August 23, 1906J 



NA TURE 



421 



At first, when the displacement by sliding is exceedingly 

 small, the strain is a purely elastic one. The molecules 

 adjacent to the piano of sliding pull one another round a 

 little, but without breaking bonds, and if in this stage the 

 strain is removed, by letting the plate slide back to its 

 original position, there is no dissipation of energy. The 

 work done in displacing the molecules is recovered in the 

 return movement. We have here a representation of what 

 happens between each pair of adjoining rows in the elastic 

 straining of a metal. -So far the action is within the limit 

 of elasticity ; it leaves no permanent effect : it is completely 

 reversible. 



But now let the process of straining be carried further. 

 The opposing molecules try to preserve their rows intact, 

 but a stage is reached when their resistance is overcome ; 

 the bonds arc broken, 'and they swing back, unable to 

 exert further opposition to the slip. The limit of elasticity 

 has now been passed. Energy is dissipated ; set has been 

 produced ; the action is now no longer reversible. The 

 model shows well the general disturbance that is set up in 

 molecules adjoining the plane of slip, which we may take 

 to account for the work that is expended in a metal in 

 producing plastic strain. 



Moreover, when the slip on any plane stops and the 

 molecules settle down again, the chances are much against 

 their all taking up the normal orientation which they had 

 before the disturbance. What I have called dissenting 

 groups or unstable coteries are formed as a result of the 

 disturbance. Here and there like poles are found in juxta- 

 position. Viewed as a whole, the molecular constitution 

 of the metal in the region adjacent to the plane of slip 



f^^^^^^ 



is now uncertain and palchy. It includes parts the stability 

 of which is much less than normal. Individual molecules 

 or small groups in it are very feebly stable ; a touch would 

 make them tumble into positions of greater stability. 



Observe how all this agrees with what we know about 

 the nature of plastic strain through experiments on iron 

 or 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 sub- 

 sidence 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 

 recognise 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 

 inversible movements, which dissipate energy and produce 

 hysteresis in the relation of the strain to the stress although 

 the strain is quasi-elastic. .At the ordinary temperature these 

 unstable groups are gradually becoming resolved, no doubt 



1 J. M.iir, "On the Recovery of Iron from Overstrain," Pkil. Trans., 

 vol. cxciii. A, lyoo. 



NO. 192 1, VOL. 74] 



through the action of the molecular movements that are 

 associated with heat, and hence the slow progressive re- 

 covery of perfect, or nearly perfect, elasticity shown by 

 the experiments of Muir. Let the temperature be raised 

 and they disappear much more quickly ; in warm surround- 

 ings the rest-cure for elastic fatigue does not need to be 

 nearly so long. 



Rosenhain ' has recently shown that after the slip-bands 

 on the surface of an overstrained specimen have been 

 obliterated by polishing, tr.aces of them will reappear on 

 etching if only a short interval of time is allowed to lapse 

 since the overstraining ; but if time is given for complete 

 recovery no traces are found. This is in remarkable 

 .igreement with the view now put forward, that the layers 

 contiguous to the surface of slip contain for a time com- 

 paratively unstable groups. They are consequently different 

 from the normal metal until the unstable groups are 

 resolved, and the temporary difference manifests itself on 

 etching, provided 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 ^ showed that this action begins with nothirig 

 more or less than slight slip on surfaces where the strain 

 is locally sufficient to exceed the limit of elasticity. \n 

 alternating stress, which makes the surfaces slip back- 

 wards and forwards many thousands, or it may be millions 

 of times alternately, produces an effect which is seen on 

 ihe polished surface as a development of the slip lines into 

 actual cracks, and this soon leads to rupture. 



We have, therefore, to look for an effect equivalent to 

 ,in interruption of continuity across part or the whole of 

 a surface of slip, an effect progressive in its char.acter, 

 becoming important after a few rubbings to and fro if the 

 movement is violent, but only after very many rubbings 

 if the movement is slight. 



That there is a progressive action which spreads more 

 or less into the substance of the grain on each side of the 

 original surface of slip was clearly seen in the experiments 

 referred to. It was found that a slip-band visible on the 

 polished surface of the piece broadened out from a sharply 

 defined line into a comparatively wide band with hazy 

 edges, and this was traced to an .actual heaping up of 

 material on each side of the step which constituted the 

 original line. 



I think this suggests that under alternating stresses 

 which cause repeated backward and forward slips, these 

 do not occur strictly on the same surface in the successive 

 repetitions, and hence the disturbance spreads to some 

 extent laterally. It may be conjectured that slip on any 

 surface leaves a more or less defective alignment of the 

 molecular centres ; that is to say, the rows on one side 

 of the plane of slip cease to lie strictly in line with those 

 on the other side. If this occurs over neighbouring sur- 

 faces, as a result of slips or a number of parallel planes 

 verv close together, the metal throughout the affected 

 region loses its strictly crystalline character, and with it 

 loses the cohesion which is due to strict alignment. 



Mr. G. T. Beilby, in a very suggestive paper,' has 

 advanced grounds for believing that portions of a metal 

 mav pass from a crystalline to an amorphous formation 

 under the mechanical influence of severe strain, as in the 

 hammering of gold leaf or the drawing of wire, and that 

 this occurs in the polishing of a metallic surface, and also 

 in the internal rubbing which takes place at a surface of 

 slip within the grain. In both cases he suggests the form- 

 ation of an altered layer. W'hen a polished metal surface 

 is etched, the altered Layer is dissolved away, and the 

 normal structure below it is revealed. 



Without accepting all Mr. Beilby's conclusions, I think 

 the idea of an altered and more or less amorphous laver 

 is supported bv the considerations I am now putting for- 

 ward. We have assumed that in normal crystallisation 

 the intermolecular forces lead to a normal piling, in which 



1 fonrn. iron and SUel Institute. rgoO. 



- Ewine anH Humf'ry. "The Fracture of Metals under Repeated 

 Aliernationsof Stre«," Pliil. Trrtns., vol. re. A, igo!. 



3 Beilby, "The H.-rd and Soft States ini Melals," /"/"V. jl/«j-., August, 



