Sea Ice
Mary Williams
measuring the flex-
ural strength of ice
beams.
When ice fails against the ship hull, the relationship between the average
ice pressure and the loaded area depends, in part, on the maximum pressure
sustained over actual contact areas, or hot spots. In materials that deform
plastically, this pressure, called the hardness, may be considered a material
property. Hardness was measured using a 136 pyramid Vickers Indenter.
Hardness decreased as contact area increased, from maxima near 60 MPa over
105 m2 to values near 15 MPa over 104 m2. The effects of temperature and
of work done on the ice are being investigated.
Since ice thickness varies with date and location, a ship-based measure-
ment is not representative of the entire Arctic. However, such a measurement
is appropriate in an analysis of ship performance and hull loads. A video cam-
era aimed at the ice surface near the starboard shoulder of the ship ran contin-
uously throughout the trip. A computer-generated scale, calibrated at the ice
surface, overlaid the image on the video record and measured the thickness
and length of ice pieces turned up by the ship. Statistical results will not be
available until these data have been carefully analyzed.
To determine the ice crystal structure, rough thin sections of ice from some
of the cores and all of the beams were prepared in the coldroom and photo-
graphed through crossed polaroid filters. Most of the photos show highly
metamorphosed, unoriented, largely granular ice in a range of grain sizes. One
surprise was an ice sample with zero salinity and very large grains (>8 cm in
diameter) inadvertently taken from a concealed, refrozen melt pond.
Snow reduces ship performance through the inertial, buoyant and friction
forces it exerts on a moving ship. The water content and grain size of snow
affect imagery collected by satellite-borne microwave sensors by changing the
apparent brightness temperature of the ice surface. Thus, snow density and
type are key links between remotely sensed ice imagery and actual ship perfor-
mance. Global snow density was measured in small full-thickness samples,
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