64 



ROYAL SOCIETY OF CANADA 

 TABLE I. 



The readings obtained are shown in the accompanying table and 

 are exhibited graphically in Figure 4. 



It is interesting to note that McFarlane^ has shown that the effect of 

 tension is to lower the yield point for shear stress and since bending 

 produces tensional stress, this lowering of the yield point by bending 

 might be expected. Although compression stress is also produced by 

 the effect of bending, yet even if it tends to raise the yield point as Lord 

 Kelvin^ supposes, it is unlikely to have any influence here. The larger 

 question of the reason for the failure of a material when subjected to 

 stress of various kinds has been the subject of much speculation by 

 mathematicians and physicists and is still debatable. It may be inter- 

 esting to point out what bearing the various theories have upon the par- 

 ticular case of compound stress considered here, and the differences 

 which arise in applying the theories to obtain formulae for computing 

 the working strength of a material. 



Theories of compound strength. — All cases of stress can be reduced 

 to the general case of three principal stresses in planes at right angles to 

 one another, and if the behaviour of a material under these simultaneous 

 stresses was accurately known, a correct theory could be formulated. In 

 the absence of such information various theories have been proposed from 



^ Art Elasticity, Enc. Brit. 

 ^ Ibid. 



