Load-extension/strain curves

Tensile Test consists of stretching a test specimen in the grips of a test machine. An extensometer, a device for measuring the extension or elongation of the test specimen, is fitted to the test specimen and removed before it breaks. A load-extension (strain) curve is plotted. See Figure 6-7. 

Another aspect of plasticity is the time dependent progressive deformation under constant load, known as creep. This process occurs when a fiber is loaded above the yield value and continues over several logarithmic decades of time. The extension under fixed load, or creep, is analogous to the relaxation of stress under fixed extension. Stress relaxation is the process whereby the stress that is generated as a result of a deformation is dissipated as a function of time. Both of these time dependent processes are reflections of plastic flow resulting from various molecular motions in the fiber. As a direct consequence of creep and stress relaxation, the shape of a stress—strain curve is in many cases strongly dependent on the rate of deformation, as is illustrated in Figure 6. 

Yield strength as determined in tensile tests [53] at ambient temperature was plotted in Fig. 6.1 against M 1, the inverse molecular mass between crosslinks. All the samples of polymer A (the most crosslinked polymer) failed before the polymer started to yield. Therefore, load-extension-curves were extrapolated up to a hypothetical yield strain in this case. The extrapolated tensile is marked by brackets (Table 6.1). 

The second group exhibits the phenomenon of drawability. This manifests itself in the stress-strain behavior (curve II in Fig. 2.21) as follows At first these materials behave in a similar way to those of curve I. The proportionality limit lies at low values, and the deformation with increasing load is also quite small. Then, suddenly, a large extension occurs, even though the load remains constant or becomes smaller. The material begins to flow and the stress-strain curve sometimes runs nearly parallel to the abscissa. The point at which the... 

Natural rubber exhibits unique physical and chemical properties. Rubbers stress-strain behavior exhibits the Mullins effect and the Payne effect. It strain crystallizes. Under repeated tensile strain, many filler reinforced rubbers exhibit a reduction in stress after the initial extension, and this is the so-called Mullins Effect which is technically understood as stress decay or relaxation. The phenomenon is named after the British rubber scientist Leonard Mullins, working at MBL Group in Leyland, and can be applied for many purposes as an instantaneous and irreversible softening of the stress-strain curve that occurs whenever the load increases beyond... 

Fig. 19. Typical load-extension curve of a Corriedale fiber in water at about 20°C after boiling for less than 20 min at a constant strain of 10% (Feughelman, 1960).

Figure 7 A typical load/extension (stress versus strain) curve for a yam from the Victory sail. Typically, a test length of 2.5cm was set...

Load-elongation [45] and stress-relaxation [46] measurements may be used to follow the course of the reduction of keratin fibers by mercaptans. Extension in the postyield region is resisted primarily by the disulfide bonds [45], Therefore, this region of the stress/strain curve holds special significance to the reduction reaction. 

Creep is usually displayed as a graph of strain against time, in a creep curve (Figure 10.28). The section of the curve OA is the extension that occurs when the specimen is first stressed. This is elastic deformation for most ordinary loads and corresponds to the straight-line part of a stress-strain curve. Creep as such is indicated by the remaining parts of the curve. 

Various possible load-extension curves for polymers are shown schematically in fig. 6.2. The whole range of behaviour shown in fig. 6.2 can be displayed by a single polymer, depending on the temperature and the strain-rate, i.e. how fast the deformation is performed, and whether tensile or compressive stress is used. These curves are discussed further in sections 8.1, 8.2 and 10.2.2. 

As discussed in section 6.2.2, the values of Young s modulus for isotropic glassy and semicrystalline polymers are typically two orders of magnitude lower than those of metals. These materials can be either brittle, leading to fracture at strains of a few per cent, or ductile, leading to large but non-recoverable deformation (see chapter 8). In contrast, for rubbers. Young s moduli are typically of order 1 MPa for small strains (fig. 6.6 shows that the load-extension curve is non-linear) and elastic, i.e. recoverable, extensions up to about 1000% are often possible. This shows that the fundamental mechanism for the elastic behaviour of rubbers must be quite different from that for metals and other types of solids. 

There remains the question of whether the drop in load observed at yielding arises from the purely geometrical strain softening associated with a true-stress-strain curve of the form shown in fig. 8.4(c), where there is no drop in the true stress but merely a reduction in slope of the stress train curve, or whether there is actually a maximum in the true-stress strain curve as shown in fig. 8.4(d). Experiments on polystyrene and PMMA in compression, under which the geometrical effect cannot take place, show that a drop in load is still observed. Results from extensive studies of PET under a variety of loading conditions also support the idea that a maximum in the true-stress train curve may occur in a number of polymers. 

In the Hookean region of the load-extension curve, up to about 3% extension, hydrogen bonds between the turns of the a-helices may be strained, but there may be no decrease in the total proportion of the total length of the protein chains in crystallites in the a conformation. 

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