ELASTIC LIMIT OF METALS 65 



importance; it takes us a good deal further, how- 

 ever, for close study of this problem has shown 

 very clearly that the value of this "elastic limit" 

 depends very much upon the nature of the metal 

 in question, and even for a given kind of metal 

 such as any particular grade of steel its value 

 depends upon the exact treatment which the 

 metal has undergone. Our knowledge of the 

 crystalline structure of metals here helps us again. 

 Plastic deformation takes the form of slip within 

 the crystals, but the individual crystals are not free 

 to undergo slip or to resist it without reference to 

 their neighbours, and the manner and extent to 

 which a given crystal is interlocked with its neigh- 

 bours and supported by them will determine the 

 load which it can bear before slip commences. 

 An isolated crystal standing by itself receives no 

 support from neighbours; in a piece of metal 

 consisting of large crystals, the amount of support 

 which these can afford one another is small, but 

 in a metal consisting of a very large number of 

 very minute crystals, slip occurs with much greater 

 difficulty, since movement in any one individual 

 crystal can only take place by the aid of a com- 

 plicated system of mutual adjustments involving 

 a very great number of crystals. Observation 

 shows clearly that a metal having a fine structure 

 invariably exhibits a higher elastic limit and a 

 correspondingly greater resistance to fatigue than 

 a metal of the same chemical composition having 



S. S. N C 



