Author(s): P. R. Kry
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Abstract: Continuous crushing of ice sheets, the steady and continued penetration of an ice sheet by an indentor, has been studied at two scales with semi-cylindrical indentors in S2 ice sheets. Geometric similarity and strain rate similarity were achieved by matching the ratios of indentor width to ice sheet thickness and penetration rate to ice sheet thickness. The small-scale tests in a cold room used indentors of 0.12 m and 0.25 m diameter penetrating machined, confined ice sheets 0.01-0.05 m thick. The bubble-free ice had typical grain diameters of about 1 mm, and was tested at various penetration rates and at temperatures of -10°C and -2°C. The large-scale tests were performed on a natural lake using a 1.2 m diameter indentor to penetrate 0.25 m thick ice sheets. The typical grain size increased with depth through the thickness from 1 mm to 5 mm. The ice sheets were all between -3°C and 0°C during the tests and penetration rates spanned more than an order of magnitude. Standard strength tests which were performed on ice grown in the cold room and on ice sampled from the lake test sheets showed the two ice types were equally strong. Each test sheet was penetrated for a distance equal to 5 - 20 ice sheet thicknesses. In the large-scale tests, the highest forces were generated when the ice failed in a mode termed ductile flaking. In this mode, the ice failed by the formation of one or more in-plane cleavage cracks which continued to open as the indentor advanced. The ice pieces underwent a ductile deformation until they eventually broke off forming flakes. Large flakes were 1/4 to 2/3 as thick as the ice sheet and extended 2-3 ice sheet thicknesses away from the indentor and 3-5 thicknesses along the indentor. During ductile flaking, the effective stress on the indentor was generally a continuous but variable function of the penetration. Its median value was usually continuously exceeded for penetration lengths equal to 1/2 - 1 ice sheet thickness. Four factors influence continuous crushing in the ductile flaking failure mode: the indentor width (penetrating a constant ice thickness), the scale (preserving geometric similarity), temperature of the ice, and effective strain rate of the penetration. In general, the effective stresses on a wide indentor are less variable than those on a narrow indentor penetrating an ice sheet under the same conditions. There were two manifestations of a scale effect. At the larger scale, ductile flaking occurred over a wider range of penetration rates, and for similar conditions, the effective stresses on the larger scale indentors were significantly less than those on the smaller scale indentors. The effects of temperature and strain rate are interrelated. As the penetration rate decreases or the temperature increases, the ductile aspects of the failure process become more important than the brittle aspects. In ductile flaking, this leads to less variability in the effective stresses, and generally an increase in the mean stress.
Year: 1981