REPEATED STRESSES. 



323 



tensile stress on one side and an equal compressive stress 

 on the other. 



The general conclusion arrived at from the results of 

 these tests was that the fracturing of the bars depended, 

 in the first place, on the range of stress ; and, secondly, on 

 the number of times the stresses were repeated. When 

 a certain minimum stress was reached, it was found that 

 the number of repetitions required to produce fracture 

 was practically infinite. 



The following figures, taken from Wohler's results, will 

 serve to give an idea of the kind obtained. They com- 

 prise the results of tests of eight bars of Krupp cast steel 

 for axles, under repeated bending stresses by rotation. 



Wohler's Tests on Repeated Stresses in Bending 

 by Means of Rotation. 



Wohler himself summarised the results of all his tests 

 in the following law : 



Wohler's Law : Rupture of material may be caused 

 by repeated vibrations, none of which attains the absolute 

 breaking limit. The differences of the limiting strains 

 are sufficient for the rupture of the material. 



As a further result of his tests, Wohler gave the 

 following values for the stresses on steel and wrought 

 iron, which may be imposed for an unlimited number of 

 repetitions without any damage being sustained by the 

 material : 



