Natural rubber stress-strain behavior

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... 

Price,C., Allen,G., de Candia,F., Kirkham,M.C., Subramaniam,A. Stress-strain behavior of natural rubber vulcanized in the swollen state. Polymer (London) 11, 486-491 (1970). 

Bristow, G.M. Relation between stress-strain behavior and equilibrium volume swelling for peroxide vulcanizates of natural rubber and cis-1,4-polyisoprene. J. Appl. Polymer Sci. 9, 1571-1578 (1965). 

Although the dynamic mechanical properties and the stress-strain behavior iV of block copolymers have been studied extensively, very little creep data are available on these materials (1-17). A number of block copolymers are now commercially available as thermoplastic elastomers to replace crosslinked rubber formulations and other plastics (16). For applications in which the finished object must bear loads for extended periods of time, it is important to know how these new materials compare with conventional crosslinked rubbers and more rigid plastics in dimensional stability or creep behavior. The creep of five commercial block polymers was measured as a function of temperature and molding conditions. Four of the polymers had crystalline hard blocks, and one had a glassy polystyrene hard block. The soft blocks were various kinds of elastomeric materials. The creep of the block polymers was also compared with that of a normal, crosslinked natural rubber and crystalline poly(tetra-methylene terephthalate) (PTMT). 

The reinforcing effect of carbon black on stress-strain behavior of natural rubber is depicted in Figure 10.1. The reinforced material has a higher modulus (is stiffer) and is less extensible. 

The stress-strain behavior of uncured EP copolymers with highly stereo- and regioregular propylene placements resembles that of natural rubber. Both elastomers possess all the requisites discussed in Section 12.10. In particular, they have negligible crystallinity at rest and are able to develop crystallinity under stretching. 

Figure 9.5 Stress-strain behavior of iightly cross-linked natural rubber at 50°C. Curve (a), experimental. Theoretical is equation (9.4). Cun/e (c) illustrates the reversible nature of the extension up to a = 5.5. At higher elongation, curve (b), hysteresis effects become important. The theoretical curve has been fitted to the experimental data in the region of small extensions, with nRT= 0.39 N/mm= (47,48).

Mechanically, rubbers may be expected to lose strength rapidly with increase in temperature, to show a large hysteresis in stress strain behavior, to exhibit marked creep and set, and to be greatly affected by rates of load application or frequency of repeated stress. Heat build-up, i.e., increase in temperature in service, as well as deterioration from environment (sunlight, oils, ozone, etc.) will reduce the valuable properties of many rubbers, both natural and synthetic. 

Chen YK, Xu CH (2012) Stress-strain behaviors and crosslink networks studies of natural rubber-zinc dimethacrylate composites. J Macro Sci B Phys 51(7) 1384-1400... 

Natural rubber (NR)-rectorite nanocomposite was prepared by co-coagulating NR latex and rectorite aqueous suspension. The TEM and XRD were employed to characterize the microstructure of the nanocomposite. The results showed that the nanocomposite exhibited a higher glass transition temperature, lower tan d peak value, and slightly broader glass transition region compared with pure NR. The gas barrier properties of the NR-rectorite nanocomposites were remarkably improved by the introduction of nano scale rectorite because of the increased tortuosity of the diffusive path for a penetrant molecule. The nanocomposites have a unique stress-strain behavior due to the reinforcement and the hindrance of rectorite layers to the tensile crystallization of NR [36]. 

Our own experience, as well as that of other authors, has shown that very precise measurement for the stress-strain relationship under general biaxial deformation is required to investigate the behavior of the strain energy density function of rubber vulcanizates. Unfortunately, available biaxial extension data are still too meager to deduce the general form of the strain energy density function of rubber-like substances. We wish to take this opportunity to summarize the principal results from our recent efforts, in the hope that they may serve to illustrate the interesting and complex nature of the derivatives 31V/9/,- of such substances. 

The effects of fillers on the behavior of elastomers are summarized in Figure 3.22. In this figure the stress-strain curves of both natural rubber reinforced with 50% carbon black and a nonreinforced natural rubber are compared. An inspection of the curves highlights three important characteristics ... 

Due to the dual filler and crosslinking nature of the hard domains in TPEs, the molecular deformation process is entirely different than the Gaussian network theories used in the description of conventional rubbers. Chain entanglements, which serve as effective crosslinks, play an important role in governing TPE behavior. The stress-strain results of most TPEs have been described by the empirical Mooney-Rivlin equation ... 

One of the unique characteristics of AA-PEA hbers is their elastomeric mechanical behavior, similar to Spandex and natural rubber materials, enabling them to be stretched to several times their length. Figure 11.5 shows the breaking stress and strain of one type of Phe-based PEA monohl-ament hbers, which support their elastomeric behavior. The AA-PEA-based... 

Differential dynamic measurements have also been made with other kinds of oscillating deformations. In studies by Painter of small dynamic shear deformations superimposed on large static shear strains in the same direction, the dynamic storage modulus G of cross-linked natural rubber and poly(dimethyl sil-oxane) at 24 Hz was found to increase with increasing static strains in excess of 721 = 0.2. Here 721 is defined as u jxt in the notation of Chapter 1. After a history of large static strain, however, the change in G with static strain in a second (and subsequent) sequence of experiments was much smaller. These history-dependent effects are no doubt related to the behavior in repeated stress-strain cycles in extension mentioned in Section 3 above. Some experiments on torsional deformations of stretched rubber strips have been reported. " ... 

Natural rubber (NR) is a well studied elastomer. Of particular interest is the ability of NR to crystallize, specifically the strain-induced crystallization that takes place whilst the material is stretched. Moreover, in many elastomer applications, network chain dynamics under external stress/strain are critical for determining ultimate performance. Thus, a study on how the strain-induced crystallization affects the dynamics of a rubbery material is of outmost importance. Lee et al [1] reported their initial findings on the role of uniaxial extension on the relaxation behavior of cross-linked polyisoprene by means of dielectric spectroscopy. Nonetheless, to our best knowledge no in-depth study of the effects of strain induced crystallization on the molecular dynamics of NR has been undertaken, analyzing the relaxation spectra and correlating the molecular motion of chains with its structure. Broadband dielectric spectroscopy (BDS) has been chosen in order to study the dynamic features of segmental dynamics, because it is a comparatively simple technique for the analysis of the relaxation behaviour over a suitable frequency interval. This study is important from a basic and practical point of view, since an elongated crosslinked polymer at equilibrium may be considered as a new anisotropic material whose distribution of relaxation times could be affected by the orientation of the chains. 

Figure 2.8 (a) Influence of carbon black particle size on tensile properties of rubber vulcanizates [95] (b) Stress-strain softening behavior (Mullins effect) of carbon blackfilled natural rubber compound... 

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. 

An argument to resolve the discrepancy between the failure envelopes obtained for different modes of straining is indicated by the work of Blatz, Sharda, and Tschoegl [42]. These authors have proposed a generalized strain energy function as constitutive equation of multiaxial deformation. They incorporated more of the nonlinear behavior in the constitutive relation between the strain energy density and the strain. They were then able to describe simultaneously by four material constants the stress-strain curves of natural rubber and of styrene-butadiene rubber in simple tension, simple compression or equibiaxial tension, pure shear, and simple shear. 

First, the role of rubber modification in high rate impact is to suppress spallation by inducing the material to yield in the presence of dynamic tensile stresses arising from impact. Second, this yield-spall transition occurs at different strain rates for different rubber contents and may be predictable using quasistatic, low temperature tests of this type. These tests can also provide information concerning the basic nature of the yield process in these materials through the activation parameters which are obtained. Third, the Bauwens-Crowet equation seems to be a good model for the rate and temperature sensitive behavior of the American Cyanamid materials and is therefore a likely candidate for a yield criterion to use in the analytical code work on these materials which we hope to perform as a continuation of this work. 

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