692 Recovery of Nickel and Carbon Steel from Overstrain. 



returned to its original value. The piece was again over- 

 strained in test No. 6, destroying the elasticity. Immersion 

 in boiling water restored the elasticity (see curve 8). 



Figure 6 shows the results of tension tests made on carbon 

 steel. The piece was first overstrained and then, tested 

 again immediately, showing no elasticity in this second test. 

 It was desired to see what effect mechanical agitation of the 

 piece would have on the recovery of its elastic properties. 

 After subjecting it to 500 end blows of a twenty oz. hammer,, 

 the third test was made. This test, made 40 minutes after 

 No. 2, shows little or no recovery due to the treatment. 



The results of the reverse torsion tests are shown in figures 

 7 and 8. In the first test (curve marked direct) the piece 

 was overstrained. The curve marked " reverse immediately " 

 shows the result of an immediate torsion test in the opposite 

 direction. The test shows the elasticity destroyed. After a 

 period of rest of 7 days similar pieces were reversed. The 

 curve in this case shows an elastic limit of 9000 lbs. per 

 sq. inch at the outer fibre, and a modulus of elasticity the 

 same as originally. Fig. 8 is self-explanatory and needs no 

 comment. 



The direct torsion may be regarded as a test that should 

 bring out the same facts as those developed by the tension 

 tests, since elements on a 45 degree helix were in simple 

 tension. From this point of view the outer fibres may be 

 considered overstrained in tension. A release of the load 

 and subsequent treatment, similar to that given the tension 

 specimens, should bring out the same facts. Similarly the 

 tests in reverse torsion may be regarded as tests in which the 

 material was first overstrained in tension and then tested in 

 compression. This is seen to be the case when it is remembered 

 that a helix under tension in the first test was under com- 

 pression in the reverse test. Looking at the matter in this 

 way, it is seen that the results of the reverse torsion tests 

 are in exact agreement with facts previously known, viz., 

 that when iron or steel is overstrained in either tension or 

 compression it loses its elasticity for stress of the opposite 

 kind. 



General Conclusions. 



It is seen from the curves exhibited : — (a) That an over- 

 strain in either tension or torsion destroys the elasticity of 

 the material, but that this elasticity gradually returns when 

 the piece is allowed to rest, the elastic limit becoming, in 

 some cases, greater than its original value, and the modulus 

 of elasticity the same as its original value, (b) That the 



