DUCTILITY 



Ductility is one of the most important properties of steel required in struc- 

 tural designing. There are two ways of measuring the ductility of steel in com- 

 mon commercial use; (a) the percentage of elongation, (b) the percentage of re- 

 duction of area of cross-section. 



The percentage of elongation is found by dividing the increase of length 

 after rupture has occurred by the original length. The elongation of a test speci- 

 men may be divided into two portions; (a) that part of the elongation which is 

 uniformly distributed over the length; (b) that part of the elongation which oc- 

 curs in the close vicinity of the section which finally breaks. The accompanying 

 sketch illustrates the "necking-down" action which occurs before rupture. The 

 elongation is measured after rupture has occurred by placing the two ends togeth- 

 er and measuring the distance between the original gauge marks. 



It will be noted after a cursory inspection of these specimens that the elon- 

 gation which is locally developed in the vicinity of final rupture, is not the same 

 in all specimens but varies greatly with the diameter or thickness of the test speci- 

 men. It requires very little study to see that a piece of steel one inch in diameter 

 will elongate much more in two inches of length adjoining the plane of rupture 

 than a piece one quarter inch in diameter. In the former case almost all of the 

 two inches represents the length of the "necking-down" portion, while in the latter 

 case only a small part of the two inches represents this "necking-down" portion. 

 The percentage of elongation in the two inches is much greater in the former 

 case than in the latter; although the ductility of the latter steel may be the 

 greater. This principle holds good for all commercial lengths of test specimens 

 which usually run from two to eight inches. 



167 



