Stress induced strain

In contrast to pipelines and harbor installations, platforms are dynamically loaded. Therefore in the choice of steels, in addition to strength and types of machinability, the risk of corrosion fatigue and strain-induced stress corrosion must be taken into account in combination with cathodic protection (see Sections 2.3.3 to 2.3.5). 

Secondary stress. The basic characteristic of a secondary stress is that it is self-limiting. As defined earlier, this means that local yielding and minor distortions can satisfy the conditions which caused the stress to occur. Application of a secondary stress cannot cause structural failure due to the restraints offered by the body to which the part is attached. Secondar) mean stresses are developed at the junctions of major components of a pressure vessel. Secondary mean stresses are also produced by sustained loads other than internal or external pressure. Radial loads on nozzles produce secondary mean stresses in the shell at the junction of the nozzle. Secondary stresses are strain-induced stresses. 

Strain-induced stress corrosion cracking for nonclassical stress corrosion ... 

Another commonly used elastic constant is the Poisson s ratio V, which relates the lateral contraction to longitudinal extension in uniaxial tension. Typical Poisson s ratios are also given in Table 1. Other less commonly used elastic moduH include the shear modulus G, which describes the amount of strain induced by a shear stress, and the bulk modulus K, which is a proportionaHty constant between hydrostatic pressure and the negative of the volume... 

The referential formulation is translated into an equivalent current spatial description in terms of the Cauchy stress tensor and Almansi strain tensor, which have components relative to the current spatial configuration. The spatial constitutive equations take a form similar to the referential equations, but the moduli and elastic limit functions depend on the deformation, showing effects that have misleadingly been called strain-induced hardening and anisotropy. Since the components of spatial tensors change with relative rigid rotation between the coordinate frame and the material, it is relatively difficult to construct specific constitutive functions to represent particular materials. 

The deformation may be viewed as composed of a pure stretch followed by a rigid rotation. Stress and strain tensors may be defined whose components are referred to an intermediate stretched but unrotated spatial configuration. The referential formulation may be translated into an unrotated spatial description by using the equations relating the unrotated stress and strain tensors to their referential counterparts. Again, the unrotated spatial constitutive equations take a form similar to their referential and current spatial counterparts. The unrotated moduli and elastic limit functions depend on the stretch and exhibit so-called strain-induced hardening and anisotropy, but without the effects of rotation. 

Graphite will creep imder neutron irradiation and stress at temperatures where thermal creep is normally negligible. The phenomenon of irradiation creep has been widely studied because of its significance to the operation of graphite moderated fission reactors. Indeed, if irradiation induced stresses in graphite moderators could not relax via radiation creep, rapid core disintegration would result. The observed creep strain has traditionally been separated into a primary reversible component ( ,) and a secondary irreversible component (Ej), both proportional to stress and to the appropriate unirradiated elastic compliance (inverse modulus) [69]. The total irradiation-induced creep strain (ej is thus ... 

If these changes in length take place freely then we will have a thermally induced strain in the material (= 0.3 x 100/50 = 0.6% in the polypropylene) but no stress. However, if the polypropylene was constrained in some way so that the 0.3 mm expansion could not happen when it is heated by 60°C, then there would be a thermally induced stress in the material, i.e. 

This example illustrates the simplified approach to film blowing. Unfortunately in practice the situation is more complex in that the film thickness is influenced by draw-down, relaxation of induced stresses/strains and melt flow phenomena such as die swell. In fact the situation is similar to that described for blow moulding (see below) and the type of analysis outlined in that section could be used to allow for the effects of die swell. However, since the most practical problems in film blowing require iterative type solutions involving melt flow characteristics, volume flow rates, swell ratios, etc the study of these is delayed until Chapter 5 where a more rigorous approach to polymer flow has been adopted. 

As reviewed thermoplastics (TPs) being viscoelastic materials respond to induced stress by two mechanisms viscous flow and elastic deformation. Viscous flow ultimately dissipates the applied mechanical energy as frictional heat and results in permanent material deformation. Elastic deformation stores the applied mechanical energy as completely recoverable material deformation. The extent to which one or the other of these mechanisms dominates the overall response of the material is determined by the temperature and by the duration and magnitude of the stress or strain. The higher the temperature, the most freedom of movement of the individual plastic molecules that comprise the... 

When a viscoelastic material is subjected to a constant strain, the stress initially induced within it decays in a time-dependent manner. This behavior is called stress relaxation. The viscoelastic stress relaxation behavior is typical of many TPs. The material specimen is a system to which a strain-versus-time profile is applied as input and from which a stress-versus-time profile is obtained as an output. Initially the material is subjected to a constant strain that is maintained for a long period of time. An immediate initial stress gradually approaches zero as time passes. The material responds with an immediate initial stress that decreases with time. When the applied strain is removed, the material responds with an immediate decrease in stress that may result in a change from tensile to compressive stress. The residual stress then gradually approaches zero. 

Testing mode Basically material fatigue failure is the result of damage caused by repeated loading or deformation of a structure. The magnitudes of the stresses and strains induced by this repeated loading or deformation are typically so low that they would not be expected to cause failure if they were applied only once. 

Frequently, a product becomes loaded when it is subjected to a defined deflection. The actual load then is a result of the structural reaction of the product to the applied strain. Unlike directly applied loads, strain-induced loads are dependent on the modulus of elasticity and, with TPs, will generally decrease in magnitude over time. Many assembly and thermal stresses could be the result of strain-induced loads. They include metal insert press fits in the plastic and clamping or screw attachments. 

Caustic embrittlement corrosion (caustic induced, stress corrosion cracking), which occurs as an intergranular form of corrosion where localized stresses and strains are present (and some silicate, which acts as a general corrosion inhibitor that protects grains at the expense of the grain boundaries). 

Thermal expansion induced by insolation may be important in desert areas where rocky outcrops and soil surfaces are barren. In a desert, daily temperature excursions are wide and rocks are heated and cooled rapidly. Each type of mineral in a rock has a different coefficient of thermal expansion. Consequently, when a rock is heated or cooled, its minerals differentially expand and contract, thereby inducing stresses and strains in the rock and causing fractures. Ollier (1969) discussed examples of rock weathering due to insolation. Fire can develop temperatures far in excess of insolation and be quite effective in fracturing rocks (Black-welder, 1927). 

One effect of this strain-induced crystallisation is that there is a characteristic upswing in the plot of stress against strain for natural rubbers, as illustrated in Figure 7.11. 

Density is also found to increase in this region, thus providing additional evidence of crystallisation. Certain synthetic elastomers do not undergo this strain-induced crystallisation. Styrene-butadiene, for example, is a random copolymer and hence lacks the molecular regularity necessary to form crystallites on extension. For this material, the stress-strain curve has a different appearance, as seen in Figure 7.12. 

Secondly, as we discussed in Section 6.5.2.1, inducing stress or strain in the metal by supporting a monolayer pseudomorfically on another metal is a powerful way to change its reactivity. Figure 6.33 gives guidance on what to expect. 

In a previous work, we found an increased pectinase production under extreme acidic pH conditions. Since very acidic pH induced stress conditions in the cell it is interesting to evaluate the response of different Aspergillus strains toward stress induced by the pH of culture medium. In our laboratory we have studied the effect of extreme acidic pH on growth and secretion of pectinases by a vtrild white strain of Aspergillus and by Aspergillus niger N-402. 

Figure 6.3 Shear strain Induced by shear stress...

Figure 12 shows the stress-strain curves of IER at various temperatures. A strain-induced reinforcing effect is not observed at temperatures above -10 °C. This fact may be due to network inhomogeneities caused by imperfect crosslinking. 

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