[ 241 ] 
VI. The Fracture of Metals under Repeated Alternations of Stress. 
By J. A. Ewing, LL.D., F.R.S., Professor of Mechanism and Applied Mechanics 'in 
the University of Canihridge, and J. C. W, Humfrey, B.A., St. John’s CoUeye, 
Cambridge, 1851 Exhibition Research Scholar, University College, Liverpool. 
Received August 11,—Head November 20, 1902. 
[Plates .7-9,] 
It is well known that metals will break down under repeated application, and 
especially under repeated reversal, of stresses greatly less than those that have to he 
applied when the “ ultimate strength ” of the material is tested in the ordinary way. 
The researches of Wohler have shown, for example, that iron capable of bearing 
about 20 tons per sq. inch of steady load will break when it is exposed to some 
millions of reversals of a stress of 8 or 9 tons per sq. inch, alternately in compression 
and extension. When the alternating stress is increased a smaller number of 
reversals suffices to produce rupture. On the other hand, examples such as are 
furnished in the balance-spring of a watch, or in a railway axle, show that very many 
million repetitions may lie applied wdth impunity, provided the limit of greatest stress 
be kept sufficiently low. The mild steel axle of a railway carriage is exposed to 
many million reversals of a stress which, in some cases, approaches as high a value as 
5 tons on the sq. inch, apparently -with perfect impunity, foi- it seems proliable that 
in the rai'e instances where fracture of such axles has occurred an explanation is to lie 
found in the gradual spreading of a crack from an origin supplied by an air-bubble or 
other primitive defect in the material. But Wooler’s researches, wdiich have been 
confirmed by other observers,^ give evidence that a stress not very much greater 
than this, and far below not only the idtimate strengtli but even the “yield-point ” of 
the metal, will produce what is called “fatigue” and bring about fracture when 
reversal of the stress is rejieated many times. 
The jmrpose of this paper is to describe experiments in which the microscope has 
been applied to study the nature of the process of fatigue by which breakdown occurs 
under repeated reversals of stress. The experiments have been made during the past 
year in the Engineering Laboratory at Cambridge. The metal chosen for experiment 
was Swedish iron, of high and very uniform quality. It had the further advantage for 
* Particulars of the researches on this subject of Wohler, Spaxgenberg, Baker, and Bauschixger 
will be found in Professor Unwix’s ‘ Treatise on the Testing of ^laterials of Construction,’ chap. xiii. 
VOL. CC.—A. 326 2 I 30.12.02 
