STRENGTH OF WROUGHT IRON. 



353 



The elastic limit follows somewhat the same kind of 

 variation, except that the maximum strength is reached 

 earlier, at about 120. 



The ductility of wrought iron, as shown by the ulti- 

 mate extension, decreases until a temperature of nearly 

 300 is attained, when it again begins to increase. 



The modulus of elasticity continually diminishes to a 

 small extent as the temperature increases. Johnson 

 gives this variation as 2 per cent, decrease for every 100 

 increase in temperature. 



FIG. 174. 



FIG. 175. 



184. The Effect on Wrought Iron of Previous Com- 

 pression beyond the Elastic Limit. This point has been 

 investigated recently by Mr. F. Grover, by means of 

 experiments on iron which had previously been compressed 

 during the process of closing tyres on wooden wheels by 

 West's process.* In this process, the tyre, which is slightly 

 larger than the wheel to begin with, is "set" or 

 compressed in place to the proper diameter by a series of 

 radial hydraulic rams pressing uniformly on the outer 

 periphery. The results of tension tests made by Mr. 

 Grover on bars of the uncompressed metal, and also 

 upon bars cut from the compressed tyres, are given in 

 the accompanying table : 



See pamphlet reprinted from "Engineer," September 3rd, 1897. 



Y2 



