ELASTIC PEOPERTIES OF MATERIALS 5 



deformation also increases ; or, in other words, the tendency of the 

 body to return to its original form grows less and less until rupture 

 occurs. 



For example, suppose that a rod of steel or wrought iron is 

 stretched by a tensile force applied at its ends. Then if the unit 

 tensile forces acting on the rod are plotted as ordinates and the cor- 

 responding unit elongations of the rod as abscissas, a curve will be 

 obtained, as shown in Fig. 2.* 



Consider the curve for wrought iron obtained in this way. For 

 stresses less than a certain amount, indicated by the ordinate at A in 



55 



8 10 12 14 16 



EXTENSION, PER CENT 



FIG. 2 



18 20 



24 



Fig. 2, the deformation is very slight and is proportional to the stress 

 which produces it, so that this portion OA of the strain diagram is a 

 straight line. For stresses above A the deformation increases more 

 rapidly than the stress which produces it, and consequently the strain 

 diagram becomes curved. When the stress reaches a certain point B 

 the material suddenly yields, the deformation increasing to a marked 

 extent without any increase in the stress. Beyond this point the 

 deformation increases with growing rapidity until rupture is about 



* Drawn from data given in the United States Government Reports on Tests of Metals. 



