TENSION TESTING. 143 



The following are the details of the calculations in- 

 volved. The calculation of the areas of cross-section need 

 not be detailed here. 



82. Stress at Elastic Limit. The commercial elastic 

 limit, or, as it is also called, the "yield point," is estimated 

 from an inspection of the above figures, or better still, 

 from the plotted diagram of the test. The load at the 

 limit as here taken is that load which has just succeeded in 

 producing an extension emphatically greater than the 

 uniform elastic extensions which have been taking place 

 regularly, or nearly so, up to this point. The increments 

 of extension for equal increments of load near the limit 

 were as follow : 



From lO'O to 10'5 tons, increment is O'OOOo in. 

 10-5 to 11-0 0-0005 in. 



11-0 to 11-5 ;, 0-0005 in. 



11-5 to 12-0 0-0006 in. 



12-0 to 12-5 ., 0-0009 in. 



12-5 to 13-0 0-0370 in. 



At 12'0 tons the stretch begins to increase from 5 ten- 

 thousandths of an inch to 6, and at 12'5 tons to 9, and 

 then when 13'0 tons is reached there has been a sudden 

 and vastly increased extension to 370 ten-thousandths. 



* 10"- - -* 



i 



| 15-06 



FIG. (5(5. 



Between these last two loads the yield-point must be, and 

 it is here taken as the load where the yield has really taken 

 place. The true limit of elasticity is nearer 12'0 than 13'0 

 tons. It being then determined that the elastic limit has 

 been reached at a load of 13 tons, the stress corresponding 

 to this is : 



