BEHAVIOUR OF OVER-STRAINED MATERIALS 469 



of increasing duration. If the point b approaches o, the curve 

 de becomes less steep, until, ultimately, when b is practically 

 coincident with o, the vertical line c d represents the whole 

 of the recovery observable ; or, provided the stress is only 

 momentary, the wire under test immediately regains its original 

 form, and behaves like a perfectly elastic body. Again, it is 

 obvious that some kind of connection must exist between the 

 curves ac and de. It has been shown that the immediate 

 deformation oa corresponds and is equal to the immediate re- 

 covery c d, and the question arises, " Is the curve d e identical 

 in shape- with a c, and will they coincide when superposed ; or, 

 in other words, is the recovery from strain the exact counterpart 

 of the whole deformation effected by the application of the 

 stress ? " If the answer be in the affirmative, it is clear that 

 the curve d e will cut the time axis at a point at double the 

 distance o b from o, which would mean that the strain will 

 be zero after a time reckoned from the moment of release equal 

 to the time during which the wire was under stress. Athough 

 there appear to be certain metal wires for which this is true, 

 there are also many other materials for which it is not the 

 case, and therefore a general rule cannot be stated. In Weber's 

 account of his experiments on the stretching of silk fibres, he 

 records that a fibre subjected to the action of a weight for 

 thirty-six hours was still visibly shortening twenty days subse- 

 quent to release. The gradual after-recovery was, in this case, 

 therefore, very much less rapid than the increase of strain 

 following the initial elastic strain. Similar results have also 

 recently been found by Prof. Trouton and the writer 1 in 

 the case of lead wires under longitudinal stress. On the other 

 hand, Mr. Phillips 2 has found that for some metal wires, 

 including copper and gold, the recovery corresponds exactly to 

 the strain, recovery is complete after a fixed finite time, and 

 there is no permanent deformation. 



In the light of this knowledge, it seems that we may in 

 general divide the strain occurring in over-stressed wires into 

 three parts, one of which may, in certain cases, be absent. 

 There is the initial elastic deformation oa (fig. 1), which has 

 its counterpart in the immediate recovery c d. Besides this, 

 there is a secondary elastic strain included in a c, which 



1 Prof. Trouton and A. O. Rankine, Phil. Mag. Oct. 1904. 

 1 P. Phillips, Phil. Mag. April 1905. 



