ON STRESS DISTRIBUTIONS IN ENGINEERING MATERIALS. 



167 



numbers of cycles endured. It is clear that each series of experiments 

 indicates a limiting range of stress below which the endurance increases 

 with very great rapidity. It may be doubted whether even the longest 

 tests indicate an absolute fatigue limit, but the asymptotes are at least 

 sufficiently clear to indicate a fairly definite value. These values are 

 summarised for the several series in the following table : 



In Fig. 4, curve F, the limiting range of stress is plotted as a 

 function of the steady stress applied to the specimen. It is interesting 

 to note that, especially for the tension side of the diagram, the form of 



10 



Compression < — 



Tfns/on 



Fia. 4. 



the curve is approximately in accordance with the hypothesis expressed 

 by ' Gerber's parabola.' ' Thus the results of the tests with combined 

 alternating and steady stresses may be expressed by the formula 



fm«=A/2 + v/P^df or 



f^, =f2-7tAf 



' See Unwin's Testing of Materials of Construction, p. 388, 3rd edition. 



