Tensile stress, elastomer

Elongation. The extension produced by a tensile stress appHed to an elastomer, ie, elongation, is almost always reduced by fillers. Regardless of what type of filler is used, elongation decreases with increased loading above approximately 5 vol % (13). 

A term descriptive of the extent to which an elastomer can be deformed by the application of a tensile stress. 

Predictions of the mechanical response of filled elastomers are further aggravated by the phenomenon of strain dilatation. As soon as dilatation commences, the tensile stress lag behind elongation, the degree of dilatation for a given composite being roughly a measure for the deviation from the expected mechanical response. Dilatation increases with particle size and volume fraction of filler—it decreases somewhat if the filler is bonded to the matrix. Farris (16,17) showed that dilatation can account well for the mechanical behavior of solid propellants and his equation ... 

Rubber, vulcanized or thermoplastic Determination of tensile stress-strain properties Standard test methods for vulcanized rubber and thermoplastic elastomers-tension... 

Both series of polyurethanes were prepared using a prepolymer technique in which reactants were mixed at 70 °C/1 hour, cast into molds at 105 °C/2 hours, and cured at 80 °C/14 hours. The BD/MDI hard segment contents ranged from 0% (transparent, colorless homopolyurethanes) to 30% w/w (opaque, white copolyurethanes). All elastomers were characterized using DSC, dynamic mechanical, and tensile stress-strain measurements. 

Think about what happens when, say, an elastomer is under tensile stress. The elastic constants, s and c, cannot be scalar quantities, otherwise Eqs. 10.5 and 10.6 would not completely describe the elastic response. When the elastomer is stretched, a contraction... 

The distinction between elastomers, fibers, and plastics is most easily made in terms of the characteristics of tensile stress-strain curves of representative samples. The parameters of such curves are nominal stress (force on Ihe specimen divided by the original cross-sectional area), the corresponding nominal strain (increase in length divided by original length), and the modulus (slope of the stress-strain curve). We refer below to the initial modulus, which is this slope near zero strain. 

When a material is subjected to small deformations, the cross-sectional area of the unstrained sample, Aq, coincides with the cross-sectional area of the strained sample, A. However, in the case of elastomers, in which the deformations can be extremely high, account has to be taken of the change in the cross section of the sample. Consequently, the value of the stress a, calculated by using Eq. (3,33) and called nominal stress, does not coincide with the true tensile stress (A (Fig. 3.10). 

Basically, four types of tensile stress-strain curve are found for cross-linked elastomers deformed below Tg. These are shown schematically in Fig. 20 where pre-... 

The mechanical and thermal properties of a range of poly(ethylene)/po-ly(ethylene propylene) (PE/PEP) copolymers with different architectures have been compared [2]. The tensile stress-strain properties of PE-PEP-PE and PEP-PE-PEP triblocks and a PE-PEP diblock are similar to each other at high PE content. This is because the mechanical properties are determined predominantly by the behaviour of the more continuous PE phase. For lower PE contents there are major differences in the mechanical properties of polymers with different architectures, that form a cubic-packed sphere phase. PE-PEP-PE triblocks were found to be thermoplastic elastomers, whereas PEP-PE-PEP triblocks behaved like particulate filled rubber. The difference was proposed to result from bridging of PE domains across spheres in PE-PEP-PE triblocks, which acted as physical crosslinks due to anchorage of the PE blocks in the semicrystalline domains. No such arrangement is possible for the PEP-PE-PEP or PE-PEP copolymers [2]. 

The chemical cross-link density is a central quantity of interest in the elastomer industry, which, in a car tyre for instance, may vary in a well-defined fashion across the tyre tread. Both types of cross-links together determine the moduli of shear and elasticity, G and E, respectively. At small elongations A = L/Lq the tensile stress [Pg.440]

Example 1, As can be seen from Figure 6-6, the Mooney elastic material is softer" than an ideal elastomer with the same modulus 6 and thus can describe the observed negative deviations from ideal behavior such as shown in Figure 6-4, If C and C2 are equal, what is the ratio (Moonev/ldea ) of the tensile stresses at an elongation of 100% ... 

If an elastomer sample in the form of a unit cube is deformed by pure shear, then the three principal extension ratios are A, = X, = 1, X = MX. [Compare with the case of simple extension where X = X = 1 NX.] Following the arguments of Section B, derive an expression relating aE and A, where [Pg.208]

The tensile stress-strain curves, for the four microstructural types, cover the range from elastomers to typical semi-crystalline thermoplastics (Fig. 3.23). The lowest crystallinity material is a competitor with thermoplastic elastomers . 

The nominal tensile stress is given by the tensile force divided by the unstrained cross-sectional area of the specimen. It has been commonly used in the literature dealing with deformation and fracture of elastomers.) This reduction scheme is clearly quite successful in dealing with a wide range of crosslinking (see Figure 10.25) (Smith, 1969). 

Fig. 7 (a) Picture of a mechanochromic elastomer made by integrating C120H-RG into a thermoplastic polyurethane backbone in the unstretched state, (b) Picture of the same material in the stretched state. Both pictures were taken under illumination with ultraviolet light, (c) Ratio of monomer to excimer emission 7m//e (circles) and tensile stress (solid line) under a triangular strain cycle between 0% and 500% at a frequency of 0.0125 Hz. Adapted with permission from [41]. Copyright 2006 American Chemical Society... 

Figure 11-14. Schematic representation of the tensile stress aw as a function of strain e at constant temperature for an elastomer E, a partially crystalline thermoplast T, and a hard-elastic thermoplast HT. The ductile region is la-II-III. The necking effect shown below the diagram is typical of normal thermoplasts, but does not occur with elastomers or hard-elastic thermpolasts. The diagram is not drawn to scale for example, elastomers show a much larger elongation at break than do thermoplasts.

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