Elongation in tension

Apparatus for the measurement of this property according to ASTM D32 [57] are available from ATS FAAR (Table 2.6). 

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When a load is placed on a specimen, stress and strain result. Stress is the internal resistance to the load as the applied force. Strain is the amount of deformation caused by this stress, such as deflection in bending, contraction in compression, and elongation in tension. 

The apparent difference between the curves for tension and compression is due solely to the geometry of testing. If, instead of plotting load, we plot load divided by the actual area of the specimen, A, at any particular elongation or compression, the two curves become much more like one another. In other words, we simply plot true stress (see Chapter 3) as our vertical co-ordinate (Fig. 8.7). This method of plotting allows for the thinning of the material when pulled in tension, or the fattening of the material when compressed. 

One of the most informative properties of any material is their mechanical behavior specifically the determination of its stress-strain curve in tension (ASTM D 638). This is usually accomplished in a testing machine by measuring continuously the elongation (strain) in a test sample as it is stretched by an... 

Although the creep behavior of a material could be measured in any mode, such experiments are most often run in tension or flexure. In the first, a test specimen is subjected to a constant tensile load and its elongation is measured as a function of time. After a sufficiently long period of time, the specimen will fracture that is a phenomenon called tensile creep failure. In general, the higher the applied tensile stress, the shorter the time and the greater the total strain to specimen failure. Furthermore, as the stress level decreases, the fracture mode changes from ductile to brittle. With flexural, a test specimen... 

Methods employed to determine the impact resistance of plastics include pendulum methods (Izod, Charpy, tensile impact, falling dart, Gardner, Dynatup, etc.) and instrumented techniques. In the case of the Izod test, what is measured is the energy required to break a test specimen transversely struck (the test can be done either with the specimen notched or unnotched). The tensile impact test has a bar loaded in tension and the striking force tends to elongate the bar (Chapter 5, Impact Strength). 

When a solid bar is loaded axially in tension, it elongates more and more with increasing load. The mechanism by which such elongation occurs can be visualised as a progressive increase in the separation of the atoms of the bar in the direction of loading, i.e., in the axial direction. The displacement of the atoms from their equilibrium positions results in the development of attractive forces between them these forces balance the applied load. This elementary picture can be considered to be applicable to the initial stages of deformation of many types of materials, crystalline as well as amorphous. 

The elongation remaining after a rubber test piece has been stretched for a given time and allowed to rest for a given time it is expressed as a percentage of the original distance between the bench marks. The term permanent set (correctly, permanent set in tension) is still popularly used to denote tension set, see standard BS903-A5. 

A relation between rupture phenomena and (specific) surface energy 7 was postulated by Dupre 4 almost simultaneously with the hypothesis of Quincke. Let a cylindrical rod be broken in tension. After rupture, two new gas — solid interfaces of vr2 each are present, r being the radius of the rupture surface. Consequently, the work of rupture ought to contain a term 2 nr2 7 . Dupre did not indicate how to separate this term from the main component of the work of rupture, which is the work required to extend the rod to its maximum elongation (or strain). The modern development of Dupre s ideas is reviewed in Section III.3. below. 

The tensile strength with sole leather is usually 2-3 kilos per sq. mm. and with good belting leather should be at least 3 kilos in the latter case, the less the elongation on tension the higher the quality. The leather should not change or crack when bent to a curve with a diameter ten times the thickness. 

In practice, up to 90% of polyurethanes are used in compression, a few percent in torsion, and very little in tension. There is considerable data on the tensile stress against tensile strain (elongation) for polyurethanes. Most polyurethane specification sheets provide this data. Figure 7.3 and Figure 7.4 show typical stress-strain curves for both polyester and polyether polyurethanes. 

Elongation The fractional increase in length of a material stressed in tension. 

A cylindrical specimen of brass (an alloy of copper and zinc) with a diameter of 20 mm and length of 200 mm is pulled in tension elastically with a force of 50,000 N. If the Young s modulus is 97 GPa and Poisson s ratio is 0.34, determine a) the amount of longitudinal extension (the elongation in the direction of the uniaxial stress) and b) the transverse contraction in diameter. 

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