Stress-strain fatigue test

Increasing temperature decreases the modulus of elasticity of elastomers at the same time that accelerated chemical effects occur. In our constant strain fatigue tests it has been found that the effect on the modulus (lower stress at the same strain) is significantly more important than chemical degradation effects, as shown in Figure 2. Thus, failure occurred much more rapidly at 4°C than at 37°C or 60°C. This may be one manner in which fatigue... 

Fatigue is the decay of mechanical properties after repeated application of stress and strain. Fatigue tests given information about the ability of a material to resist the development of cracks or crazes resulting from a large number of deformation cycles. 

Stress-controlled fatigue tests and the properties generated by these tests are useful for HCF analysis where elastic strains are dominant. One... 

Testing Strain-controlled fatigue test Stress-controlled fatigue test... 

Fig. 8. Stress-strain hysteresis as a function of the number of number, measured in a tensile/compression fatigue test at 150 Hz and 0.13 % strain (SiC/SiC)...

Standard fatigue tests normally require extended periods of time to complete regardless of the type of stress/strain used. Accelerated fatigue tests change some conditions of the test in the hope of reaching the same end point in less time. Potential means of accelerating the tests include changing factors such as the temperature, frequency, chemical environment, levels of stress or strain, or the addition of stress concentrators. 

Dynamic properties are taken to mean the results from mechanical tests in which the plastic is subjected to a deformation pattern from which the cyclic stress-strain behaviour is calculated. These do not include cyclic tests in which the main objective is to fatigue the material. 

Thermoplastics being sensitive to creep, the fatigue tests at defined strain are less severe than those at defined stress for comparable original stresses. 

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. 

Most fatigue tests apply a fixed pre-stress or strain, partly because without bonding of the test piece it is necessary to hold the rubber in place. The amount of pre-stress or strain will affect the fatigue life in particular the fatigue life is appreciably shortened if the cyclic deformation passes near to or through zero strain. 

Multiple stress-strain tests on the same sample of human skin is shown in Fig. 2.7. The stress necessary to stretch (stain) the same skin is less with each successive pulling or stressing of the skin sample. This phenomenon is typical of the skin to decrease in strength or modulus with continued stretching - it is the physical nature of skin to fatigue with repeated stretching or pulling. 

Figure 12.16 Evolution of stress versus time for different fatigue tests depending on the stress ratio RCT and the corresponding stress-strain cycles crm is the average stress.

Dynamic mechanical measurements are performed at very small strains in order to ensure that linear viscoelasticity relations can be applied to the data. Stress-strain data involve large strain behavior and are accumulated in the nonlinear region. In other words, the tensile test itself alters the structure of the test specimen, which usually cannot be cycled back to its initial state. (Similarly, dynamic deformations at large strains test the fatigue resistance of the material.)... 

TensUe tests, at controlled strain rates, were performed on an Instron tensile tester. Samples were rectangular, 6.35 mm by 3.17 mm in cross-section, or cylindrical, with a diameter of 5.1 mm. Both types had a gauge length of 12.7 mm. Fatigue tests were carried out on similar cylindrical samples, or on rectangular specimens, 5.1 mm by 3.17 nun in cross-section, at various selected stress amplitudes and at frequencies... 

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