Stress-Strain Test Apparatus

Dynamic properties are more relevant than the more usual quasi-static stress-strain tests for any application where the dynamic response is important. For example, the dynamic modulus at low strain may not undergo the same proportionate change as the quasi-static tensile modulus. Dynamic properties are not measured as frequently as they should be simply because of high apparatus costs. However, the introduction of dynamic thermomechanical analysis (DMTA) has greatly widened the availability of dynamic property measurement. 

Figure 6.3 Schematic representation of the apparatus used to conduct tensile stress-strain tests. The specimen is elongated by the moving crosshead load cell and extensom-eter measure, respectively, the magnitude of the applied load and the elongation.

Sinusoidal stress/strain testing have also been used to examine the temperature dependent changes in rheological properties that occur in the real complex system the dough. Dreese, Faubion, and Hoseney [113,114,102] used a homemade apparatus, assembled from commercially available transducers and related electronics (Figure 49). 

The stress-relaxation behavior of a material is normally determined in either the tensile or the flexural mode. In these experiments, a material specimen is rapidly elongated or compressed to produce a specified strain level and the load exerted by the specimen on the test apparatus is measured as a function of time. Specimens of certain plastics may fail during tensile or flexural stress-relaxation experiments. 

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. 

Modulus of rupture, MOR. was obtained on polished samples by four point bend tests (4 X 20 mm) with 1 mmmin deformation rate using Instron equipment. The elastic modulus, E, was measured at room temperature, by the acoustic resonance method using a self-assembled apparatus and, at high temperature, from the stress-strain curve obtained in the four point bend tests. 

The test condition for seams in the long-term relaxation test is closer to actual mechanical effects occurring under field condition of an installed geomembrane. The relaxation test was described in Sect. 3.2.10. This test can also be performed as a long-term test. The test apparatus is modified in such a way that the specimen can be kept in a test liquid at an elevated test temperature while a specified constant strain is applied and the eonseeutive stress relaxation is measured. There are still no standards or guidelines available, which describe the long-term relaxation test on seams. The specimens are prepared as for the long-term tensile ereep test and the test... 

A cross-section of the test apparatus employing a strain gauge to determine en-capsulant stress is given in Fig. 5 The measuring device is a tube with known... 

Compression Stress Relaxation The definition of compression stress relaxation is that when a constant strain is applied to the gasket sample, the force necessary to maintain that strain is not constant but decreases with time this behavior is called stress relaxation. The test apparatus used for compression stress relaxation measiuements is the Wykeham Farrance device. It provides information for the prediction of the service life of materials by measuring the sealing force decay of a sealing gasket sample as a function of time, temperature, and environment. 

FIGURE 16.2 An automated materials testing apparatus typically applies a known stress on a dogbone-shaped sample and measures the resulting stretch (strain). A fixed or linear strain can also be programmed with the required force (or stress) measured as the sample is stretched. 

Testing under tension at high strain rate presents particular problems. Apart from the above mentioned modifications to the apparatus required to apply positive stress, the measurement is hindered by the different response of the material. Indeed, the material usually behaves as brittle and the specimen cracks soon after the stress-strain curve has reached its maximum. The only value that can be obtained is in practice the 0.2% (or other chosen percentage) proof stress. However, this value is also strongly dependent on the strain rate. 

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