Stress-strain test

Strength and Stiffness. Thermoplastic materials are viscoelastic which means that their mechanical properties reflect the characteristics of both viscous liquids and elastic solids. Thus when a thermoplastic is stressed it responds by exhibiting viscous flow (which dissipates energy) and by elastic displacement (which stores energy). The properties of viscoelastic materials are time, temperature and strain rate dependent. Nevertheless the conventional stress-strain test is frequently used to describe the (short-term) mechanical properties of plastics. It must be remembered, however, that as described in detail in Chapter 2 the information obtained from such tests may only be used for an initial sorting of materials. It is not suitable, or intended, to provide design data which must usually be obtained from long term tests. 

As shown in Fig. 3.4 stress-strain tests on uniaxially aligned fibre composites show that their behaviour lies somewhere between that of the fibres and that of the matrix. In regard to the strength of the composite, Ocu, the rule of mixtures has to be modified to relate to the matrix stress, o at the fracture strain of the fibres rather than the ultimate tensile strength, o u for the matrix. 

Stress-strain tests may be made in compression as well as tension. A modulus may... 

Stress-strain tests of these perfectly alternating PDMS-PSF copolymers show that the mechanical behavior is dictated by the volume fraction of PDMS present in the system. At high siloxane content (> 70 wt %), copolymers show elastomeric behavior Hue to the presence of continuous PDMS matrix. An increase in the PSF content resulted in an increase in the initial modulus and the ultimate tensile strength of these materials, while a decrease in the ultimate elongation was also observed, as expected. 

The different results in X-ray and stress-strain tests are probably attributable to differences in the preparation of the... 

Figure 4 helps illustrate the terminology used for stress-strain testing. The slope of the initial straight-line portion of the curve is the elastic modulus of the material, In a tensile test this modulus is Young s modulus,... 

Tensile stress-strain tests give another elastic constant, called Poisson s ratio, v. Poisson s ratio is defined for very small elongations as the decrease in width of the specimen per unit initial width divided by the increase in. length per unit initial length on the application of a tensile load ... 

Numerous methods have been used to measure elastic moduli. Probably the most common test is the tensile stress-strain test (8-10). For isotropic materials. Young s modulus is the initial slope of the true stress vs. strain curve. That is,... 

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. 

Fig. 4.11 Time frames obtained by simulation of stress-strain tests on (a) N-doped SWCNTs, (b) B-doped SWCNTs and (c) N-doped single layer graphene.

There are two reasons for using a tensile stress/strain test other than the standard method as typified by ISO 37. First, it can be sensibly argued that a more useful measure of stiffness is the so-called relaxed modulus, i.e. the stress at a given elongation after a fixed time of relaxation this is essentially a short term stress relaxation test. Secondly, it may be more convenient for quality control purposes to have a simple test in which only one parameter is measured. 

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. 

The dumb-bells specified are the same as those for tensile stress/strain tests except the preferred thickness is 1.5 mm. The ring is also the same as the tensile ring, which means that the bulk of the two types of test piece are different. 

Figure 12. Results of the second stress-strain test for the remaining portion of the strips after the first test.

Table 2.1 lists and defines the terminology of mechanical stress-strain testing. Table 2.2 shows the values for procine skin. The typical stress-strain relationship for human skin is shown in Fig. 2.6 and the E for modulus value is shown as the slope on the linear segment of the curve. 

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 8.5. Plots of elastic modulus versus mineral content (b) and days of mineralization (a) from incremental stress-strain tests performed on mineralized self-assembled type I collagen fibers. Slopes were obtained from the straight portions of the elastic and viscous stress-strain curves.

The inplane shear stress-strain tests reported here have been well demonstrated to be a reliable test for matrix-dominated properties in composites 141). For the selected mechanical properties that were monitored, their sensitivity to the thermal history was well demonstrated. In particular, the embrittlement process during the sub-Tg annealing or physical aging has been clearly observed. This decrease in molecular mobility, which gives rise to an increase in relaxation time and hence a decrease in toughness, can be rationalized as a decrease in free volume in an approach towards the equilibrium glassy state. 

Many impact tests measure the energy required to break a standard sample under certain specified conditions. The most widely used tests are the lzod test (pendulum-type instrument with notched sample, which is struck on the free end), the Charpy test (pendulum-type instrument with sample supported at the two ends and struck in the middle), the falling-weight test (standard ball dropped from known height), and the high-speed stress-strain test. 

Stress-strain tests were mentioned on page 24 and in Fig. 11-12. In such a tensile lest a parallel-sided strip is held in two clamps that are separated at aconstant speed, and the force needed to effect this is recorded as a function of clamp separation. The test specimens are usually dogbone shaped to promote deformation between the clamps and deter flow in the clamped portions of the material. The load-elongation data are converted to a stress-strain curve using the relations mentioned on p. 24. These are probably the most widely used of all mechanical tests on polymers. They provide useful information on the behavior of isotropic specimens, but their... 

Commercial polymer films are usually produced with some orientation. The orientation is generally different in the longitudinal (machine) direction than in the transverse direction. How could you tell which is the machi ne direction from a stress-strain test Hint Refer back to the elTeets of orientation mentioned in Section 1.8.)... 

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