664 TRANSACTIONS OF SECTION O. 



groups. They are consequently different from the normal metal uutil the unstable 

 groups ai'e resolved, and the temporary diflferenco manifests itself on etching, 

 provided that is done while the difference still exists. 



Fiom 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 nothing more or less than slight slip on 

 surfaces where the strain is locally sufficient to exceed the limit of elasticity. An 

 alternating stress, which makes the surfaces slip backwards and forwards many 

 thousands, or it may be millions of times alternately, produces an effect which is 

 seen on the 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 eftect equivalent to an interruption of con- 

 tinuity across part or the whole of a surface of slip, an effect progressive in its 

 character, 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 sub- 

 stance 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 back- 

 ward and forward slips, these do not occur strictly on the same surface in the suc- 

 cessive 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 align- 

 ment 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 surfaces, as a result of slips or a number of parallel planes very 

 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 may 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 iu 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 formation of an altered layer. 

 When 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 layer is supported by the considerations I am now 

 putting forward. We have assumed that in normal crystallisation the inter- 

 molecular forces lead to a normal piling, in which each molecule touches six 

 neighbours. But it may be conjectured that some of them may take up pyramidal 

 piling (touching twelve others) under the compulsion of strong forces — such 

 forces, for example, as act on the superficial molecules of a surface that is being 

 polished. 



If this also occurs at a surface of slip, it gives us a clue to several known facts. 

 It at least assists in explaining the familiar result that metal is hardened by 

 straining in the sense of being made less plastic. Again, it accounts for the 

 general increase of density which is found to take place in such an operation as 

 wire drawing. Further, if a local increase of density occurs in the interior of a 

 grain through piling of some molecules iu the closer manner where repeated slips 

 are going on, the concentration of material at one place requires it to be taken from 

 another ; in other words, the closer piling tends to produce a gap or crack in 



' Ewing and Humfrey, ' The Fracture of Metals under Repeated Alternation^ of 

 Stress,' Phil. Trans., vol. cc. A, 1902. 



- Beilby, ' The Hard and Soft States in Metals,' Phil. Mag., August 1904. 



