Yield point elongation

How do we determine the tensile modulus, tensile yield point, elongation at break and tensile strength of a polymer What characteristics of the polymer define these properties ... 

Y = wye pipe or duct fitting—takes less pressure drop than a T. yield = fraction or percent of the total charged weight that becomes shipped product, yield point elongation = the point in tensile testing where the elasticity of the test piece is deformed and does not return to its original shape and dimension when the strain is removed (i.e., beyond its elastic limit). 

Another source of surface problems is Luder s lines, Luder s hands, or stretcher strains (see Section IV for more detail) and is associated with yield-point elongation (YPE). These defects are particularly apparent in regions where the strain or deformation is very low, and they disappear at moderate and high... 

Figure 15 Schematic. showing yield-point elongation. (From Ref. 32.)...

Yield strength is the stress at the yield point. For materials whose stress-strain curve does not exhibit a yield point, yield strength is sometimes defined in terms of a specified limiting stress value of the departure of linearity. The yield point elongation is the difference between the elongation at the completion and at the start of discontinuous yielding. 

Coefficient of thermal expansion a (20-100°C) Tensile strength ctb Yield point Elongation. 5... 

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

Fig. 41. Typical stress—strain curve. Points is the yield point of the material the sample breaks at point B. Mechanical properties are identified as follows a = Aa/Ae, modulus b = tensile strength c = yield strength d = elongation at break. The toughness or work to break is the area under the curve.

When a fiber is stressed, the instantaneous elongation that occurs is defined as instantaneous elastic deformation. The subsequent delayed additional elongation that occurs with increasing time is creep deformation. Upon stress removal, the instantaneous recovery that occurs is called instantaneous elastic recovery and is approximately equal to the instantaneous elastic deformation. If the subsequent creep recovery is 100%, ie, equal to the creep deformation, the specimen exhibits primary creep only and is thus completely elastic. In such a case, the specimen has probably not been extended beyond its yield point. If after loading and load removal, the specimen fails to recover to its original length, the portion of creep deformation that is recoverable is still called primary creep the portion that is nonrecoverable is called secondary creep. This nonrecoverable elongation is typically called permanent set. 

Ferrous valves are also available in nodular (ductile) iron, which has tensile strength and yield point approximately equal to cast carbon steel at temperatures of 343°C (6.50°F) and below and only slightly less elongation. 

For stainless steel, the stress-strain curve (see Fig. 26-37) has no sharp yield point at the upper stress limit of elastic deformation. Yield strength is generally defined as the stress at 2 percent elongation. 

Stress-strain curves at the conditions of product application. If applicable, this would usually indicate the toughness of material by sizing up the area under the curve (Chapter 2). It would also show the proportional limit, yield point, corresponding elongations, and other relevant data. 

Mechanical properties, such as elastic modulus and yield point, that depend on crystallinity per se are not seriously affected by low to moderate doses of ionizing radiation. On the other hand, those mechanical properties that are sensitive to interlamellar activity are most dramatically affected by the low to moderate radiation doses. This is seen in the ultimate tensile strength and elongation at failure of the polyolefins. It is also reflected in the large change in melt index between 0 and 18 Mrad, which indicates formation of cross-links that increase with increasing... 

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. 

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