62 MODERN SCIENCE OF METALS 



this is a matter of the highest practical importance, 

 and a remedy can only be found when the reason 

 for this apparent " fatigue" is known. It so happens 

 that the fracture which results from such a case 

 of fatigue often exhibits curiously bright crystalline 

 facets, and it was long supposed that the cause of 

 the failure was due to the metal changing from a 

 supposed normal "fibrous" to a supposedly abnor- 

 mal, weak and brittle "crystalline" condition. 

 But the New Metallurgy has already taught us 

 that the metal is essentially crystalline to begin 

 with, and that its crystalline nature is not incon- 

 sistent with strength and ductility; microscopic 

 examination of pieces which have failed under 

 "fatigue" next serves to show most clearly that 

 their structure has not undergone any change of 

 that kind they are neither more nor less crystal- 

 line than before. The old "explanation," therefore, 

 is entirely disproved; what, then, is the correct 

 explanation ? 



It is really very simple, and arises naturally out 

 of the fact that when metal undergoes plastic 

 deformation as when our bar is even slightly 

 bent under its load this occurs by slipping of 

 layers within some of the component crystals of 

 the metal. If this slip is left undisturbed i.e. if 

 the load is left quietly on, then nothing further 

 occurs. If, however, the load is reversed, a new 

 series of events begins. The reversal of the load 

 tends to reverse the process of slipping by which 



