6 Foster, Phenomena due to Repetitions of Stress. 



by the results of repeated torsions can hardly be due to 

 the difference in the ranges of stress. 



Table I. 



Materials. 



Simple 



Bending. 



Bending : Rotating Bars. 



Stress Limits. 



Repetitions. 



Stress Limits. 



Repetitions 



Iron for axles... 

 Phoenix Co...... 



O, ( + )l5-28 

 o>( + )i4-3 



[ 3,400,000] 

 [48,000,000] 



( + )i5-3>(-)i5-3 

 { + )i4-3. (-)i4-3 



56,430 

 99,000 



Homogeneous 

 Iron 



o, ( + )i9-i 



[34,500,000] 



( + )i8-2,(-)i8-2 



31,586 



[ ] signifies not broken. 



Stresses are in tons per square inch. 



It may be thought that to effect a true comparison 

 equal ranges of stress should be chosen. That this is not 

 so is perhaps best shown by an illustrative example. 

 Suppose the total elastic strain either (not total of both) 

 in tension or compression to be --^ inch, and the extension 

 or compression due to a load of 8 tons is 2^^ inch. 

 Then for a stress range of either o to 8 tons or + 8 to — 8 

 tons the amount of lag or hysteresis required to give the 

 bar a permanent extension is abont ^\^ inch. If how- 

 ever the unidirectional range of stress is made equal to 

 that for the alternating stresses, that is 16 tons, the former 

 will only have -^\-^ inch or y^ ^^^^ o^ "^^ alternating 

 stresses to make up in lag and it is practically certain 

 that the difficulty of making up this lag increases very 

 rapidly as its amount increases, for as pointed out earlier it 

 is very unlikely — and experiment bears this out — that the 

 lag of, let us say, loading cycle number 10,000 will be any- 



