202 Mr. C. A. Cams-Wilson on the Behaviour 



the limit of resistance along that plane has been raised, by the 

 sliding, to that of the rest of the bar, when the sliding will 

 henceforth be uniform. 



In a bar cut into pieces the required increase is the same 

 in each piece, only in the harder bar less permanent strain is 

 required than in the softer. Hence X is the same in each 

 piece and the yield is a measure of the hardness. 



Steel bars and plates can be shown in which the permanent 

 strain at the yield-point has clearly taken place as a sliding 

 parallel to one plane only. 



It would seem that there was an apparent discontinuity in 

 the form of the stress-strain curve at the yield-point. 



The question arises whether the curve may not actually be 

 continued through a double inflexion, and be one with the 

 rest of the curve as shown by the dotted lines, fig. 1, Plate III. 



Let us imagine a solid model made so as to represent at 

 any point of its surface a particular condition of the bar with 

 respect to stress, strain, and hardness. Let the curves L, II.,, 

 III., IV. (fig. 1, Plate III.) be placed parallel to one another 

 at distances from the plane X Y proportional to the hardness 

 which each curve represents, with their origins in the axis Z 

 (perpendicular to X Y), and let the surface of the model 

 be made to pass evenly through all these curves and other 

 similar curves which would be obtained at higher and lower 

 degrees of hardness. 



We see that from the curve which just passes outside the 

 yield-line there is a part of the solid figure entirely wanting. 

 It would seem a much more natural view to suppose that in 

 some sense the successive curves are theoretically continuous, 

 as shown by the dotted line above. 



Such a supposition involves a state of affairs in which the 

 limit of elastic resistance decreases as the permanent strain 

 increases. 



This would be the case if the sliding at the yield-point 

 instead of taking place simultaneously over the whole bar 

 parallel to one plane only took place as a strain wave passing 

 up or down the bar with a definite velocity without any 

 further increase of load. 



This is what actually takes place. Sliding is started at 

 one end and takes place along a definite plane, generally at 

 45° to the axis ; this disturbs the equilibrium of the mole- 

 cules in the parallel plane next to it, and sliding is induced in 

 this plane, so that a wave of strain passes the whole way up the 

 bar ; or sometimes starting from both ends two waves meet 

 in the centre. 



The velocity of the wave varies greatly with the quality of 

 the steel ; sometimes the wave appears to be instantaneous, 



