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It is known that carbon and some alloy steels, when fractured by a sudden application of load will, under certain conditions, show a fibrous fracture, whereas under other conditions the fracture will be granular and be produced by a considerably smaller shock load. The transition from " fibrous " to " granular " fracture may occur with a temperature drop of the order of 100' F. to o" F. Other contributing factors are change in size, form, composition, and previous mechanical and thermal treatment.
Some of these factors may cause the transition to occur at a higher temperature and some at a lower one. For a structure exposed to atmospheric ranges of temperature, the lower the temperature at which the transition from fibrous to granular fracture of the material occurs, the greater the margin of safety to be expected under shock loads.
Charpy impact tests made on notched-bar specimens of the steels under consideration for the chains, at various temperatures ranging from -40' C. (-40' F.) to 100' C. (212' F.), showed that the transition from fibrous to granular fracture occurred at an appreciably lower temperature in the carbon steel than in the alloy steel.
hIany factors must, of course, be given due consideration in the choice of the steel to be used, but to the extent that the relative shock-resistance of the carbon steel and alloy steel chains can be judged from the results, the tests indicated a greater margin of safety for the carbon steel.
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