Sponsored by: Jt. SMD/MSD Nuclear Materials and MSD Flow and Fracture Committees and FEMS (Federation of European Materials Societies)
Program Organizers: R.J. Arsenault, Deptartment of Materials Science and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115; David Cole, CRREL, 72 Lyme Rd., Hanover, NH 03755; Todd Gross, Department of Mechanical Engineering, University of New Hampshire, Durham, NH 03824; Gernot Kostorz, Institut für Angewandte Physik, ETH Hönggerberg, CH-8093 Zürich, Switzerland; Peter Liaw, Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200; Sivan Parameswaran, NRC-Institute for Aerospace Research, Ottawa, Canada K1A 0R6; Howard Sizek, Inco Alloys International Inc., Huntington, WV 25705-1771
Tuesday. PM Room:Orange County 3
February 6, 1996 Location: Anaheim Marriott
Session Chairpersons: David M. Cole, US Army Cold Regions Research and Engineering Laboratory, Hanover, NH 03755; Kathryn A. Forland, GMI Engineering and Management Institute, Flint, MI 48504-4898
2:00 pm Invited
VALIDATION OF WEERTMAN'S THEORY OF BASAL MORAINE FORMATION BY BOTTOM FREEZING BENEATH POLAR ICE SHEETS: A.J. Gow, US Army Cold Regions Research and Engineering Laboratory, Hanover, NH 03755
Weertman postulated on theoretical grounds that wet-bottomed ice sheets could, by refreezing of basal meltwater generated by geothermal and frictional heating incorporate debris from the underlying bed. This "freeze-on" of basal moraine was offered as an alternative mechanism to that of "shearing-in" to explain the origin of so-called shear moraines at the margin of the Greenland Ice sheet. A viable test of Weertman's "freeze-on" hypothesis came several years later with the successful drilling to bedrock of the Antarctic Ice Sheet at Byrd Station. Penetration of the ice/rock interface at this location was accompanied by significant up-welling of water into the 2164 meter-deep drill hole. The bottom 4.83 meters of ice core from Byrd Station contained abundant stratified debris testifying to episodic incorporation of bed sediment into the base of the Antarctic Ice Sheet. In this paper I will describe the nature and disposition of this basal moraine, which together with stable isotope and entrapped gas analyses of the enclosing ice are consistent only with a "freeze-on" mechanism as first postulated by Weertman. Formation of debris-laden ice in cores from the bottom of the Greenland Ice Sheet at Camp Century is also attributed to "freeze-on" of bed sediment. "Freeze-on" is the likely predominant mechanism of basal moraine formation in ice sheets at their pressure melting points.
2:30 pm Invited
ON CRYSTAL ORIENTATIONS IN FLOATING ICE SHEETS: W. F. Weeks, Geophysical Institute, University of Alaska Fairbanks 99775-7320. J. Wettlaufer, Applied Physics Laboratory, University of Washington, Seattle, WA 98195
The facts that crystals of glacier ice show pronounced c-axis orientations associated with deformation and that ice crystals forming in the atmosphere show striking morphological changes associated with changes in temperature and vapor pressure are well known. Less well known and less studied are the striking c-axis orientations that are commonly observed in lake and sea ice and presumably also when ice forms on the bottoms of ice shelves; a situation that occurs at a number of locations in the Antarctic. The general process at work here is commonly referred to as geometric selection; it is a process that also occurs during the solidification of a number of other materials; and that in ice can proceed to several different end points. In addition, if the salinity of the water is greater than ~1 0/00, the c-axes are parallel to the current and, if the salinity is less than ~10/00, the c-axes are perpendicular to the current. Finally these apparently contradictory experiments and observations are examined in view of recent developments in crystal growth theory and observations in the Russian literature in an attempt to develop a consistent, overall explanation.
3:00 pm Invited
SCALE EFFECTS IN THE FRACTURE OF ICE: J.P. Dempsey, Department of Civil and Environmental Engineering, Clarkson University, Potsdam, NY 13699-5710
Do the fracture properties at large scale differ significantly form those at small scale? Can laboratory-scale testing be used to predict properties at large scale? What particular problems are faced with large scale testing? How useful are the size-effect theories developed to date? The answers to these questions are key to the applicability of fracture mechanics in the fields of ice engineering, concrete design and rock mechanics. The present focus is to determine the size-scale effect from nonlinear fracture models in terms of the ice microstructure, rate of loading and thermal regime. The required large-scale in-situ arctic experiments posed very challenging logistical and mechanical problems. A very important objective is to evaluate the scale effect over the range 10-1 m (laboratory) to 102 m (full thickness field experiments) employing a combination of on-site and laboratory experiments linked to theoretical fracture and constitutive models as well as theoretical ice property models. Nonlinear fracture models can then be predict the scale effect over the range 10-1m to approximately 103m.
3:30 pm Invited
THE COMPRESSIVE FAILURE OF ICE: E.M. Schulson, Thayer School of Engineering, Dartmouth College, Hanover, NH 03755
Compressive failure limits the forces exerted by floating ice sheets and other ice features on engineered structures and is thus important in design. This paper considers the failure process in terms of its dependence on both environmental (temperature, loading rate, confinement) and microstructural factors (e.g., grain size, shape and texture), and discusses the deformation mechanisms. Particular attention is paid to brittle failure under multiaxial loading and to the transition from ductile to brittle behavior which occurs upon exceeding a critical strain rate. Evidence is presented for crack nucleation via grain boundary sliding, for the frictional crack sliding/wing crack mechanism of crack growth, and for the role of crack-tip dislocation creep in suppressing brittle failure. It is suggested that lessons learned from a study of ice may be relevant to the failure of other materials.
4:00 pm Invited
THE STRUCTURE OF A PLEISTOCENE GLACIATION CYCLE: T. Hughes, Department of Geological Sciences, Institute for Quaternary Studies, University of Maine, Orono, ME 04469
The structure of a typical Pleistocene glaciation cycle, as revealed by proxy data for ice volume, consists of a rather abrupt drop in sea level of about 50 m, followed by an irregular "sawtooth" drop of another 70 m or so over a period of about 80,000 years, and then terminating abruptly in less than 10,000 years. During the last glaciation cycle, the "teeth" of the sawtooth drop averaged about 10,000 years in duration, roughly corresponding to precession hemicycles of Earth's rotation axis. Each "tooth" displayed its own "sawtooth" pattern, with peaks every 1000 to 3000 years. In all cases, changes were abrupt, occurring in a few decades to a few centuries. This structure is interpreted as resulting from Northern Hemisphere ice sheets that formed rapidly when sea ice grounded on Arctic continental shelves, thereby damming rivers flowing into the Arctic. These marine ice sheets advanced onto land from relatively stable ice domes over Hudson Bay, the Gulf of Bothnia, and the Barents, Kara, and East Siberian Seas, lowering sea level some 50 m. Unstable advances and retreats from these domes, caused by thawing and freezing of the bed beneath them, caused "sawtooth" sea-level drops of another 70 m, with overall lowering caused by continuing glacio-isostatic depression under the ice sheets. Termination of the glaciation cycle took place when this depression passed a critical threshold, causing the ice sheets to collapse. Computer ice-sheet reconstructions of a typical glaciation cycle are presented.
CREEP OF A COMPOSITE AND FLOW OF DIRTY ICE IN GLACIER: T. Mori Ryobi Ltd., Toshima, Kita, Tokyo 114, Japan; K. Wakashima, Tokyo Inst. Tech. Nagatsuta, Midori, Yokohama 226, Japan
Using a micromechanics approach, creep of a composite, a material consisting of a plastic matrix and reinforcements (inclusions), is examined. It is shown that if no relaxation process is operative, the creep of a composite eventually stops, contrary to the prediction of a model used by continuum plasticity. This is due to the accumulation of internal stresses. The combination of sliding and mass transport on the interfaces of inclusions plays a role, equal to or more effective than plastic deformation allowed in the inclusions, to relax stresses. If these two relaxation processes operate, the balanced state of stresses is achieved in the matrix and inclusions mass transport and sliding, leading to a stationary creep state.
RHEOLOGY, MELT MIGRATION AND DILATATIONAL ATTENUATION IN EQUILIBRATED PARTIAL MELTS: R.F. Cooper, Department of Materials Science and Engineering and Geology and Geophysics, University of Wisconsin, Madison, WI 53706
The rheology of partial melts is first-order dependent on the (quasi)
equilibrium distribution of the melt phase at the grain scale. The primary
rheologic effect of partial melting is to enhance the rates of diffusional
creep mechanisms far greater than those of dislocation mechanisms: for a given
set of ([[sigma]], T) conditions, change of dominant mechanism and strain rate
increases from 2 to 104 are possible, depending on liquid-solid
interfacial equilibrium. The partially molten aggregate is thus considered as
two interpenetrating fluids, and potentials for relative motion of the solid
and liquid phases accompanies plastic deformation. The melt-migration response
is distinctly anelastic; as a consequence, dilatational attenuation becomes a
significant aspect of the internal friction signature of partial melts.
Illustration of these phenomena will be made with data on silicate assemblages.
Applications of these ideas range from the characterization of rheology of, and
magma motion in, Earth's upper mantle and lower crust, to the forging of
ceramics, to the high-temperature creep of ceramics composites.
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