visible striations of yielding, plastic flow or slip lines (Luder s bands) at an approximate angle of 54.7° to the load axis also occur in each for stresses equivalent to the upper yield point stress. (For a description and an example of Luder s band formation in polycarbonate, see Fig. 3.7(c)). It has been argued that this instability point (and the appearance of an upper and lower yield point) in metals is a result of the testing procedure and is related to the evolution of internal damage. That this is the case for polycarbonate will be shown in Chapter 3. For a discussion of these factors for metals, see Drucker (1962) and Kachanov (1986). 

We now turn to the other end of the stress-strain curve and explain why, in tensile straining, materials eventually start to neck, a name for plastic instability. It means that flow becomes localised across one section of the specimen or component, as shown in Fig. 11.5, and (if straining continues) the material fractures there. Plasticine necks readily chewing gum is very resistant to necking. 

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