BEHAVIOUR OF OVER-STRAINED MATERIALS 467 



with time, and the subsequent initial recovery by a gradual 

 untwisting. 



The general nature of the effect of a constant stress which 

 exceeds the elastic limit is shown in fig. 1, where the strain 

 or deformation produced at any time is given by the ordinate 

 to the curve. The time is reckoned from the moment when the 

 stress is applied, and the length oa represents the initial 

 immediate strain. This is followed by the gradual increase of 

 deformation denoted by the part of the curve ac. At the time 

 corresponding to the point b the stress is removed, with the 



< 



I- 

 co 



A 



B 



Tl ME 



Fig. 1. 



result that an immediate recovery of magnitude cd occurs, and 

 afterwards a slow diminution of strain shown by the curve de. 

 The chief method of research on this question of over-strain 

 has been to observe the changes in deformation taking place in 

 wires of various substances when stretched by a constant force 

 or twisted by a constant couple, particularly the gradual 

 recovery from strain subsequent to the cessation of the stress 

 indicated by the curve de. Another means of investigation 

 has, however, also been much used. Let us again suppose a 

 wire being gradually stretched by a weight, but let this weight 

 be in such a form that it can be removed a little at a time. 

 When the wire attains a certain length, remove a small portion 



