TENSION TESTING. 



145 



86. Reduction of Area per Cent. After the test the 

 diameter of the bar at the point of fracture is measured, 

 and the corresponding area of the cross-section at this 

 point calculated. Then the required percentage reduc- 

 tion is : 



Reduction of Area per Cent 



original area final area 



original area 

 0-788 - 0-341 



x 100 = 



x 100 

 447 



0-788 0-788 



= 56'5 per cent. 



In a previous chapter* it has been shown that " so long 

 as a bar remains parallel, and the volume is constant, the 

 percentage of reduction of area, calculated on the original 

 area, is always less than the percentage of elongation 

 reckoned on the original length." 



But this is not quite a case in point, because there is 

 considerable local contraction, and the portion in question 

 of the bar has not remained parallel, and, moreover, the 

 local contraction is probably accompanied by a flow of metal 

 into the 2-in. portion, causing the volume not to be constant. 



87. The Modulus of Elasticity. -The first step in this 

 calculation is the choosing of that set of elastic readings 

 which is the most regular. In the two sets in question, if 

 each reading be subtracted from that following it, the 

 increments of length from \ ton increase of load will be 

 obtained. These are : 



L2 



p. 40. 



