﻿264 Dr. J. A. Ewino- on the 



ft 



nature of plastic strain through experiments on iron and 

 other metals. Its beginning is characteristically jerky. Once 

 the critical force is reached which is enough to start it, there 

 is a big yield, which will not be stopped even by reducing the 

 amount of the straining force. 



Again, we know that there is a slow creeping action that 

 continues after the straining force has done its main work. I 

 ascribe this to the gradual breaking up of the more unstable 

 groups which have been formed during the subsidence of 

 disturbance in the earlier stage of the slip. 



Further, we know that overstrained iron is very imperfectly 

 elastic until it has had a long rest, or until it has been raised 

 for a short time to a temperature such as that of boiling water *. 

 This is to be expected when we recognize the presence of 

 unstable individuals or groups resulting from the overstrain. 

 When the elasticity of the overstrained piece is tested by 

 removing and reapplying the load, some of these tumble into 

 new positions, making irreversible movements, which dis- 

 sipate energy and produce hysteresis in the relation of the 

 strain to the stress although the strain is quasi-elastic. At 

 ordinary temperatures these unstable groups are gradually 

 becoming resolved, no doubt through the action of the mole- 

 cular movements that are associated with heat, and hence the 

 slow progressive recovery of perfect, or nearly perfect, elas- 

 ticity shown by the experiments of Muir. Let the tempe- 

 rature be raised and they disappear much more quickly ; in 

 warm surroundings the rest-cure for elastic fatigue does not 

 need to be nearly so long. 



Hosenhain f has recently shown that after the slip-bands on 

 the surface of an overstrained specimen have been obliterated 

 by polishing, traces of them will reappear on etching if only 

 a short interval of time is allowed to lapse since the over- 

 straining ; but if time is given for complete recovery no 

 traces are found. This is in remarkable agreement with the 

 view now put forward, that the layers contiguous to the surface 

 ot slip contain for a time comparatively unstable groups. They 

 are consequently different irom the normal metal until the 

 unstable groups are resolved, and the temporary difference 

 manifests itself on etching, providing that is done while the 

 difference still exists. 



From the engineer's point of view a much more important 

 matter than this fatigue of elasticity is the fatigue of strength 

 that causes fracture when a straining action is very frequently 

 repeated. Experiments which I made with Mr. Humfrey J 



* J. Muir, " On the Recovery of Iron from Overstrain," Phil. Trans. 

 vol. cxciii.'A, 1900. 



f Journ. Iron and Steel Institute, 1906. 



j Ewing and Humfrey, "The Fracture of Metals under Repeated 

 Alternations of Stress," Phil. Trans, vol. cc. A, 1902. 



