102 Adams and Coker — Elastic Constants of Rooks. 



we obtain for the modulus of cubical compression (or bulk 

 modulus) D, the value 21,300,000, a constant for the material, 

 the reciprocal of which gives the decrease in volume of 1 

 cubic inch for 1 pound of pressure. 



While certain rocks, such as many of the marbles, have a 

 structure identical with that of wrought iron, most of the 

 rocks constituting the earth's crust are composed of several 

 minerals, and thus rather resemble cast iron in character, the 

 gray variety of this substance being an aggregate of crystals 

 or individuals of the metal iron (wrought iron), graphite, etc. 



It will therefore be of interest to ascertain how a specimen 

 of cast iron behaves under compression stress, and how far its 

 elasticity falls short of that which would be exhibited by a 

 perfectly elastic body. 



9000 



. 7000 



5000 



3000 



IOOO 



I 





A 



/ 

































4-0 



80 120 



S TRAIN 



160 



200 



Fig. 3. Cast iron stress strain curves. 



For this purpose a 

 cast iron was faced 

 plotted in figure 3. 

 II lateral extension. 



The behavior of cast iron as 

 mental results shows a falling 

 standard of perfect elasticity, but 

 elastic bodies there is probably 

 that we are justified in using the 



fine-grained specimen of somewhat hard 

 and tested. The stress strain curves are 

 I represents longitudinal compression and 



exhibited by these experi- 

 away from the theoretical 

 even in the most perfectly 

 a slight hysteresis effect, so 

 results obtained to calculate 



the modulus of compressibility, if the error introduced thereby 

 is negligible or very small. 



It may be pointed out that this method and others of the 

 indirect type have been freely used to obtain values of the 

 bulk modulus for cast iron and metals of like character, and 

 it will be shown that the composite crystalline rocks are very 

 similar to cast iron in their behavior under stress, although 

 generally more perfectly elastic. 



