Stress field, distortion

C Distortion of halo around glass sphere in the stress field of the non-releasing resin particle... 

Let us return to the reduction of shear stress at the crack tip due to the emission of dislocations. Figure 14-9 illustrates a possible stress reduction mechanism. It can be seen that the tip of a crack is no longer atomically sharp after a dislocation has been emitted. It is the interaction of the external stress field with that of the newly formed dislocations which creates the local stress responsible for further crack growth. Thus, the plastic deformation normally impedes embrittlement because the dislocations screen the crack from the external stress. Theoretical calculations are difficult because the lattice distortions of both tension and shear near the crack tip are large so that nonlinear behavior is expected. In addition, surface effects have to be included. 

Equation (12.61) is also true for residual stresses in metal. The latter kind of stress usually will have tensile and compressive stress fields associated with it. As far as the solubility of hydrogen is concerned, the effect of the tensile stress field (which increases solubility) overwhelms the counter-effect due to the compressive stress field (which tends only to decrease the already small solubility). Therefore, the larger the lattice strain or distortion, the larger the concentration of hydrogen (Fig. 12.74). All imperfections in crystals are regions of distortion or strain. Hence, absorbed hydrogen finds its way to, and concentrates at, such imperfections. 

Moreover, the decrease of the stress field ahead of the crack tip is explained by the development of a damage zone with load increase (Fig. 9). Loading is made prior to reaching the experimental fracture energy evaluated by the Rice Integral (as a crack stop criterion). Nevertheless, the mesh distortion at the crack tip does not allow to increase more loading because Ottosen model is not usually applied on notched beams. 

Stress fields result in a distortion of the lattice. In a tensile stressed region, hydrogen migrates toward and is trapped by centers of dilation. The stress field around a crack, for example, creates a high triaxial stress that dilates the metal lattice, with the result that hydrogen is attracted towards the crack. Traps associated with stress fields also tend to have a physical character. 

Blow molding is complicated by the complex stress field set up in the materials when the parison is inflated. This amounts to a biaxial stretching of the molten polymer and it is difficult to obtain material data under these conditions so that simulation may be performed. Despite this, much work on the inflation stage has been done, mostly with the aim of determining the final thickness distribution. Recently parison inflation has been simulated using three-dimensional finite elements and with remeshing of the parison as it inflates to minimize error from element distortion. ... 

Stress-Driven Diffusion Stress and diffusion can be coupled in a number of ways. In a uniform stress field, the dijfusivity of the diffusing species can become directionally dependent. This is because the stress field can affect the amount work required for the species to move in different directions (e.g., parallel vs. perpendicular to the stress field). Movements in directions that cause the greatest distortions to the stress field will be penalized, while movements in directions that minimize the distortion to the stress field will be favored. 

In contrast to other methods of measuring elastic stress, the X-ray method is not subject to distortion of the results by plastic deformation. Furthermore, highly inhomogeneous stress fields can be measured, because the irradiated surface can be as small as l-10mm . The technique has the disadvantage that X rays penetrate only some 10" cm into steels, so that only stresses at the surface can be determined. 

Because dislocations distort the crystal lattice, an elastic stress field forms around the dislocation line. This will now be shown using the example of an... 

In addition, the carbon further strengthens the alloy considerably. The solubility of carbon is much smaller in the body-centred than in the face-centred crystal structure because the interstitial spaces are smaller. If 7 iron with a sufficient amount of dissolved carbon (> 0.008 wt-%) is quenched, the carbon remains dissolved in the body-centred lattice. The carbon atoms strongly distort the body-centred cubic cell to a tetragonal one (figure 6.54). This distorted lattice structure has an extreme strength when highly oversaturated because the stress field cannot be passed by dislocations. As a rule of thumb. 

To rationalize this inconsistency, a hypothesis of local distortion of stress field has een put forward and attributed to defects, which are always present. These defects are assumed to cause a local stress concentration and, consequently, a premature failure. In the distorted stress field, some stress components are inversely proportional to the defect radius. This means that some stress components are very high in the neighbourhood of the crack tip, and approach infinity as the radius of the crack tip tends to zero, so that the rubber should hardly sustain any stress in the presence of very sharp flaw tips. In conclusion, these models foresee a too strong or a too weak material, in contrast with experimental results. 

The capping layer tends to stabilize the configuration against dislocation formation. The basis for this expectation is that the formation of the dislocation at the film-substrate interface requires distortion of the material over the full depth /if. On the other hand, the background mismatch stress field does work as the dislocation is formed only over a fraction of the total thickness /igi < /if. To what extent can the capping layer stabilize the structure against dislocation formation ... 

The capillary rheometer can be used for estimating the normal stress difference using the total ends pressure loss [65,68] and the exit pressure loss [69-71], wherein the latter has a more rigorous theoretical basis. However, the assumption of fully developed flow existing up to the tube exit may not hold true, especially in slow flows [72] and Ae errors introduced by the velocity field distortions at the exit may prove significant. 

Orientation shrinkage occurs in addition to thermal shrinkage. Residual stresses have been locked into the product during cooling against the mold surface. They relax over time. As the stress field is not uniform, the product may exhibit distortion, warp, and dimensional variation. Residual stress relaxation is a vexation in the trimming operation. It is often the cause of product rejection by the customer. 

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