Tensile samples, uniaxial

Even plastics with fairly linear stress-strain curves to failure, for example short-fiber reinforced TSs (RPs), usually display moduli of rupture values that are higher than the tensile strength obtained in uniaxial tests wood behaves much the same. Qualitatively, this can be explained from statistically considering flaws and fractures and the fracture energy available in flexural samples under a constant rate of deflection as compared to tensile samples under the same load conditions. These differences become less as the... 

In the following, the relationship between fracture statistics and defect size distribution is discussed for the simple case of tensile tests (uniaxial and homogeneous stress state) on a homogeneous brittle material. The tests are performed on specimens of equal size. It is assumed that the volume of the specimens is V = V. The number of tested specimens (the sample size) isX. In each test the load is increased up to the moment of failure. The strength is the stress at the moment of failure. In each sample the strength values of the individual specimens are different, i.e. the strength is distributed. 

The first group of tests is carried out on specimens generally fabricated into a dumb-bell shape, with forces applied uniaxially. The usual apparatus consists of a machine with a pair of jaws, which during the test are moved relative to each other, either together or apart, in a controlled manner. A chart recorder is employed to give a permanent record of the results obtained, so that the force at fracture can be determined. Whether this kind of set up measures tensile, compressive, or flexural strength depends on how the sample is oriented between the jaws, and on the direction that the jaws are set to travel relative to one another. 

Change in length of a sample in uniaxial tensile or compressive deformation divided by its initial length... 

Integral over the total change in length of a sample of the incremental strain in uniaxial tensile deformation... 

Force resulting from an applied tensile or compressive uniaxial deformation divided by the initial cross-sectional area of the sample normal to the applied deformation. 

Fig. 48 Schematics showing (a) the tensile testing rig holding the sample between clamps for uniaxial deformation and (b) deformation stages in the sample, recorded with an optical imaging system. L0 is the original length of the sample, and L the length after deformation...

All the uniaxial tensile loads described above assume that the load is shared uniformly across the sample cross-section and that the sample maintains a constant cross-section (over the measured region at least). The tensile modulus, E, is then taken to be defined as ... 

The fracture properties of thermosets are often very difficult to measure the brittleness of these materials. If a thermoset is tested in uniaxial tensile mode, the stress and strain at break, strain rate, e = df/dt, and also on the sample dimensions (length and cross section). Thus, the parameters intrinsic values of the materials because they depend on the... 

The test method consists of uniaxially straining a sample of the film-substrate couple as shown schematically in Figure 1. The film thickness is t, and the specimen width is w. Under tensile strain, an interfacial shear stress, x(x), is produced. While the film is bonded to the substrate, the shear stress, x(x), at the interface causes a tensile stress, metal film. When the strain is sufficient, the tensile stress will reach the ultimate tensile strength of the film, tr. Then, if the film fails by brittle... 

A particular well known solution to Equations 2 and 7 is uniaxial tension (4r — 0) applied to a sample containing an ellipse at right angle to the tensile axis (ft = tt/2). In this case, the roots of Equation 7 are given by 77 = 0 and 77 = tt/2. From Equation 2 one obtains a maximum tensile stress for 77 = 0, corresponding to the major axis of the ellipse (Figure 1), which is given by ... 

Although the uniaxial tension test is the one most widely used, it has two drawbacks when it is used to provide information on the yielding of polymers. First, the tensile stress applied can lead to brittle fracture before yield takes place, and second, yield occurs in an inhomogeneous way due to the formation of a neck accompanying the tensile test. In any case, given that the section of sample decreases as the stress increases, cj cy . 

Figure 14.5b represents the uniaxial compression test, which uses samples with cylindrical or rectangular cross section. The stress and strain are defined in an analogous way to that of the tensile test. This test overcomes the disadvantages mentioned in relation to a tensile test. The stress is compressive, and consequently there is no possibility of the brittle fracture observed in tensile deformation. Plastic yield can even be seen in thermostable materials, which, under other conditions, can be brittle. In addition, the determination of the yield stress is made under conditions of stable deformation since there is no geometrical reason for the formation of a neck such as occurs in tension. A problem that can arise in this test concerns the diameter/height ratio of the sample. If this ratio is too large friction between plates and sample will introduce a constraint, and if it is very small... 

For measurements at 77 K we used a simple optical cryostat with an inserted uniaxial stress device that transfers the applied tensile stress to an elastic ring with the firmly fixed sample inside [2]. The ring converts the stretching force in one direction into a compressive one along the sample. The axial distribution of the stress in the ring and a solid junction secured the sample of a premature... 

Orientation of commercial POM can be carried out using cylindrical samples which are obtained by extruding POM into a thermostatically controlled form. After the form is filled with the melt, it is removed from the extruder and gradually cooled down in a compression-molding press. Cylindrical specimens are then uniaxially stretched in a tensile test instrument at a temperature of 130°C and a stretching rate... 

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