42 - ME. LEONARD BAIRSTOW ON THE ELASTIC LIMITS OF 



As the number of repetitions becajne greater, the " cyclical permanent set," curve 

 No. 1, fig. 1, b, became measurable and gradually increased, until after nearly 19,000 

 reversals of stress it had become about 1 1 per cent, of the original elastic extension. 

 The hysteresis loop marked 18,750 was then taken. Starting from A, a no-load 

 point, the curve was traced to B as the tensile stress increased and fell to C as the 

 tensile stress was removed. Exactly similar curves, CD and DA, apply to the 

 compressive stresses. The parts BC and DA are parallel to the elastic line O, and it 

 will be seen that there was no elastic limit in either tension or compression for 

 increasing loads. 



Raising the stresses to 15'0 tons per sq. inch produced an immediate increase in 

 the " cyclical permanent set." The hysteresis loop at 23,260 reversals is similar to 

 the one previously described. 



Finally, stresses of 20'2 tons per sq. inch were imposed, and at 29,280 reversals 

 diagram EFGH represented the condition of the specimen. The width of the 

 hysteresis loop was then very great, but even for that case the lines FG and HE are 

 parallel to the original elastic line O. It therefore follows, as has been previously 

 stated, that the width of the hysteresis loop, i.e. EG, is equal to the change of length 

 from H to F, minus the elastic extension, which can be calculated from the slope of 

 the curve O and the range of stress applied. 



The behaviour of this specimen illustrates the necessity for BAUSCHINGER'S hypo- 

 thesis relating to "primitive" elastic limits, as the extensometer was incapable of 

 showing the first deviations from elasticity. At a slightly lower range, probably 

 1:5 tons per sq. inch, the specimen would have been really elastic, as no number of 

 reversals would have produced a hysteresis loop. The general method of estimating 

 this limiting range of elasticity will be fully given after the description of the results. 



Some features of the Curve No. 1 are curious. At the lower ranges of stress the 

 width of the hysteresis loop was scarcely affected by a considerable increase in the 

 number of repetitions, whilst at the highest stress the specimen would appear to 

 have supported reversals better after 1000 repetitions than at the earlier applications. 



Another specimen (Curve No. '2, fig. 1, b) repeated the observations faithfully, and 

 the type of result has been supported again and again. The figures near the curve 

 give the stresses for each section. At stresses from 16'6 tons per sq. inch to 

 18 - 4 tons per sq. inch reversals during at least 24 hours produced no measurably 

 increased effect, whilst at 19'3 and 20'2 tons per sq. inch recovery actually 

 occurred. After only 1200 reversals at 21'1 tons per sq. inch the specimen broke. 

 Half the fracture showed the absence of extension peculiar to alternating stress 

 fractures. 



Although the observations were continued almost to the actual breaking-point, the 

 extensometer had given no special warning of the deterioration, and as extensions oi 

 about the 100,000th part of the length of the specimen could be detected, it will be 

 realised how extremely local is the actual damage. Further, it would appear that 



