46 



MR. LEONARD BAIRSTOW ON THE ELASTIC LIMITS OF 



extension due to the yield. The actual hysteresis loop was very narrow and was not 

 symmetrical about the mean load. Probably, after the lapse of a long time, the 

 specimen would have become elastic for this range. The " permanent extension " is 

 much greater than that described in fig. 3, and the corresponding change in the 

 position of the superior elastic limit is also greater, amounting to 1 5 tons per sq. inch. 



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STRESS. TONS <-ca Sa IN 



Fig. 5. 



Fig. 5. The maximum stress to which the specimen was subjected was in the 

 neighbourhood of the maximum tensile stress, and it will be seen that very great 

 differences in the minimum stress were necessary to produce measurable effects. The 

 first large extension was accompanied by a hysteresis loop, fig. 5, c, which gradually 

 decreased in width until at 6000 repetitions the differences from elasticity were not 

 greater than the errors of observation. A new elastic line was therefore found which 

 corresponded with the extended length, and this has been taken as the standard for 

 the calculation of the modulus of elasticity for the higher loads. 



The slope of this line again agreed with the corrected value obtained from the new 

 specimen. 



The " permanent extension," fig. 5, a, ceased at 6000 reversals and was not affected 

 by an increase of range of 3 - 6 tons per sq. inch, nor was any hysteresis loop produced, 

 fig. 5, c. Further increase of range immediately produced a " cyclical permanent set," 

 fig. 5, b, and slow extension commenced. Finally, with a range of stress of 27 '0 tons 

 per sq. inch, very rapid extension set in. Even for this case, however, the hysteresis 

 loop was not very great. 



The superior elastic limit has now been raised by extension, almost up to the 

 maximum tensile stress, and further extension would raise the inferior limit of 



