Mechanical Behaviour of Antarctic Ice

Duval, Paul; Gac, H. Le

doi:10.1017/s0260305500002585

Cited by 28 publications

(8 citation statements)

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“…Studies on several deep ice cores from Antarctica and Greenland have revealed the development of highly preferred crystal orientation with depth (Gow and Williamson, 1976;Russell-Head and Budd, 1979;Herron and Langway, 1982;Herron and others, 1985). A clear relationship between the c-axis orientation and strain-rates has been verified both by ill-situ measurements (Russell-Head and Budd, 1979;Gundestrup and Hansen, 1984) and by laboratory studies (Lile, 1978;Duval, 1981;Duval and Le Gac, 1982;Pimienta and others, 1987;Shoji and Langway, 1987).…”

Section: Introductionmentioning

confidence: 73%

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Crystalline Texture of the 2083 m Ice Core at Vostok Station, Antarctica

et al. 1989

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Crystalline texture and c-axis orientation of the 2083 m ice core at Vostok Station, covering more than 150kyear, reveal the existence of strong anisotropics. Changes in crystal size with depth are compatible with the growth of grains driven by the free energy of grain boundaries. A smaller growth rate appears to be associated with cold periods. A gradual increase in the horizontal elongation of grains was observed between 350 and 680 m. But, the mean value of the coefficient of the linear dimensional orientation of grains does not change below 700 m.The c-axis orientation of ice grains tends to orientate perpendicular to the direction of the elongation of grains, forming a vertical girdle pattern. This characteristic fabric has been interpreted as resulting from the gradual rotation of grains by basal glide under uniaxial longitudinal tension. The rotation of grains was calculated with respect to the total strain, simulating the formation of the girdle fabric pattern. The fabric-enhancement factor was calculated at various depths. It appears that Vostok ice hardens gradually with depth when considering the transverse convergent flow. No significant variation of the enhancement factor was observed with changes in climate and impurity content.

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Section: Introductionmentioning

confidence: 73%

“…With a surface slope of I x 10-3 , the horizontal shear stress at 2000 m is lower than 0.02 MPa. By adopting the flow-law parameter of Duval and Le Gac (1982), the shear strain-rate is of the order of 4 x 10-6 year-l at -36°C, i.e. at the temperature of the bottom of the core (Vostretsov and others, 1984).…”

Section: Forma Tion Processes Of Cr Yst Al-orientmentioning

confidence: 99%

Crystalline Texture of the 2083 m Ice Core at Vostok Station, Antarctica

et al. 1989

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“…Fabric has been well documented in ice sheets by extensive thin-section measurements on ice from ice cores (Alley and others,1995;Gow and others, 1997;Thorsteinsson and others, 1997), and sonic logging both in boreholes and on the ice cores themselves (Kohnen and Gow, 1979;Taylor, 1982;Anandakrishnan and others,1994;Thorsteinsson and others,1999). A consequence of fabric development is that bulk physical properties become anisotropic, as shown by experiments and theory (Steinemann,1958; Russell-Head and Budd,1979;Duval,1981;Duval and le Gac, 1982;Budd and Jacka, 1989;Van der Veen and Whillans, 1990;Alley, 1992;Anandakrishnan and others, 1994;Azuma, 1994Azuma, , 1995Azuma and Goto-Azuma, 1996;Castelnau and others, 1996). At Dye 3 in Greenland, fabric accounts for roughly 70% of the difference between measured deformation rates and those predicted by Glen's flow law (Thorsteinsson and others, 1999;Cuffey and others, 2000).…”

Section: Introductionmentioning

confidence: 97%

An analytical approach to deformation of anisotropic ice-crystal aggregates

Þorsteinsson

2001

J. Glaciol.

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Deformation rates of single hexagonal crystals, deforming by glide on the basal plane, are described as a function of stress state and crystal orientation. These results are used to infer the deformation rate of crystal aggregates assuming that the stress distribution within the crystal aggregate is homogeneous. Analytical equations for the deformation rate of anisotropic ice aggregates are derived for vertically symmetric girdle fabric. This type of fabric is approximated by a uniform distribution of c-axis orientations between a cone angle and a smaller girdle angle relative to the symmetry axis. For simple shear stress acting on a single-maximum fabric there is a slight de-enhancement for cone angles of 60–90°. In uniaxial compression a maximum enhancement of ∼1.7 occurs at a cone angle of 57°. A pure shear stress state has similar features, with the additional complication that it causes a non-zero transverse strain rate, except for perfect vertical alignment of crystals and isotropic fabric. In combined states of stress the contribution of each stress component to the strain rate depends on fabric. A single enhancement factor is not adequate to describe the effects of anisotropy for complex stress states.

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“…Owing to the very large anisotropy of ice crystals and the preponderance of intracrystalline dislocation glide in polar ice [Pimienta and Duval, 1987], initially isotropic ice near the surface becomes anisotropic when fabric develops, exhibiting a different viscous response to shear stress on different planes. Very large variations of strain rates with the applied stress direction have been found with both laboratory and in situ measurements [Russell-Head and Budd, 1979;Duval and Le Copyright 1996 by the American Geophysical Union.…”

Section: Introductionmentioning

confidence: 99%

“…Distorted grain shape is found to slightly slow down fabric development. Gac, 1982;Gundestrup and Hansen, 1984;Shoji and Langway, 1988;Budd and Jacka, 1989]. Such anisotropy renders the isotropic model for polar ice inadequate for ice sheet flow modeling.Since fabrics develop with strain, the anisotropic constitutive laws must be associated with a model giving the evolution of lattice preferred orientations.…”

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confidence: 99%

Viscoplastic modeling of texture development in polycrystalline ice with a self‐consistent approach: Comparison with bound estimates

Castelnau¹,

Duval²,

Lebensohn³

et al. 1996

J. Geophys. Res.

129

101

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Ice crystals deform easily by dislocation glide on basal planes, which provides only two independent easy slip systems. The necessary slip on other systems limits the strain rate of polycrystalline ice. The preferred c axis orientation of ice from polar ice sheets develops as a result of intracrystalline slip. An anisotropic viscoplastic self-consistent (VPSC) approach is used for predicting texture development and mechanical behavior of polycrystalline ice. Results are compared with lower and upper bound estimations. It is assumed that ice crystals deform by basal, prismatic, and pyramidal slip. The resistance of each slip system is determined from experimental data on monocrystals and isotropic polycrystals. The VPSC model can predict the behavior of isotropic polycrystalline ice on both the macroscopic and microscopic scale. This is not the case for the lower and upper bounds. Fabrics simulated in uniaxial extension and compression are qualitatively similar for all models. However, large differences in the rate of fabric development are found. This is explained by the different interaction stiffness between grain and matrix. Fabric concentration obtained with the VPSC model for uniaxial deformation is in close agreement with those observed in polar ices. In simple shear, the single maximum fabric found in situ cannot be reproduced without an extensive (and probably unrealistic) activity of nonbasal systems. The preferential growth of grains well oriented for basal glide associated with rotation recrystallization could be at the origin of the discrepancy between model results and natural simple shear fabrics. Distorted grain shape is found to slightly slow down fabric development. Gac, 1982;Gundestrup and Hansen, 1984;Shoji and Langway, 1988;Budd and Jacka, 1989]. Such anisotropy renders the isotropic model for polar ice inadequate for ice sheet flow modeling.Since fabrics develop with strain, the anisotropic constitutive laws must be associated with a model giving the evolution of lattice preferred orientations. Physically based modeling of plastic deformation in polycrystals is the normal way to simulate fabric development. In this paper we present results from several homogenization methods. The viscoplastic macroscopic response (i.e., of the polycrystal) is calculated by averaging microscopic responses (i.e., of grains). A microscopic constitutive relation is chosen, but the form of the macroscopic constitutive relation is not known in advance. The general assumption is that grains deform by dislocation slip only, where different resistance is attributed to each slip system. Deformation mechanisms such as diffusional creep, grain boundary sliding, and climb of basal dislocations [Duval et al., 1983; Lliboutry and Duval, 1985] are not taken into account. Since polar ice deforms essentially by dislocation glide in basal planes, the effects of these 13,851 13,852 CASTELNAU ET AL.' TEXTURE DEVELOPMENT IN ICEmechanisms are expected to be very small. Furthermore, the elastic strain is neglected in the models; app...

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Mechanical Behaviour of Antarctic Ice

Cited by 28 publications

References 14 publications

Crystalline Texture of the 2083 m Ice Core at Vostok Station, Antarctica

Crystalline Texture of the 2083 m Ice Core at Vostok Station, Antarctica

An analytical approach to deformation of anisotropic ice-crystal aggregates

Viscoplastic modeling of texture development in polycrystalline ice with a self‐consistent approach: Comparison with bound estimates

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