300 



SIMILAR BARS. 



retains its parallel shape, the reduction in area reckoned 

 on the original area must be equal to the percentage 

 extension reckoned on the final length (see p. 40). In the 

 great majority of tensile tests of ductile materials 

 this relation does not hold after the maximum load owing 

 to the local extension, though, if measurements be 

 taken of the bar at a point previous to the maximum 

 load, it will generally be found that the conditions are 

 satisfied. 



In the case also of some classes of steel there is little 

 or no local extension and the relation is again found to 

 hold. As an example of this the following, given by 

 Mr. Hadfield in his paper on manganese steel,* may be 

 quoted. 



The test results given on the report for one bar 

 were : 



From these results the final length of the specimen 

 can be determined, and from it the extension, calculated 

 on the final length. 



The final length of the specimen was 



So that 



and, 



o 



8 inches + 46'8. - inches. 

 = 8 + 3-74 = 11-74 inches. 



3-74 



L + I 11-74 



= '31'9 per cent. 



A a 



= 32'3 per cent. 



Though these two figures, representing respectively the 

 extension reckoned on the final length and the reduction 

 in area, are not quite identical, they only differ by about 

 one per cent., which is easily accounted for by want of 



* Mm. Proc. Inst. C.E., Vol. xciii. 



