140 Prof. J. A. E wing. On Measurements of small [May 16, 



These figures show that even after three weeks the elastic recovery 

 is still somewhat incomplete. This is the more remarkable when it 

 is borne in mind that the testing is done here with a load of 8 tons 

 only, a considerably smaller load than that which had been applied 

 to produce permanent set (namely 11 tons). The imperfection which 

 the elasticity still shows after three weeks of rest would be more 

 conspicuous if the loading were extended beyond 8 tons. 



Throughout this group of tests evidences were seen of much elastic 

 " nachwirkung." If a load exceeding, say, 4 tons, was left on for a 

 few minutes there was continued extension, amounting sometimes to 

 as much as five scale divisions. And when the load of 8 tons was 

 removed there was, as the table indicates, a gradual retraction which 

 in a few minutes destroyed the apparent set observable at the 

 moment of removing the load. In the final observation (4n) this 

 action had almost wholly disappeared, but even then a trace of it 

 could be detected. 



As elastic recovery goes on in the days or weeks following over- 

 strain there is a gradual return towards Hooke's law, or as it might 

 be described in other terms, a gradual straightening out of the stress- 

 strain curve. So long as the recovery is incomplete there can scarcely 

 be said to be any elastic limit, in the sense of a point below which 

 there is strict proportionality of strain to stress. 



For the Toung's modulus of this bar the tests give the following 

 values : 



(4A.) Primitive condition (before overstrain). 

 Section 0'890 square inch. 



Mean extension from to 8 tons = 85'6 divisions per ton. 

 E = 13480 tons per square inch. 



(4B.) Immediately after stretching past the yield-point by 

 11 tons. Section O385 square inch. 



Extension for the first ton = 87 ; E = 13440. 

 Mean extension from to 8 tons = 97 per ton ; E = 12060. 



(4a.) After 5 days' rest. 



Extension for the first 2 tons = 86 per ton ; E = 13600, 

 Mean extension from to 8 tons = 93' 7 per ton; E = 12480. 



(4n.) After 21 days' rest. 



Extension for the first 2 tons = 85'5 per ton ; E = 13670. 

 Mean extension from to 8 tons = 87'5 per ton; E = 13370. 



The behaviour of iron or steel when " fatigued " by overstrain, and 

 before recovery of elasticity has taken place through prolonged rest, 



