Fracture of Metals under repeated Alternations of Stress. 70 



" The Fracture of Metals under repeated Alternations of Stress." 

 By J. A. Ewing, LL.D., F.B.S., Professor of Mechanism and 

 Applied Mechanics in the University of Cambridge, and 

 J. C. W. Humfrey, B.A., St. John's College, Cambridge, 1851 

 Exhibition Besearch Scholar, University College, Liverpool. 

 Beceived August 11, 1902. 



(Abstract.) 



The paper describes an investigation by means of the microscope of 

 the process by which iron becomes " fatigued " and breaks down when 

 subjected to repeated reversals of stress, as in Wohler's experiments. 

 It is shown that although the greatest stress is much within the limit 

 of elasticity (as determined by the proportionality of strain to stress 

 in an ordinary tensile test), it produces rupture after many reversals. 

 The first visible effect is the production of slip-bands here and there 

 on individual crystals. These gradually become more numerous : 

 they also become accentuated and broadened and their edges turn 

 rough and burred, apparently as a result of grinding of one surface on 

 the other over the plane in which slip has occurred. At a later stage 

 certain of the slip-bands develop into cracks, whose existence can be 

 demonstrated by repolishing the specimen, when the slip bands which 

 have not opened into cracks are obliterated, but the cracks remain 

 visible as actual fissures. As the process of reversals goes on, the- 

 cracks spread from crystal to crystal, and fracture ensues. In the 

 particular material dealt with, Swedish iron, having an elastic limit in 

 tension of about 13 tons per square inch and a breaking strength of 

 2 3 -6 tons per square inch, it was found that a stress not exceeding 

 9 tons per square inch, when reversed some millions of times, was 

 sufficient to develop cracks and to bring about the fracture of the 

 piece. Stresses of 8 and even 7 tons per square inch were found to- 

 develop slip bands which would probably turn into cracks under a 

 sufficient number of reversals. The paper is illustrated by micro- 

 photographs taken at various stages of the destructive process. 



