VS .NET Code 128B Flow and fracture of a crystalline material in .NET Generator 3 of 9 in .NET Flow and fracture of a crystalline material

Flow and fracture of a crystalline material using none toincoporate none for web,windows generate code 128 c# Small circle GTIN-14 Boreholes Weakly oriented Single-maximum Elevation above sea level, m Multiple-maximum Single-maximum 4 Distance from divide, km Bubbly white (Pleistocene) ice Deformed superimposed ice Vertical exaggeration, 4x Figure 4.15 none for none . Vertical cross section along a ow line on Barnes Ice Cap showing zones characterized by particular fabrics.

Arrows show locations of cores used to determine fabric type. (After Hooke and Hudleston, 1980.).

by repeated ly compressing a sample and then annealing it (Huang et al., 1985). However, these processes are not consistent with the occurrence of such fabrics in ice that is actively deforming, as in Barnes Ice Cap (Figure 4.

15) (Hooke and Hudleston, 1980). Matsuda and Wakahama (1978) measured the orientations of a-axes as well as c-axes in ice with multiple-maximum fabrics. They did this by observing etch pits in the thin sections.

In ice with four-maximum fabrics, they found that the a-axes of adjacent crystals were systematically aligned in a way that suggested mechanical twinning. Noting that strong shear deformation under high temperatures is required to produce such fabrics, they suggested that the large amount of plastic strain energy thus produced can be absorbed by propagation of twin boundaries without changing the relative structural relation between crystals or the crystal-boundary structure, and without resulting in strong bubble elongation. Because the various fabrics appear to form under fairly speci c conditions of cumulative strain, strain rate, and temperature, and because these parameters all tend to increase systematically with depth in the accumulation area of a glacier, fabric type also varies with depth.

For example, in Barnes Ice Cap transitions from weakly oriented to broad single-maximum (or equivalent) fabrics occur at depths of 80 140 m, and the broad single maximum gives way to multiple-maximum fabrics at 140 200 m (Figure 4.15). At Byrd Station in Antarctica, the transition to broad single maximum fabrics (small circle variety) occurs at a depth of 350 m.

Then, a strong single-maximum fabric appears at 1200 m and multiple-maximum fabrics show up at 1830 m. Differences in temperature are probably largely responsible for the difference in depths to the transitions, although cumulative strain may also play a role; Barnes. Deformation mechanism maps Ice Cap is near or above 10 C throughout, whereas at Byrd Station the temperature exceeds 10 C only below 1900 m. In Barnes Ice Cap, as the various layers are advected outward they become exposed at the surface in the ablation area (Figure 4.15).

. Summary. Given these various processes of recrystallization and crystal deformation, one may well ask how we should visualize the deformation of polycrystalline ice on an intergranular scale. Available evidence suggests that stresses are heterogeneous, that intracrystalline glide takes place on basal planes within individual grains, that this glide results in internal rotation of the crystal structure, and that nucleation of grains with basal planes parallel to the maximum resolved shear stress and resorption of grains that have rotated out of this orientation results in the development of fabrics with preferred orientations. Mismatches between adjacent grains resulting from unequal slip at grain boundaries are accommodated by grain-boundary migration and by rotation and translation of grains.

These grain-boundary processes are thus likely to be rate limiting. Computer models incorporating these principles successfully simulate many characteristics of fabric evolution in ice sheets (Etchecopar, 1977; Van der Veen and Whillans, 1994)..

Deformation mechanism maps Our discuss ion so far has focused on the type of creep most commonly observed in glaciers, called power-law creep because the creep rate is proportional to the stress raised to some power >1 (Equation (4.4)). The dominant processes in power-law creep are dislocation glide and climb.

For completeness, some other types of creep should be mentioned. In recent years, scientists working on ice deformation mechanisms have found it useful to plot maps showing the deformation mechanisms operating at different temperatures and stresses (Figure 4.16).

The temperature is usually normalized by dividing by the melting temperature in Kelvin, Km . This is called the homologous temperature. Similarly, the stress is normalized by dividing by Young s modulus.

In Figure 4.16 the stress used is 3 e . Note that the equivalent octahedral stress is shown on the right ordinate.

The heavy lines in Figure 4.16 divide the diagram into elds in which a single deformation mechanism is dominant. Power-law creep occupies much of the right side of the diagram.

Below and to the left of the powerlaw creep eld is the eld of diffusional ow. In this type of ow, atoms move from crystal boundaries that are under compression to ones that are.
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