Stress-strain curves plastics mechanical behavior

In fundamental solid mechanics including fracture mechanics studies, the requirement is to determine the stress-strain curves and to study the propagation of a crack in a substance. Understanding the behavior of crack propagation was essential for the modem day development of plastic products. 

Similar stress-strain curves have been obtained for polystyrene crazes. However, these results do not necessarily reveal the real mechanical behavior of the craze. The removal of the solvent from samples will cause shrinkage and have a significant plasticizing effect on the craze fibrils. This has to... 

Figure 11. Typical stress-strain curve showing the three theoretically identifiable regions of mechanical behavior. Key A, elastic region B, elastic (Bj-plastic (BJ region and C, plastic region.

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. 

The most important mechanical property of a plastic is its tensile stress-strain curve (Figs. 10-1 and 7-4). This curve is obtained by stretching a sample in a testing machine and measuring its extension and the load required to reach this extension. Plastics show viscoelastic behavior (as reviewed in Chapter 1) that is highly sensitive to temperature and, in some materials, to relative humidity variations so it is important to use samples of standard shapes, preconditioned at constant and standard temperature and relative humidity before testing. Requirements are explained in the ASTM specifications. 

The obtaining of good mechanical properties is obviously an important issue because the end of a fuel cell test is directly related to a general or local membrane breaking. To our knowledge, the study of SPI mechanical properties is limited in the literature to the measurement of stress-strain curves in the dry and water-swollen states [39,110,112,126]. These curves present an elastic behavior limited to few percent of deformation followed by a plastic linear behavior (Fig. 44). Despite the polymer being in a glassy state, the water sorption induces plasticization. The maximum tensile stress at break is of the order of 70 MPa for SPI compared to less than 20 MPa for Nafion and the elon-... 

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...

Figure 10 is a stress-strain-temperature diagram for a Ni-Ti shape-memory alloy that summarizes its mechanical behavior. At the extreme rear the stress-strain curve shown in the a-t plane corresponds to the deformation of martensite below Mf. The induced strain, about 4%, recovers between A and Af after the applied stress has been removed and the specimen heated, as seen in the e-T plane. At a temperature above Mj (and Af) SIM is formed, leading to a superelastic loop with an upper and lower plateau, the middle o-e plane. At a still higher temperature and above M, the front a-e plane, no SIM is formed. Instead, the parent phase undergoes ordinary plastic deformation. 

Stress—Strain Behavior. The determination of stress-strain behavior in tension is one of the most important test methods for mechanical properties of engineering plastics and is of high importance to the design engineer. The tensile test is usually performed by monitoring the force that develops as the sample is elongated at a constant rate of extension. An often encountered stress-strain curve of a plastics material at equilibrium with any environment can be represented as depicted in Figure 2. 

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