Rubber stress-strain tests

A compression stress/strain test is in many ways easier to carry out than a tensile test, and in view of the large number of applications of rubber in compression, should be more often used. Frequently, it would be logical for the test piece to be the complete product and a compressive force applied as it would be in service. Usually a constant rate of deformation would be appropriate and the force and corresponding deformation recorded without attempts at calculating the resultant stresses and strains. 

Short-term stress-strain testing is widely practised in the rubber industry, especially in the form of indentation hardness, tensile strength, and elongation at break. Applications range from quality control and measurement of the state of cure to material specification and a convenient means of monitoring aging resistance. 

Tensile stress-strain tests with amorphous rubbers over a range of strain rates and temperatures have shown that for a given rubber the failure point lay along an envelope of the stress-strain diagram (Smith, 1962) (Fig. 4.8) and that the data could be superimposed by a WLF-type shift operation (Fig. 4.9). 

The mechanical properties of polymers are specified with many of the same parameters that are used for metals—that is, modulus of elasticity and yield and tensile strengths. For many polymeric materials, the simple stress-strain test is used to characterize some of these mechanical parameters. The mechanical characteristics of polymers, for the most part, are highly sensitive to the rate of deformation (strain rate), the temperature, and the chemical nature of the environment (the presence of water, oxygen, organic solvents, etc.). Some modifications of the testing techniques and specimen configurations used for metals (Chapter 6) are necessary with polymers, especially for highly elastic materials, such as rubbers. 

In TPE, the hard domains can act both as filler and intermolecular tie points thus, the toughness results from the inhibition of catastrophic failure from slow crack growth. Hard domains are effective fillers above a volume fraction of 0.2 and a size <100 nm [200]. The fracture energy of TPE is characteristic of the materials and independent of the test methods as observed for rubbers. It is, however, not a single-valued property and depends on the rate of tearing and test temperature [201]. The stress-strain properties of most TPEs have been described by the empirical Mooney-Rivlin equation... 

Stress-Strain Data. Tensile tests were made with an Instron tester at some seven crosshead speeds from 0.02 to 20 inches per minute at five or six temperatures from 30° to —46°C. The tests were made on rings cut with a special rotary cutter from the circular sheets of the elastomers. The dimensions of each ring were determined from the weights of the ring and the disc from its center, the thickness of the ring, accurately measured, and the density of the rubber. Typically, the outside and inside diameters were 1.45 and 1.25 inches, respectively, and the thickness was about 0.085 inch. The test procedure used is described elsewhere (11), and the cubic equation, eq 4 in ref. j 2, was used to compute the average strain in a ring from the crosshead displacement. 

There are various test methods, one being the De Mattia Flex Test method which is suitable for rubbers that have reasonably stable stress-strain properties, at least after a period of cycling, and do not show undue stress softening or set, or highly viscous behaviour. The results obtained for some thermoplastic rubber should be treated with caution if the elongation at break is below,... 

ISO 844 2001 Rigid cellular plastics - Determination of compression properties ISO 3386-1 1986 Polymeric materials, cellular flexible - Determination of stress-strain characteristics in compression - Part 1 Low-density materials ISO 3386-2 1997 Flexible cellular polymeric materials - Determination of stress-strain characteristics in compression - Part 2 High-density materials ISO 5893 2002 Rubber and plastics test equipment - Tensile, flexural and compression types (constant rate of traverse) - Specification ISO 7743 2004 Rubber, vulcanized or thermoplastic - Determination of compression stress-strain properties... 

The classical means for following vulcanization by physical methods is to vulcanize a series of sheets for increasing time intervals and then measure the stress strain properties of each and plot the results as a function of vulcanization time. A modification of this test generally called a rapid modulus test is widely used in the industry as a production control test. A single sample taken from a production batch of compounded rubber is vulcanized at a high temperature and its tensile modulus is measured. Temperatures as high as 380°F are used to reduce the vulcanization test time to only a few minutes. Any modulus value deviating from a predetermined acceptance limit indicates that the batch is defective and is to be rejected. 

It is unfortunate that test methods for soft plastics and for rubbers, although very similar, are not identical, for example differences in tensile stress strain, tear and hardness methods. If they were aligned, much of debate about which method to use would be eliminated. For some properties, there is a distinct difference in approach. For example, glass transition temperature is frequently determined for plastics whilst various low temperature tests have been specifically developed for rubbers. 

Stress/strain relationships are commonly studied in tension, compression, shear or indentation. Because in theory all stress/strain relationships except those at breaking point are a function of elastic modulus, it can be questioned as to why so many modes of test are required. The answer is partly because some tests have persisted by tradition, partly because certain tests are very convenient for particular geometry of specimens and partly because at high strains the physics of rubber elasticity is even now not fully understood so that exact relationships between the various moduli are not known. A practical extension of the third reason is that it is logical to test using the mode of deformation to be found in practice. 

There is a British standard19 giving guidance on the application of rubber testing to finite element analysis. Several of the models for stress strain behaviour are appraised and advice given on selection. The point is made that the models considered treat the rubber as a perfectly elastic material,... 

The effect of the difference between El and E2 on the force at break is shown in Figure 8.7. The approximate average stress is /2(A+B) which is the apparent tensile strength and may be considerably less than the true tensile strength, A. The discrepancy may be as much as 33%66 and will vary with the steepness of the final part of the stress/strain curve. Although this may not be serious in pure quality control testing, the variation with slope of the stress/strain curve could make comparisons between different rubbers invalid. 

Whilst it is generally held that an extensometer is necessary, it would be rather less expensive if elongation of dumbbells could be obtained from crosshead movement. Tay and Teoh76 devised a numerical scheme whereby the stress strain characteristics could be derived from measured load versus total elongation data from a finite element analysis of the dumb-bell shape. Their method was shown to work to within 10% of values measured with an infra red extensometer for two fairly soft plastics and a silicone rubber. To be effective, the tensile test must be carried out with grips which essentially prevent any slippage and it is, of course, necessary to have the computing facility set up to carry out the analysis. 

It was not until 1989, which in relative terms is quite late, that an international standard for compression stress strain was published. This is perhaps a sad reflection on the order of priorities that existed within the standardisation of rubber testing. However, ISO 7743104 is now well established. 

The stress strain curve is recorded and the modulus determined at a shear strain of 25 %. For the quadruple test piece, the shear strain is half the measured deformation divided by the thickness of one rubber block. The shear stress is the applied force divided by twice the area of a bonded face of one block. 

The term dynamic test is used here to describe the type of mechanical test in which the rubber is subjected to a cyclic deformation pattern from which the stress strain behaviour is calculated. It does not include cyclic tests in which the main objective is to fatigue the rubber, as these are considered in Chapter 12. Dynamic properties are important in a large number of engineering applications of rubber including springs and dampers and are generally much more useful from a design point of view than the results of many of the simpler static tests considered in Chapter 8. Nevertheless, they are even today very much less used than the "static" tests, principally because of the increased complexity and apparatus cost. 

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

Determination of tensile strength at break, tensile stress at yield, elongation at break, and stress values of rubber in a tensile test Physical testing of rubber Part A2 Method for determination of tensile stress-strain properties... 

To relate the physical properties of carbon black to rubber properties, we tested these tread blacks in the ASTM natural rubber recipe and in an SBR 1500 test recipe. In both elastomers, we checked standard stress/strain properties of modulus, tensile strength, and hardness. In the natural rubber recipe we also tested Firestone running temperature and rebound, and Goodyear rebound. In the SBR we checked percent swell, extrusion rate, viscosity, and laboratory abrasion. 

Figure 6a shows the effect of rubber content on the stress-strain behavior of these materials. All the rubber-modified materials exhibited a load maximum under all testing conditions. In most tests the unmodified material failed before it exhibited a yield maximum, although the data were reduced in the same manner used for the other samples. In all cases the yield stress decreased as the rubber content was increased, but the... 

Goodyear material with the same rubber content as the 13% American Cyanamid material exhibited more ductile stress-strain behavior. Possible reasons for this difference will be discussed later in the paper. Figure 6b shows the effect of rate on the behavior of the 13% American Cyanamid material. The yield stress increases with strain rate as was the case in all materials tested. Figure 6c shows the effect of temperature on the behavior of the same material. Again, as was the case in all materials tested, the yield stress decreases as the temperature is increased. 

However, the correlation with the theory is complicated by the fact that one does not test only the branching theory but also the rubber-elastidty theory. Several variants are offered for explaining the dependence of the stress-strain data on the network structure (cf. e.g. Refs jt is assumed that the equilibriiun force... 

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