﻿Metals in Reverse Torsion. 429 



Results of series (a), where the torsion was not carried to 

 the elastic limit in the first test, show that this test had no 

 appreciable effect upon the elastic properties of the material 

 in the opposite direction. But when the stress in the first 

 test (direct test) is carried to, or beyond, the elastic limit, the 

 elastic properties are changed. From fig. 1 it is seen that 

 the elastic limit in the second test was changed very little by 

 the first test to the elastic limit, but the elastic limit in the 

 4th test, due to the overstrain in the 3rd, is much lower 

 (about one-third as much), even after a rest of 6 days. The 

 5th test gave a higher elastic limit than the 4th, due probably 

 to the fact that the stress was in the same direction as the 

 original. All curves are nearly parallel, showing that there 

 was little, if any, change in the modulus of elasticity. 

 Fig. 2 shows that the first stress to the elastic limit had 

 little, if any, effect upon the elastic limit in the reverse 

 direction. A slight overstrain is here indicated by the set 

 curve (4th curve from left to right). The overstrain in this 

 test, even after a rest of 158 days, caused a considerable 

 lowering of the elastic limit. The overstrain in the third 

 test reduced the elastic limit in the fourth test almost one- 

 half. In this case the stress was reversed after five days. 



"When the piece was stressed to the yield-point, as shown 

 in fig. 3. the elastic limit in the reverse direction is less 

 than one-half of what it was at first, even after a rest of 

 30 days. Overstrained in this direction, and allowed to rest 

 151 days, the specimen shows an elastic limit somewhat 

 higher than in the second test. The third overstrain, with a 

 period of rest of 113 days, shows the elastic limit raised to 

 about what it was in the third test. The fourth overstrain 

 in the reverse direction after 4 days shows a lowering of 

 about one-third in the elastic limit. The behaviour of several 

 pieces of nickel-steel is shown by the table on p. 430. 



(Note. — The abbreviations, e.L, y.p., and rup. refer to the 

 elastic limit, yield-point, and rupture respectively.) 



The effect on the elastic limit of immediately reversing the 

 stress after overstrain to the yield-point is shown for certain 

 pieces of nickel-steel in fig. 4. The overstrain destroved 

 the elasticity of these pieces. Low carbon-steel stressed to 

 the yield-point and reversed after seven days, shows an 

 elastic limit of less than one-third the original ; when reversed 

 immediately no elasticity is exhibited (see fig. 5). When 

 the original stress is carried to, or a little beyond, the elastic 

 limit, the specimens show some elasticity when reversed 

 immediately (see fig. (3). Wrought iron stressed to the 

 yield-point and reversed exhibits the same properties as steel 

 (see figs. 7 and 8). 



