Yield point phenomenon

Plastic fluids behave more like plastic solids until a specific minimum force is applied to overcome the yield point. Gels and ketchup are extreme examples. Once the yield point has been reached, the liquids begin to approach Newtonian behavior as shear rate is increased. Although plastic behavior is of no benefit to ketchup, it has some beneflt in paints. Actually, it is the yield point phenomenon that is of practical value as illustrated by no-drip paints. When the brush stroke force has been removed the paint s viscosity builds quickly until the flow stops. Dripping is prevented because the yield point exceeds the force of gravity. 

The plastic behaviour of some metals, especially plain carbon steels, deviates from that described so far. Their stress-strain curve shows a so-called apparent yield point (also known as yield point phenomenon) (figure 3.5(b)) ... 

The yield point phenomenon is often not desired because it causes an inhomogeneous plastic deformation. In deep drawing of metal sheets for car bodies, for instance, the surface of the sheets becomes rough (so-called orange peel ). Counter measures are therefore needed. Steel sheets, for instance, are usually temper rolled before the deep drawing process to tear off the dislocations by minor plastic deformation. 

Figure 6.10 (a) Typical stress-strain behavior for a metal showing elastic and plastic deformations, the proportional limit P, and the yield strength a-y, as determined using the 0.002 strain offset method, (b) Representative stress-strain behavior found for some steels demonstrating the yield point phenomenon. 

Some steels and other materials exhibit the tensile stress-strain behavior shown in Figure 6.10b. The elastic-plastic transition is very well defined and occurs abruptly in what is termed a yield point phenomenon. At the upper yield point, plastic deformation is initiated with an apparent decrease in engineering stress. Continued deformation fluctuates slightly about some constant stress value, termed the lower yield point, stress subsequently rises with increasing strain. For metals that display this effect, the yield strength is taken as the average stress that is associated with the lower yield point because it is well defined and relatively insensitive to the testing procedure. Thus, it is not necessary to employ the strain offset method for these materials. 

Note that to observe the yield point phenomenon, a stiff tensile-testing apiparatus must be used by stiff, it is meant that there is very little elastic deformation of the machine during loading. 

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 may be pointed out that the term yield point is sometimes erroneously used as a synonym for elastic limit and proportional limit As it has been described in the paragraphs above it is actually a phenomenon that occurs in only a very small number of cases in tensile testing. As it has also been observed in the description that graphically and experimentally, it is an anomalous behaviour in which there is a strain occurring with no increase in stress. 

The continuous chain model includes a description of the yielding phenomenon that occurs in the tensile curve of polymer fibres between a strain of 0.005 and 0.025 [ 1 ]. Up to the yield point the fibre extension is practically elastic. For larger strains, the extension is composed of an elastic, viscoelastic and plastic contribution. The yield of the tensile curve is explained by a simple yield mechanism based on Schmid s law for shear deformation of the domains. This law states that, for an anisotropic material, plastic deformation starts at a critical value of the resolved shear stress, ry =/g, along a slip plane. It has been... 

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. 

In metals the above phenomenon manifests as shown in figure 3.1 [7]. For instance, when a vertical metallic bar is subjected to a load "p for a given period of time "to" it produces an elongation equal to "8o" which is the yield point. Subsequent to the time "to" if the load is kept constant or without further application and the time duration increased, the bar gradually and slowly lengthens. Since this extension takes place only in a portion of the sample... 

Under dilatational stresses and in contact with solvents, polymers exhibit a cavitational mode of plasticity called environmental crazing. This phenomenon occurs at small strains in the order of a few percent well below the yield point of the polymer. Environmental crazes are normally observed at the surface of a specimen where the penetrating solvent produces a polymer-solvent mixture. Environmental crazing has been extensively discussed in the literature (see e.g. However, one basic problem in studying this phenomenon arises from the fact that the macroscopic state of the sample at craze initiation may differ considerably from the local one which is, in general, poorly defined. 

Thixotropy of casting slips is characterized by the time dependence of both viscosity and yield point. This phenomenon is caused by reversible sol-gel transition in the clay component, which can be affected by mechanical means (stirring, vibration). The effect is much more distinct with enamel slips than with ceramic casting slips. A more marked yield point is required with the former, while ceramic casting suspensions are usually characterized by a pseudoplastic rheological behaviour without a distinct yield point. 

The same phenomenon occurs in the deflection of a column imder a compression loading. In this type of failure, a critical load is reached beyond which collapse occurs as a result of a rapid increase in the stresses beyond the yield point of the material. The critical pressure that causes collapse is not a simple function of the induced stress, as with tensile loads. In fact, it is directly proportional to the modulus of elasticity of the material and the moment of inertia of the shell and is inversely related to the cube of the radius of the curvature. 

Experiments on water filtered through 0.22-pm filters showed a decreasing yield, pointing out the role of bacteria. This may explain that particle formation is rapidly inhibited unless a removable process is active in the water. Nevertheless, the phenomenon remains operative even in sterilized samples. 

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... 

Experiments of the second type, in which the strain is increased linearly with time, give the sort of results shown in fig. 8.10, from work by Higashi et al. (1964), this time for glacier crystals in tension with the basal plane at 45° to the strain axis closely similar results have been found by Ready KLingery (1964). There is a clear yield point at a strain between 0-5 and i per cent, after which the stress decreases markedly. This is a familiar phenomenon in metals but, in distinction from the metallic case, the stress in... 

I2, T2) attributed to positrons trapped at the crystalline-amorphous interface had T2 0.32 ns and h exhibited a precipitous decrease from about 58% to about 50% at the yield point, followed by recovery back to about 58%. This phenomenon interpreted as indicating interfacial loss of defects occurs during the initial deformation process and then some unknown recovery process takes place subsequently. 

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