8 Foster, Phenomena due to Repetitions of Stress. 





< 

 3 



/ 



— 



/ / 



^ EXTENSION 



^ 



^ 



+ 



Fig. 4. 

 maximum to a negative maximum is continuous, and 

 during such reversal the lag of the extension is con- 

 tinuously increasing, so that except just at the commence- 

 ment of the loadings, which will in practice be applied in 

 an indeterminate manner, the procedure is similar to that 

 of a unidirectional stress and failure might result — apart 

 from the formation of cracks — either by pulling out or 

 crushing depending on the initial loadings. This is borne 

 out by those specimens which have not failed by the 

 formation of cracks. 



Our theory leads us to several interesting conclusions. 

 For instance, referring to Fig. i, it is obvious that the 

 greater the magnitude of the stress range the greater will 

 be the lag per cycle and the less the total lag required, 

 and hence the number of repetitions required to produce 

 fracture will increase at an increasing rate as the stress 

 range is decreased. 



