Tensile stress engineering

The engineering tensile stress, cr, is defined as the tensile force divided by the initial cross-sectional area of the sample A and is, therefore. 

The conventional engineering tensile stress in the specimen is calculated from... 

The effect of temperature on PSF tensile stress—strain behavior is depicted in Figure 4. The resin continues to exhibit useful mechanical properties at temperatures up to 160°C under prolonged or repeated thermal exposure. PES and PPSF extend this temperature limit to about 180°C. The dependence of flexural moduli on temperature for polysulfones is shown in Figure 5 with comparison to other engineering thermoplastics. 

Both Watts and sulfamate baths are used for engineering appHcation. The principal difference in the deposits is in the much lower internal stress obtained, without additives, from the sulfamate solution. Tensile stress can be reduced through zero to a high compressive stress with the addition of proprietary sulfur-bearing organic chemicals which may also contain saccharin or the sodium salt of naphthalene-1,3,6-trisulfonic acid. These materials can be very effective in small amounts, and difficult to remove if overadded, eg, about 100 mg/L of saccharin reduced stress of a Watts bath from 240 MPa (34,800 psi) tensile to about 10 MPa (1450 psi) compressive. Internal stress value vary with many factors (22,71) and numbers should only be compared when derived under the same conditions. 

F(FG = normal (shear) component of force A = area u(w) = normal (shear) component of displacement o-(e ) = true tensile stress (nominal tensile strain) t(7) = true shear stress (true engineering shear strain) p(A) = external pressure (dilatation) v = Poisson s ratio = Young s modulus G = shear modulus K = bulk modulus. 

The Swedish engineer, Weibull, invented the following way of handling the statistics of strength. Fie defined the survival probability PJ.Vg) as the fraction of identical samples, each of volume Vg, which survive loading to a tensile stress a. Fie then proposed that... 

The engineer visualizes a soil mass as an ideal, real, physical body incapable of resisting tensile stresses. 

Figure 5.86 Fiber-matrix coupling in continuous, unidirectional fiber-reinforced composites (a) Coupling for tensile stress parallel to fiber axis, (b) Coupling for stress perpendicular to fiber axis. Reprinted, by permission, from N. G. McCrum, C. P. Buckley, and C. B. BucknaU, Principles of Polymer Engineering, 2nd ed., p. 258. Copyright 1997 by Oxford University Press.

Suppose a cylindrical aluminium rod in a construction will have to be able to withstand a maximum tensile force of 3 x 105 N. The engineer can use the o-e curve to determine that the safe tensile stress is maximally 170 N/mm2 with an accompanying e = 0.003. The rod should at least be 5 m long and should not elongate more than 5 mm. What is the maximum length of the rod ... 

Here, we used the mechanical engineering convention that takes as positive the forces that are pulling the element (tensile stresses) and negative the forces that push on the surface... 

In the equations above a is the true tensile stress, i.e. F/A. In practice in general use is made of engineering stress, which is equal to F/Aa, where F is the tensile load and A and A0 are the cross-sectional surface areas of the sample in the deformed and non-deformed state, respectively. Because the Poisson constant Vi for rubbers A = A0/A, so that the equations for the tensile stress become ... 

This approach is the most useful for engineering purposes since it expresses fracture events in terms of equations containing measurable parameters such as stress, strain and linear dimensions. It treats a body as a mechanical continuum rather than an assembly of atoms or molecules. However, our discussion can begin with the atomic assembly as the following argument will show. If a solid is subjected to a uniform tensile stress, its interatomic bonds will deform until the forces of atomic cohesion balance the applied forces. Interatomic potential energies have the form shown in Fig. 1 and consequently the interatomic force, whidi is the differential of energy with respect to linear separation, must pass throt a maximum value at the point of inflection, P in Fig. 1. 

Although measures of engineering stress and engineering strain assume constant cross-sectional area of the rod being stressed, a material deformed elastically longitudinally (in compression or tension) has an accompanying lateral dimensional change. This is described by Poisson s ratio, i/. If a tensile stress produces an axial strain +e and... 

In engineering and testing bulk polymers, the term stress is taken to be the ratio of applied force to the cross sectional area. The SI unit of stress is the newton per square meter (N,-m"). This is also numerically equivalent to the pascal (Pa), albeit that the Pa is usually employed as a unit of compression stress (i.c., pressure) rather than tensile stress. 

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