IRON AND STEEL UNDER CYCLICAL VARIATIONS OF STRESS. 



45 



In the case of complete recovery an extension of about - 2 mm. would have been 

 produced in the process of raising the superior elastic limit from 1 3 tons per sq. inch to 

 23 tons per sq. inch. In the actual case the range is just too great and the specimen 

 is incapable of completely adjusting itself to an elastic state. 



An increase in the range, by making the stresses 2 '2 tons per sq. inch and 

 + 23'2 tons per sq. inch, immediately put up the "cyclical permanent set" and 

 increased the rate of extension. A further increase in the range produced further 

 increases in both quantities. 



The hysteresis loop and the " cyclical permanent set " again agreed with those of 

 figs. 1 and 2, the loop being symmetrical about the mean load. 



The numbers plotted under " cyclical permanent set," fig. 3, b, are now greater than 

 the width of the hysteresis loop. Obviously, the increased length of the specimen, 

 and consequent decreased diameter, will affect the elastic extension so as to make it 

 greater than that calculated from the original specimen. When correction is made 

 for this, the lines BC and DA become parallel to the elastic line, within the probable 

 errors of observation, and, although scarcely any compressive stress occurs, the 

 hysteresis loop could not easily be distinguished from a similar loop produced by equal 

 stresses. 



-.3000 



I 



1000 

 100 



o 10,000 20,000 



NUMBER or REPETITIONS o- STRESS 



O IO 2O 20 



STRESS. TONS t" SQ IN 



Fig. 4. 



Fig. 4. Except that the maximum stress is now 28 '4 tons per sq. inch, and that the 

 yield occurred with the first load, the description of the last figure applies very closely 

 to the general features. The first range of stress applied was insufficient to produce 

 progressive extension, and the "permanent extension" curve even receded slightly. 

 It will be noticed, also, that some recovery occurs in the " cyclical permanent set." 

 About half the remainder of this quantity is accounted for by the increased elastic 



