IKON AND STEEL 223 



more slowly than deformation, until finally rupture is about to 

 occur, at which point the load attains its maximum value, called 

 the ultimate load. If the stress be continued, the piece begins to 

 neck and breaks at a load somewhat less than the maximum (see 

 Article 7). This necking is due to the fact that the metal under 

 great strain becomes plastic and flows. Brittle materials, such as 

 cast iron and hard steel, show very little, if any, necking. In com- 

 puting the fiber stress at the maximum load the original cross section 

 is used. 



In commercial tests the load at the yield point (commercial elastic 

 limit) and the maximum load are noted; also the percentage of 

 elongation and the percentage of reduction of cross section. The per- 

 centage of elongation is the increase in length divided by the original 

 length multiplied by 100. This percentage varies with the original 

 length taken (see Article 20), and therefore is usually computed for an 

 original length of eight inches. The percentage of reduction of cross 

 section is the decrease in area of the cross section divided by the 

 original area of the cross section multiplied by 100. In some com- 

 mercial laboratories provision is made for making as many as sixty 

 tests per hour on one machine. 



172. Effect of overstrain on wrought iron and mild steel. If 

 wrought iron and mild steel are strained just beyond the elastic limit 

 in tension or compression, then released and tested again in the same 

 direction, it has been found that this second test shows that the 

 elastic limit is higher than at first, and almost as high as the load 

 in the first test. Eepeated overstrain of this kind, with subsequent 

 annealing, makes it possible to raise the elastic limit considerably 

 above what it was originally. When further strained the metal 

 loses its elasticity and takes on a permanent set ; that is to say, it 

 does not return to its original length when the stress is removed. 

 The elastic properties, however, can be restored by annealing (see 

 Article 18). Overstrain in either tension or compression destroys 

 almost entirely the elasticity of the material for strain of the opposite 

 kind ; for instance, a piece of mild steel overstrained in tension has its 

 elastic properties in compression almost entirely destroyed, and vice 

 versa. Overstraining in torsion produces much the same effect as 

 overstraining in tension or compression. 



