Art. 26 



SHIFTING OF THE ELASTIC LIMIT. 



29 



little 1 . The period of rest allows the metal to harden, and this 

 hardening will be greater if, in place of relieving the stress, it is 

 held stationary. It is thus possible, by repeated applications of 

 increased loads, to raise the yield point nearly up to the point of 

 rupture, at the same time raising the ultimate strength and de- 

 creasing the ductility so that the fracture of a piece of soft steel 

 will resemble that of hard steel. 



If the stress exceeds the elastic limit but not the yield point, 

 the elastic limit is immediately raised, and by a repetition of 

 such stress, the yield point is also raised. 



It has been found that iP there has been overstrain in ten- 

 sion, the elastic limit in compression is lowered and vice versa. 

 If this lowered elastic limit is raised again by a stress producing 

 a small permanent set, the elastic limit for the opposite kind of 

 stress falls to zero 2 . Ordinary periods of rest do not seem to 

 have any effect in this case, but gradually increasing stresses, 

 alternating between tension and compression, establish new 

 elastic limits considerably lower than the original elastic limits. 

 These have been called natural elastic limits, the original elastic 

 limits being supposed to be artificially raised by the rolling of 

 the metal. 



A very interesting method of restoring elasticity and raising 

 the elastic limit is given in Ewing's Strength of Materials. 

 ' ' When a piece of iron or steel has had its elasticity broken down 

 by overstraining, it will make a very complete recovery if heated 

 for a few minutes to a temperature such as that of boiling water. 

 "When the overstrained piece has been immersed in a bath of 

 boiling water it is found to have practically perfect elasticity 

 up to a new yield point, which is higher than the load used in 

 the process of overstraining." 



"A remarkable experiment may be made by taking a bar of 

 mild steel and stretching it in the first instance jnst up to the 

 primitive yield point, then heating it for a few minutes up to 

 100 C to produce elastic recovery, then stretching it again just 

 up to its new yield point, then heating again to 100 C and so on. 

 Each step raises the elastic limit, and notwithstanding its natur- 



*See Burr's Ela*tic*fy and Resistance of Ihe Materials of Engi- 

 neering, pp. 247-250, for results of experiments made by Professor Bausoh- 

 inger. For a statement of Bauschinger's conclusions, see Greene's Sirac- 

 tural Mechanics, p. 166. 



2 For str^P-defnrmation diagrams, showing these remarkable phe- 

 nomena, see Johnson's Materials of Construction, Figs. 436 and 437. 



