Plastic strain, plastics mechanical behavior yield stress

As an example, for room-temperature applications most metals can be considered to be truly elastic. When stresses beyond the yield point are permitted in the design, permanent deformation is considered to be a function only of applied load and can be determined directly from the stress-strain diagram. The behavior of most plastics is much more dependent on the time of application of the load, the past history of loading, the current and past temperature cycles, and the environmental conditions. Ignorance of these conditions has resulted in the appearance on the market of plastic products that were improperly designed. Fortunately, product performance has been greatly improved as the amount of technical information on the mechanical properties of plastics has increased in the past half century. More importantly, designers have become more familiar with the behavior of plastics rather than... 

A plastic material is defined as one that does not undergo a permanent deformation until a certain yield stress has been exceeded. A perfectly plastic body showing no elasticity would have the stress-strain behavior depicted in Figure 8-15. Under influence of a small stress, no deformation occurs when the stress is increased, the material will suddenly start to flow at applied stress a(t (the yield stress). The material will then continue to flow at the same stress until this is removed the material retains its total deformation. In reality, few bodies are perfectly plastic rather, they are plasto-elastic or plasto-viscoelastic. The mechanical model used to represent a plastic body, also called a St. Venant body, is a friction element. The... 

A particular distinction between the mechanical behaviors of metals and plastics (URPs and RPs) is explained in order to avoid a possible confusion that could have arisen from the preliminary review. A typical stress-strain curve for a metal exhibits a linear elastic region followed by yield at the yield stress, plastic flow, and ultimately failures at the failure stress. Yield and failure occur at corresponding strains, and one could define yield and failure in terms of these critical strains. This is not common practice because it is simpler in many cases to restrict step (a) to a stress analysis alone. By comparison, it may appear strange that it was stated that plastics failure criteria are usually defined in terms of a critical strain (rather than stress) and, by comparison with the metals case, switching back from strain to stress may appear to be a minor operation. 

In contrast to the two previous cases, plastic flow (plasticity) is characterized by the absence of proportionality between the stress and the strain, that is, plasticity represents a case of nonlinear behavior. For plastic bodies subjected to stresses below the critical value, t < t (the so-called shear yield point), the rate of strain is zero (dy/dt = 0). Plastic flow starts at the yield stress, t = t, and does not require further increase in stress (Figure 3.6). Similar to viscous flow, plastic flow is thermodynamically and mechanically irreversible. However, in contrast to the prior case, the rate of energy dissipation in plastic flow is proportional to the rate of strain ... 

Figure 3, which is a replot of the data of Fig. 1, was included to focus attention on the deformational behavior of these steels as measured by elongation. At 70 F, these steels deform by localized necking and extensive reduction of area, followed by fracture in the necked area (see Fig. 10). At -320 and -423°F, these same steels deform by a much more uniform elongation and reduction of area over the entire reduced section (see Fig. 10). This condition is most pronounced in the 62 cold-worked samples. At low temperature, the increase in strength in the reduced section due to plastic strain and associated martensite formation more than offsets the increase in stress due to the reduction in cross-sectional area (i.e., necking down) hence, the sample elongates over the entire reduced section prior to failure. At -423 F this elongation frequently occurs in a discontinuous manner, accompanied by audible clicks, serrations in the stress—strain curve, and striations in the sample, whose appearance is not unlike Luder s bands. The cross section of such a striation is shown in Fig. 12. These striations have been observed in other alloys by other investigators, and have been variously attributed to catastrophic twinning, thermal instability, and the burst-type formation of dislocations [1]. In this material another possibility exists, namely, the formation of martensite. This transformation is known to occur by an instantaneous shear mechanism and yields a volume increase which could account for the serrated stress—strain curve [5]. These effects demonstrate again that the...

It is observed that the critical stress to form martensite increases linearly with an increase in temperature from up to a temperature M above which martensite forms only after plastic deformation of the parent phase. There is another temperature, A/, above which martensite cannot be mechanically stimulated, no matter what the stress. This behavior is shown in Figure 6 (Olson and Cohen, 1972). Note that between Af, and Af the stress is below the yield strength of the parent and is thus an elastic stress. Accordingly, the nudeation is stress-assisted and existing nucleating sites are simply aided mechanically. At M" the stress surpasses the parent yield strength, and because of the plastic deformation new nucleating sites are introduced. Thus, a distinction between elastic stress-induced martensite and plastic strain-induced martensite can be made. 

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