182 ILLINOIS ACADEMY OF SCIENCE 



FATIGUE OF METALS UNDER REPEATED STRESS 

 H. F. Moore, University of Illinois 



The failure of metal under repeated stress is a familiar phe- 

 nomenon. Illustrations of such failure are furnished by the 

 bending of a wire back and forth until it breaks ; by the fail- 

 ure which takes place in railroad rails after a large number 

 of trains have passed over them; and by the failure of boiler 

 plates between riveted holes after the boiler has heated and 

 cooled many times. The failure of metal under repeated 

 stress is called failure by "fatigue." The old theory of such 

 failures was that under repeated stress metal "crystalized" 

 and became brittle, finally snapping between crystals. This 

 belief lead many engineers to consider wrought iron to be su- 

 perior to steel under repeated stresses, because wrought iron 

 seemed fibrous in its structure while steel was crystaline. 



The use of the microscope in studying metals has very gen- 

 erally discredited the "crystallization" theory. Under the 

 microscope, the structure of all metals is seen to be crystaline, 

 and no marked change in size of crystals can be detected in 

 metal which has failed under repeated stress. The appear- 

 ance of the fracture of metal to the naked eye is not a reliable 

 indication of the structure of these metals. After a piece of 

 soft steel is broken by a gradually applied tension the fracture 

 will appear silky, not crystalline. If a piece of the same soft 

 steel is nicked and bent it will break in two at the nick and the 

 fracture will appear crystalline. If a piece of the same soft 

 steel bar is bent back and forth a great many times it will 

 finally snap in two with very little warning and the fracture 

 will appear crystalline. The appearance of the fracture is de- 

 pendent not only on the nature of the metal but upon the shape 

 of the piece broken and the manner of applying load. 



Examination under the microscope gives some idea of what 

 happens when metal fails by fatigue. Figure la shows the 

 appearance under the microscope of an unstressed piece of 

 Norway iron. It is made up of crystals of pure iron and 

 fibres of slag. Figure lb shows the appearance of the same 

 piece of iron after it has been subjected to several hundred rep- 

 etitions of stress. Right across crystals appear fine lines ; these 

 are known as "slip lines" and indicate the splitting up of the 



