MECHANICAL PRINCIPLES. 103 



same, while for shear stress, it is generally assumed at one- 

 third less. It is further generally assumed that the stress 

 under tension and compression when the elastic limit is reached 

 is about six-tenths of the ultimate tenacity. 



According to William Kent, A. M. M. E., one of the most 

 recognized authorities on mechanical questions, the following 

 are the 



MODULI OF ELASTICITY FOR IRON AND STEEL. 



Cast iron 12,000,000 to 27,000,000 (?) 



Wrought iron... .22,000,000 to 29,000,000 



Steel 26,000,000 to 32,000,000. 



Quoting from " Kent's Pocket Book " : " The maximum 

 figures given by many writers for iron and steel, viz., 

 40,000,000 and 42,000,000, are undoubtedly erroneous. . . . 

 The modulus of elasticity of steel (within the elastic limit) is 

 remarkably constant, notwithstanding great variations in 

 chemical analysis, temper, etc. It rarely is found below 

 28,000,000 or above 31,000,000. It is generally taken at 

 30,000,000 in engineering calculations." 



The values given above are generally approximated as 

 follows : 



Cast iron 15,000,000 pounds per square inch 



Wrought iron. .25, 000,000 " " " " 



Steel 30,000,000 " " " " 



When under tension or compression steel will stretch or shorten 



I 



30,000,000 



part of its normal length for every pound per sectional inch 

 in change of load. 



The tendency of columns or struts under load is to fail by 

 both compression and flexure, or bending, the column yield- 



