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LX. Recovery of Nickel and Carbon Steel from Overstrain. 

 By E. L. Hancock, Assistant Professor of Applied 

 Mechanics, Purdue University *. 



[Plate XVI.] 



IT has been known for some time that wrought iron when 

 stressed to its ultimate strength, but not fractured, will, 

 when allowed to rest, show a higher ultimate strength and a 

 higher limit of elasticity. This fact was discovered, inde- 

 pendently, by Captain Beardslee and by Professor Thurston. 

 Later experimenters on the subject of overstrain of iron and 

 steel have confined their attention, for the most part, to the 

 behaviour of these materials after they have been stressed to 

 the yield-point. This yield-point is the elastic limit of the 

 contractor, and is often so designated. The true elastic limit, 

 however, or proportional limit has a value somewhat less 

 than the yield-point. 



Yield-Point Phenomena. 



The yield-point might be defined as that load at which the 

 piece elongates considerably, forming a flat place in the 

 stress-strain diagram. The material under this load seems 

 to be in a somewhat plastic condition, similar to that shown 

 by ductile materials when they have been stressed to the 

 ultimate strength, and rapid deformation takes place. The 

 yield-point has always been a subject of interest to those 

 interested in the elasticity of materials. The great deforma- 

 tion at this point without any increase of load indicates a 

 peculiarity in the molecular condition or arrangement caused 

 by the stress. The desire to, know the effect of loads great 

 enough to stress iron or steel to this yield-point, and the 

 recovery of these materials after release from such loads, 

 brought about the investigations reported in this paper. 



Previous Investigations. 



It has been known for some time that iron or steel stressed 

 to the yield-point in tension or compression exhibits no 

 elasticity when subjected to stress of the opposite kind. 

 That is, a piece overstrained in tension shows no elasticity in 

 compression and vice versa. (In this paper a piece is con- 

 sidered overstrained when the stress has reached the yield- 

 point and the deformation has been allowed to take place.) 



* Read before the Indiana Engineering Society, January 18, 1907. 

 Communicated "by the Author. 



