Stress induced dilatation

A linear decrease of KIc with an increase in crosslink density was reported for model PU based on triisocyanate and diols of various molar masses (Bos and Nusselder, 1994), and for epoxy networks (Lemay et al., 1984). It was suggested that the dilational stress field at the crack tip may induce an increase in free volume and a devitrification of the material. A linear relationship between GIc and M XJ2 was verified for these systems, although other empiric equations were found in other cases (Urbaczewski-Espuche et al., 1991). 

The concept of stress-induced dilatation affecting the relaxation time or rate has been suggested by others (5, 6, 7, 8). The density of most solids decreases under uniaxial stress because the lateral contraction of the solid body does not quite compensate for the longitudinal extension in the direction of the stress, and the body expands. The Poisson ratio, the ratio of such contraction to the extension, is about 0.35 for many polymeric solids it would be 0.5 if no change in density occurred, as in an ideal rubber. The volume increase, AV, accompanying the tensile strain of c, can be described by the following equation ... 

If an increase in free volume arising from stress-induced dilatation contributes to the relaxation process in the same manner as dilatation by raising the temperature, we can estimate the shift in relaxation time with Equation 2 by substituting for the fractional free volume, f,... 

Rensing H, Jaeschke H, Bauer 1, Patau C, Datene V, Pannen BH and Bauer M, Differential activation pattern of redox-sensitive transcription factors and stress-inducible dilator systems heme oxygenase-1 and inducible nitric oxide synthase in hemorrhagic and endotoxic shock. Cril Care Med 29(10) 1962-71,2001. 

The use of a locking stylet during transvenous removal can be due to the decision to use a traction device (as the main extraction tool or in combination with sheaths). Obviously, if the stylet cannot be inserted or if it stops very early in the coil lumen, traction cannot be performed. Regardless of the use of traction devices, the presence of a stylet into the lead is helpful during any dilatation technique. Either mechanical or powered sheaths, while advancing over the lead and freeing it from binding sites, induce mechanical stress on the lead body. The stiffer the lead, the more effective and safe is dilatation. 

In subsequent sections the Eshelby theory of shear transformations is broadened by incorporation of strain-induced dilatancy into an interaction-energy component of the transformation free energy to account for interaction of the transformation strains with mean normal stresses, to obtain specific results for the differences among shear, tension, and compression flow and strength-differential effects. 

When there is a prominent dilatancy effect in the flow phenomenon arising from the interaction of the shear-induced dilatation, with a prevailing mean normal stress (Tin, this is often considered to make the critical threshold resistance in shear T dependent on through a friction coefficient... 

In general, the development of crazes is associated with dilatational stresses (Kambour, 1973). In one case, crystalline poly(ethylene terephthalate), so-called shear crazes have been reported to lie along shear bands induced by yielding. While such crazes are not yet understood, it is reasonable to assume that a dilatational stress component must somehow be involved. If such crazes exist in other systems, however, the argument in this section should not be affected. 

Strain-Induced Dilatation. An alternative view of yield in polymers comes from the fact that a tensile strain induces a hydrostatic tension in the material and a corresponding increase in the sample volume. This in turn translates to an increase in the free volume, which increases the polymer mobility and effectively lowers the glass-transition temperature (Tg) of the polymer (alternatively it can be looked upon as increasing the free volume to the value it would have at the normal measured Tg). The increased mobility results in a lowering of the yield stress. Rnauss and Emri (35) used an integral representation of nonlinear viscoelasticity with a state-dependent variable related to free volume to model the yield behavior, with the free volume a function of temperature, time, and stress history. This model uses the concept of reduced time (see VISCOELASTICITY), where application of a tensile stress causes a volume dilatation and consequently causes the material time scale to change by a shift factor related to the magnitude of the applied stress. Yield occurs because the free-volume shift factor causes the molecular mobility to increase in such a way that yield can occur. 

Hyperhomocysteinemia, in coronary arteries increased TNF-a expression, which enhanced oxidative stress ultimately impairing flow-induced dilation that can be reversed by superoxide dismutase (SOD) (147). Also, in EC Hey inhibits dimethylarginine dimethylamino-hydrolase (DDAH) enzyme activity by direct interaction (enzyme that degrades asymetric dimethylarginine [ADMA]), causing ADMA to accumulate and inhibit NO synthesis, which might explain how Hey impairs NO dependent vasodilation (148). 

KoUer, A., Sun, D., and Kaley, G., Role of shear stress and endothelial prostaglandins in flow- and viscosity-induced dilation of arterioles in vitro, Circ. Res., 72,1276,1993. 

Equation (5-9) means that the energy necessary to introduce a hydrogen atom in the lattice is increased by the presence of previous hydrogen and decreased by the presence of hydrostatic dilatation stress. In other words, the interaction between deformations induced by hydrogen and a hydrostatic stress o rises to a binding energy Wxnt=-o V per hydrogen mole. 

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