Dilation, stress-induced

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. 

Recorded normal stresses in borehole SG2 (CT Oo. Or), are reproduced, together with predictions of several teams in Figure 26a,b,c. Measuring cells were located at radial distances of 2.59 m, 2.97 m and 3.25 m respectively. The observed increase in total stress is due to differential rock dilation effects induced by the varying temperature field and by the increasing swelling pressure at the tunnel wall. 

Figure 1 presents the three basic mechanisms of stress induced permeability change in fractured rock (a) normal closure/opening, (b) shear dilation/contraction and (c) induced anisotropy due to different orientations of fractures and anisotropic stress condition. 

The second mechanism is dilation/contraction induced permeability change during shear (Figure 1 (b)). Fractures dilate when fracture failure occurs under high differential stresses. This mechanism can play a major role in the flow field since the dilated fracture tend to have much higher transmissivity than the fracture under elastic deformation. 

Several approaches have been taken to calculating the toughening associated with a stress-induced phase transformation and there is agreement between these various analyses. The simplest calculations assume the phase transformation is purely dilatational and that it is nucleated by a critical value of the mean stress at a crack tip. In a material that contains a crack, the transformation will occur in a frontal zone at the crack tip, as shown schematically in Fig. 8.60. For this case, the transformation is found not to increase toughness. Toughening only... 

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. 

Other transformations, such as ferroelastic transformation and twin formation in a system may also induce toughening effects. The former discussion on stress-induced transformation was Martensitic, involving both dilation and shear components of the transformation strain. Twin transformation typically only has a... 

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. 

Loufrani, L., K. Matrougui, D. Gorny M. Duriez, 1. Blanc, B.l. Levy and D. Henrion. 2001. Flow (shear stress)-induced endothehmn-dependent dilation is altered in mice lacking the gene encoding for dystrophin. Circulation 103(6) 864-70. 

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