ON THE ENDURANCE OF METAL.S. 



433 



The Table VIII. contains a summary of all Bauschinger's experiments 

 on the endurance of a bar subject to repeated stresses. He constructed 

 a machine of the same kind as Wohler's, in -which a bar could be sub- 

 jected to stresses ranging from to an npper fixed limit in tension. He 

 ascertained both the initial elastic limit and the elastic limit acquired 

 under repetition of stress ; the initial breaking strength and the strength 

 after the bar had been broken in the Wohlei' machine. It -will be seen 

 that the elastic limit rises with repetition of stress to a point which is in 

 many cases a little above the load applied. When that is the case the 

 bar suffers a large number of repetitions of load before fracture. If the 

 elastic limit — observed in about a 5 in. length of bar — is very near or 

 below the load applied, the bar breaks with comparatively few repetitions 

 of load. 



Now it has been shown that a parabola, known as Gerber's parabola, 

 can be drawn, so as to fit Wohler's results extremely well. Let the lower 

 stress limit on a bar be denoted p, and let s be the range of stress to 

 which it is subjected, and /its statical breaking strength. Then Gerber's 

 equation is — 



Bauschinger's results enable us to determine the constants in this equation, 

 and Bauschinger has in fact determined the constants for each of the 

 materials on which he experimented. Using these constants, we can de- 

 termine the range of stress a bar -will bear indefinitely repeated for other 

 conditions of loading. The Table IX. below has been thus computed, and 

 it agrees singularly well with the corresponding results obtained by 

 Wohler. It is extremely valuable, because Wohler only determined 

 values of the limiting stresses for three materials, two of them steels of 

 i-ather high tenacity. Bauschinger's results extend Wohler's to materials 

 in more common use. 



For comparison the corresponding results deduced from Wohler's 

 experiments are appended in the following table (Table X.). It will be 

 understood that these stresses are the stresses which would ultimately 

 break a bar, with a sufficiently large number of repetitions of loading. 



Table IX. — Bauschinger' s Endurance Tests. 



(Tons per Square Inch. Stresses requiring a to 10 Million Repetitions to cause 



Fracture.) 



1887. 



