BEHAVIOUR OF OVER-STRAINED MATERIALS 4 73 



op be applied, the resulting increase in length, mjii, will have 

 a certain fixed value depending solely on the magnitude of 

 o m. The line o r, then, represents completely the relation 

 between stress and strain, however long the wire may have been 

 stretched. Now consider what will happen when the wire is 

 stretched by a force greater than the elastic limit o p, say o s. 

 The wire will exhibit an immediate increase in length, followed 

 by a further slow increase with time — hence the strain has 

 different values according to the duration of the stress. It 

 would be possible to determine the increase in length after a 



STRESS 



series of intervals — for example, every minute. The strain after 

 one minute might be s/3, after two minutes S7, and so on. If 

 the force were only applied momentarily, a strain, s a, less than 

 any of these, would be obtained. Similarly, a second series of 

 points a, /3\ 7', etc., could be obtained by observing the increase 

 in length produced by a still greater force o s' after the same 

 minute intervals. A whole series of such observations could be 

 made, thus obtaining several sets of points, a a a", etc., /3/3'/3", 

 etc., and 7 7' 7", etc. By joining corresponding points the curves 

 ra, r b, rc would be obtained. The curve ra will be very 

 nearly that which would represent the variations of strain with 



