IRON AND STEEL UNDER CYCLICAL VARIATIONS OF STRESS. 



43 



individual slips in the crystalline grains cannot have increased their extent due to 

 repetitions, The precise action which ultimately produced fracture is difficult to 

 imagine. 



Unequal Stresses. The question here arose as to the best conditions of test. In 

 the previous case of equal and opposite stresses the mean stress was kept constant, 

 but it will be seen that this condition makes the experimental difficulties very great 

 near the maximum tensile stress, where the greatest stress during variation cannot be 

 increased appreciably. It was finally decided, after trial, to keep the maximum 

 tensile stress constant for each experiment and to increase the range by reducing the 

 minimum stress. 



One immediate advantage of a constant maximum stress is that extensions which 

 increase when the range is increased must be ascribed to the range and not to the 

 maximum stress. 



Keeping the maximum stress constant at +18'3 tons per sq. inch, a specimen 

 (fig. 2) was subjected to repetitions of stress. The " cyclical permanent set," fig. 2, 1), 



300 



10.000 20,000 



NUMBER or REPETITIONS or 



30,000 

 STRESS 



Fig. 2. 



10 O IO 



STRESS. TONS pt SQ IN 



and the hysteresis loops, fig. 2, c, were similar to those previously obtained, the 

 greatest width of the loop being at the mean stress. 



The hysteresis loop was not, however, quite closed. The diagram, " permanent 

 extension," fig. 2, a, shows the alteration of length from the original condition, all the 

 extensometer readings having been taken when the stress was +18'3 tons per 

 sq. inch. 



The change in " cyclical permanent set " is thus seen to have been accompanied by 

 a change of length, and when the "set" had become constant the rate of extension 



2 



