ABSTRACT. The generalized (Glen) flow relation for ice, involving the second invariants of the stress deviator and strain-rate tensors, is only expected to hold for isotropic polycrystalline ice. Previous single-stress experiments have shown that for the steady-state flow, which develops at large strains, the tertiary strain rate is greater than the minimum (secondary creep) value by an enhancement factor which is larger for shear than compression. Previous experiments combining shear with compression normal to the shear plane have shown that enhancement of the tertiary octahedral strain rate increases monotonically from compression alone to shear alone. Additional experiments and analyses presented here were conducted to further investigate how the separate tertiary shear and compression strain-rate components are related in combined stress situations. It is found that tertiary compression rates are more strongly influenced by the addition of shear than is given by a Glen-type flow relation, whereas shear is less influenced by additional compression. A scalar function formulation of the flow relation is proposed, which fits the tertiary creep data well and is readily adapted to a generalized form that can be extended to other stress configurations and applied in ice mass modelling.
BACKGROUNDIn natural ice masses the most important and common state of deformation is arguably a combination of approximately bed-parallel shear and vertical compression. For deformational flow with a stationary boundary, a region of simple shear is associated in an essential way with bulk transport of ice in glaciers, ice sheets and ice shelves, and this is generally accompanied by normal deformations associated with increasing velocities along the flow and divergence or convergence transverse to the flow.For a coordinate system with x and y horizontal and z vertical, and corresponding component velocities (u, v, w), simple shear deformation in the x direction can be characterized by du/dz = c where we note that the horizontal planes on which the forces generating shear deformation act do not rotate, while compression normal to these planes is described by dw/dz = k, where c/2 and k are the respective shear and vertical compressive strain rates. The compressive flow may be confined or unconfined, and quite generally the accompanying horizontal normal strain rates are du dx ¼ ð À 1Þk and dv dy ¼ Àk where the factors involving indicate the proportions of the deformations in the horizontal directions, relative to the rate of vertical compression. Note that = 1/2 corresponds to uniaxial compression in the z direction, while = 1 corresponds to longitudinally confined compression in the experiments reported here (Fig. 1).The generalized flow relation for ice involving the second invariants of the stress deviator and strain-rate tensors (Nye, 1953;Glen, 1958) provides a useful formulation for the interactions between the individual stress and strain-rate components for isotropic ice. This relation is not expected to apply for anisotropic i...
Laboratory creep deformation experiments have been conducted on initially isotropic laboratory-made samples of polycrystalline ice. Steady-state tertiary creep rates, , were determined at strains exceeding 10% in either uniaxial-compression or simple-shear experiments. Isotropic minimum strain rates, , determined at ˜1 % strain, provide a reference for comparing the relative magnitude of tertiary creep rates in shear and compression through the use of strain-rate enhancement factors, E, defined as the ratio of corresponding tertiary and isotropic minimum creep rates, i.e. . The magnitude of strain-rate enhancement in simple shear was found to exceed that in uniaxial compression by a constant factor of 2.3. Results of experiments conducted at octahedral shear stresses of to = 0.040.80 MPa indicate a creep power-law stress exponent of n = 3 for isotropic minimum creep rates and n = 3.5 for tertiary creep rates. The difference in stress exponents for minimum and tertiary creep regimes can be interpreted as a t0 stress-dependent level of strain-rate enhancement, i.e. .The implications of these results for deformation in complex multicomponent stress configurations and at stresses below those used in the current experiments are discussed.
andT. H. Jacka (Antarctic Division, Kingston, Tasmania 71 50, Australia)
ABSTRACTThe major results from a comprehensive study of the Amery Ice Shelf are presented, following the work of a wintering expedition in 1968 and supplemented by f u r t h e r measurements during the summer seasons of 1969 to 1971. The programme included ice-core d r i l li n g , oversnow surveys f o r ice movement and optical l e v e l l i n g , ice-thickness sounding, and measurements of snow accumulation. The new data obtained provide the basis f o r a more accurate assessment of the mass balance and dynamics of the ice shelf than was possi b l e from the e a r l i e r surveys.The results indicate a substantial growth of basal ice under the ice shelf inland where the ice thickness is greater than 450 m. Further towards the ice f r o n t the high s t r a i n thinning i s approximately balanced by the horizontal ice advection.The v e l o c i t y d i s t r i b u t i o n over the ice shelf i s primarily governed by a substantial surface slope towards the ice f r o n t and high r e s t r a i n i n g shear stress along the sides.
andT. H. Jacka (Antarctic Division, Kingston, Tasmania 71 50, Australia)
ABSTRACTThe major results from a comprehensive study of the Amery Ice Shelf are presented, following the work of a wintering expedition in 1968 and supplemented by f u r t h e r measurements during the summer seasons of 1969 to 1971. The programme included ice-core d r i l li n g , oversnow surveys f o r ice movement and optical l e v e l l i n g , ice-thickness sounding, and measurements of snow accumulation. The new data obtained provide the basis f o r a more accurate assessment of the mass balance and dynamics of the ice shelf than was possi b l e from the e a r l i e r surveys.The results indicate a substantial growth of basal ice under the ice shelf inland where the ice thickness is greater than 450 m. Further towards the ice f r o n t the high s t r a i n thinning i s approximately balanced by the horizontal ice advection.The v e l o c i t y d i s t r i b u t i o n over the ice shelf i s primarily governed by a substantial surface slope towards the ice f r o n t and high r e s t r a i n i n g shear stress along the sides.
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