MI;, .i MI-IK ON mi: IM-COVKIIY OF IRON FROM OYKI;XTI;.UX. 



and. it' small, in.iv vaiiisli. provided time U> allowed for 1 Mick ward creeping to take 

 it, It may also U- shown tliat, it' tin- re-applied load l>e increased, tlie elongation 

 produced will increase in a greater proportion. Thus, if a stress-strain curve V 

 ..l.taincd from a recently overstrained bar of iron or steel, it will show, even for small 

 loads, a marked falling away from the straight line which would indicate oljedience to 

 H"KI:'S law. 



It is the recoverv from this semi-plastic state induced by overstrain to a condition 

 of jK-rfect or nearly perfect elasticity with raised elastic limit, that is referred to in 

 the title of this paper. Such recovery is known to be effected by mere lapse of time,* 

 and the object of the experiments about to be described is to show the effect of 

 moderate temperature, of mechanical vibration, and of magnetic agitation, on this slow 

 return to the elastic state, and further to illustrate this recovery by means of 

 compression tests. One section of the paper deals with +he phenomenon of hysteresis 

 in the relation of extension to stress, which is exhibited in a marked degree by iron 

 in the overstrained state. Incidentally, attention will be called to subsidiary points 

 of inten 



The experiments were carried out in the Engineering Laboratory of Cambridge 

 I'niversity. and were the outcome of suggestions by Professor Ewixo. It was on his 

 suggestion that the effect of moderate temperature on recovery from overstrain was 

 tried, and the result of that trial led to much of the work incorporated in this paper. 



Uefore going into details of the experiments it may l)e of interest to give, drawn to 

 a vma II scale, an ordinary complete stress-strain diagram, such as is obtained in the 

 testing ,,f in>n or steel. The period in the history of iron subjected to tensile stress 

 which is about to l>e investigated, may thus be more clearly indicated. The curve 

 given in Diagram No. I. was sketched by hand, roughly, from data obtained from the 

 experiments which will lie described later. It applies to steel not previously sub- 

 mitted to overstrain. 



For the portion ab of this curve HOOKE'S law is ol>eyed. At b the yield-point 



occurs, and as soon as this point is passed the material Incomes overstrained. During 



the large yielding which takes place at the yield-point the load may be reduced with- 



out pausing the extension to stop. After stretching by a large amount as compared 



with elastic extension, the material will lie found to have hardened ; so that to pro- 



duce further yielding the load must be increased. The stress-strain diagram may 



If represented by some such curve as cd. If at d the load be removed, 



and at once gradually replaced, then the stress-strain curve may follow a path such 



These curves de and ef, when obtained in such a manner that the exten- 



PIXCSI.KR'S Journal,' vol. 224, p. 5, and papers already cited. 

 KWI.M:, Wh papers cited above. 



,, K ht also 1* ,,,,1, . Lord KKLVIS'S discovery of the effect of a Sunday's rest on wires 

 Reeled to torsi.mul vibrations throughout the preceding weekSee article, "Elasticity," 



V 01. I I I* . 



