The numerous available observations are deficient due to the fact that in the majority of cases 

 it is not knovra under what conditions these determinations had been made. And, furthermore, they 

 are extremely contradictory. What I have said pertains to sea ice in particular. 



The method of obtaining ice samples which are to undergo future investigation is a great 

 source of error in determining the mechanical properties of natural ice. 



Ordinarily, a chunk is broken off from a large ice floe and this chunk is raised on to the deck 

 of the ship, or is sent to the laboratory. Sometimes the temperature is extremely different from its 

 temperature at the moment it was taken, the chunk is broken up into smaller pieces from which 

 blocks approximately 20 to 50 cm^ are cut out. It is clear that during all these operations the 

 physical properties of ice change considerably: the brine drains out of the cells, the temperature 

 of the block changes (which causes changes in the structure of the ice) , the faces of the block melt 

 during the sawing, etc. 



Another shortcoming in the existing determinations of the mechanical properties of ice is the 

 fact that all of them unintentionally are made on comparatively small samples of ice, which by its 

 very nature, is extremely nonhomogeneous. 



Ice, like any hard body, in relation to the external forces acting upon it, can be elastic, 

 plastic, and frangible. Changes in the form of a solid body caused by small external forces can 

 disappear when these forces stop actii^. Such changes are called eleastic deformations; a solid 

 body in such a case is in an elastic stage. With an increase in the external force above a certain 

 amount (determined for each solid body) called the limit of elasticity — the changes in the body no 

 longer disappear when the action of the force stops. An imprint is left in the body, so to speak, of 

 the action exerted on it — a deformation remains. If the residual deformation is destroyed by an 

 opposite action during the same time Interval, the body is in a plastic state. If the applied force 

 destroys the body, this body is in a frangible state. 



Following Veinberg, the amountof force at which a body ceases to be plastic and is destroyed, 

 i.e. , changes into the frar^ible stage, we will call the limit of plasticity. Ordinarily, this force is 

 called destructive force or breaking point. 



Experiments have shown that separate crystals are plastic only on the plane perpendicular to 

 the main axis. In other words, a crystal of ice behaves as if it consisted of a number of plates 

 piled upon each other perpendicular to the axis and moving quite easily in relation to each other 

 under the influence of an external force. At the same time, if the force is directed along the main 

 axis, the ice crystal approaches in its properties an absolutely frangible body which breaks along 

 with its deformation. As we shall see later, the elastic limit of ice, and particularly of sea ice, is 

 extremely small, and because of this, we can consider natural ice as an extremely plastic body at 

 high temperatures (near the freezing temperature) , and at low temperatures as an extremely fran- 

 gible body, which can be confirmed by the simplest observations. Inasmuch as the temperature of 

 natural ice rises very shairply during the winter along a direction from its upper surface to its 

 lower surface, the upper layers of the ice are frangible, the lower — plastic. Sea ice is consider- 

 ably more plastic than river ice. Thus, sludge, brash ice, young ice, and generally nilas ice are 

 distinguished by very high plasticity; scum* which is the characteristic initial form of fresh ice is 

 very frangible. All these facts limit the application of the formulas and conclusions of the theory 

 of the resistance of substances (based on the theory of elasticity) to natural ice. 



LITERATURE: 25, 62, 77. 



*SKlianka in Russian - Translator. 



184 



