2 Foster, Phenomena due to Repetitions of Stress. 



Professor Ewing and Mr. Humfrey have shown that 

 when a metal is subjected to an increasing stress in the 

 ordinary manner there is no visible change in the crystal- 

 line grains of which the metal is composed until the 

 elastic limit is passed and the extension has consequently 

 become permanent. At this stage nearly straight lines 

 (called by the experimenters, slip bands) appear on the 

 surface of the grains, due to a slip or shear having taken 

 place in the crystal along what is very nearly, but not 

 quite, a plane surface. It is this slipping which accounts 

 for the permanent extension of a metal stressed beyond 

 its elastic limit. 



In the account of his experiments on repeated 

 stresses* Professor Ewing showed that when a metal 

 was subjected to repetitions of stress less than the elastic 

 limit of the material, after a time these lines or slip bands 

 appeared on the crystals showing that in some way the 

 metal has passed into the condition of a metal subjected 

 to a statical stress greater than the elastic limit of the 

 material ; evidently as the repetitions continued the elastic 

 extension became greater, each repetition adding some- 

 thing to the elastic extension until this extension was 

 equal to the maximum elastic extension of the bar, after 

 which any further repetitions of the stress would produce 

 a permanent extension. According to the theory about 

 to be sketched this increase in the extension is due to 

 hysteresis in the metal. As the repetitions continued 

 more of these slip bands appeared, showing more distor- 

 tion of the metal until probably the distortion would have 

 been sufficient to produce fracture, but this was prevented 

 by a development which had not been foreseen in writing 

 the paper for the Owens College Engineering Society, 



* "Fracture of metals under repeated alternations of Stress." By J. 

 A. Ewing and J. C. W. Humfrey. Tratis. Roy. Soc. Vol. 200, Series A. 



