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 CHAPTER lY 



PHYSICAL AND CHEMICAL PROPERTIES OF SEA ICE 



STRENGTH AND HARDNESS 



The mechanical properties of sea ice can be expressed only in very 

 general terms, since they vary greatly with temperature, air content, 

 and salt content of the ice, and depend to some extent on its previous 

 history. Rough values for river ice near the freezing point can 

 be taken as 150 p. s. i. for tensile strength, 500 p. s. i. for compressive 

 strength, 100 p. s. i. for shear strength, and perhaps 50 p- s. i. for 

 torsion. These values increase as the temperature decreases. It is 

 stated that the compressive strength increases fourfold when the 

 temperature is reduced from 23° F. to —76° F. Working values for 

 ordinary sea ice are about one-third those just given for river ice. 

 However, the increase of strength with temperature may be largely 

 nullified by the strains set up by unequal cooling since, no matter 

 how cold the upper surface of a floe may be, if afloat it is in contact 

 with sea water at around 28° F. 



The hardness of ice likewise increases with decreasing temperature. 

 At 32° F. ice has a hardness of 2 on Moh's scale; this increases to 

 4 at -50° F. and 6 at about -80° F. The hardness of mild steel 

 ship plate is about 5i/^ and of glass about 6 on the same scale. 



Newly formed sea ice is weak and plastic in consistency, and does 

 not acquire its strength and characteristic brittle nature until it has 

 been cooled below 16° F. 



THERMAL PROPERTIES 



The freezing point of sea water and the temperature of maximum 

 density are shown in figure 1. The specific heat of pure ice is only 

 half that of water; but the addition of a little salt, as in sea ice, 

 greatly modifies the heat content. Since newly formed sea ice will 

 consist of a matrix of crystals of nearly pure ice surrounding cells 

 of brine, further cooling results in freezing more water out of the 

 brine. The apparent specific heat of the mixture will be made up 

 of the specific heat of the existing ice plus the latent heat of fusion 

 of the water newly frozen. This effect is illustrated by the following 

 table. 



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