Polymer properties stress-strain characteristics

Polymeric materials show a wide range of stress-strain characteristics. One characteristic of polymers that is markedly different from metals and ceramics is that their mechanical properties are highly time- and temperature-dependent. An elastomer or a rubbery polymer shows a stress-strain curve that is nonlinear. 

In Chapter 18 is discussed the measurement of mechanical, electrical and optical properties of polymers. Mechanical measurements include measurement of load bearing characteristics of polymers including stress/strain curves, stress temperature curves, recovery and rupture. Also measurement of impact strength characteristics by Izod and falling weight methods and many other polymer characteristics for polymer sheet, pipe, film, powders and rubbers and elastomers. 

Stress-strain characteristics under elongation or tensile deformation can be used to understand the mechanical behavior of polymers. Stress is defined as the force per unit area and strain is defined as dimensionless fractional increase in length. Tensile properties of a polymer can be characterized using quantities such modulus of elasticity, stiffness, elastic elongation, ultimate tensile strength, toughness, brittleness, and creep (Tripathi, 2002 Monasse and Haudin, 1995). 

One of the fascinating properties of the elastomeric materials is their rubber-like elasticity— that is, they have the ability to be deformed to quite large deformations and then elastically spring back to their original form. This results from crosslinks in the polymer that provide a force to restore the chains to their undeformed conformations. Elastomeric behavior was probably first observed in natural rubber however, the past several decades have brought about the synthesis of a large number of elastomers with a wide variety of properties. Typical stress-strain characteristics of elastomeric materials are displayed in Figure 15.1, curve C. Their moduli of elasticity are quite small, and, they vary with strain because the stress-strain curve is nonlinear. 

The most common type of stress-strain tests is that in which the response (strain) of a sample subjected to a force that increases with time, at constant rate, is measured. The shape of the stress-strain curves is used to define ductile and brittle behavior. Since the mechanical properties of polymers depend on both temperature and observation time, the shape of the stress-strain curves changes with the strain rate and temperature. Figure 14.1 illustrates different types of stress-strain curves. The curves for hard and brittle polymers (Fig. 14.1a) show that the stress increases more or less linearly with the strain. This behavior is characteristic of amorphous poly-... 

Stress is related to strain through constitutive equations. Metals and ceramics typically possess a direct relationship between stress and strain the elastic modulus (2) Polymers, however, may exhibit complex viscoelastic behavior, possessing characteristics of both liquids and solids (4.). Their stress-strain behavior depends on temperature, degree of cure, and thermal history the behavior is made even more complicated in curing systems since material properties change from a low molecular weight liquid to a highly crosslinked solid polymer (2). ... 

Stress-strain curves developed during tensile, flexural and compression tests may be quite different from each other. The moduli determined in compression are generally higher than those determined in tension. Flaws and sub-microscopic cracks significantly influence the tensile properties of brittle polymeric materials. However, they do not play such an important role in compression tests as the stresses tend to close the cracks rather than open them. Thus, while tension tests are more characteristic of the defects in the material, compression tests are characteristic of the polymeric material as it is. The ratio of compressive strength to tensile strength in the case of polymers is in the range 1.5 to 4.0 [Dukes, 1966]. 

Tensile Properties Similar to polyethylene, the stress-strain curve of JSR RB has a yield point. Above the yield point, the stress-strain curve continues to increase with elongation, then breaks. This kind of stress-strain curve is similar to EVA and indicates a characteristic property lying somewhere between amorphous and crystalline polymers. The dynamic properties of JSR RB can be improved by stretch-... 

The discussion of mechanical properties comprises the various contributions of elastic, viscoelastic and plastic deformation processes. Often two characteristic stress levels can be defined in the tensile curve of polymer fibers the yield stress, at which a significant drop in slope of the stress-strain curve occurs, and the stress at fracture, usually called the tensile strength or tenacity. In this section the relation is discussed between the morphology of fibers and films, made from lyotropic polymers, and their mechanical properties, such as modulus, tensile strength, creep, and stress relaxation. 

Viscoelastic characteristics of polymers may be measured by either static or dynamic mechanical tests. The most common static methods are by measurement of creep, the time-dependent deformation of a polymer sample under constant load, or stress relaxation, the time-dependent load required to maintain a polymer sample at a constant extent of deformation. The results of such tests are expressed as the time-dependent parameters, creep compliance J t) (instantaneous strain/stress) and stress relaxation modulus Git) (instantaneous stress/strain) respectively. The more important of these, from the point of view of adhesive joints, is creep compliance (see also Pressure-sensitive adhesives - adhesion properties). Typical curves of creep and creep recovery for an uncross-Unked rubber (approximated by a three-parameter model) and a cross-linked rubber (approximated by a Voigt element) are shown in Fig. 2. 

Polymers are used as structural materials, therefore their mechanical properties are very important. Mechanical behaviour of a polymer is its deformation and flow characteristics under stress. The generalised stress-strain curve for plastics is represented in Figure 6.11, which serves to define several useful quantities, including modulus or stiffness (the slope of the curve), yield stress, and strength and elongation at break. Polyethylene gives such a curve. 

The stress-strain curve showed good linearity over a considerable strain, but did not show a typical yield point characteristic of polymer films and macrofibers. An elastic modulus of 45 MPa was determined for the PEO nanofibers (Tan et al. 2005a). A similar approach based on using AFM for assessing the single-fiber tensile properties of PAN was reported by Buer et al. (2001). 

Viscoelasticity is a phenomenon observed in most of the polymers since they possess elastic and viscous characteristics when deformed. The properties such as creep, stress relaxation, mechanical damping, vibration absorption and hysteresis are included in viscoelasticity. If a material shows linear variation of strain upon the application of stress on it, its behavior is said to be linear viscoelastic. Elastomers and soft biological tissues undergo large deformations and exhibit time dependent stress strain behavior and are nonlinear viscoelastic materials. The non-linear viscoelastic properties of solid polymers are often based on creep and stress-... 

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