Tests of Brittle Materials under Combined Stress. 909 



not possible to find the exact stress distribution, but it 

 appeared to be reasonable to assume that the yield was 

 sufficient to cause the stresses which are directly due to the 

 bending and torsion to become uniform. For example, fig. 1 



Fig. 1. 



Uniform 

 Stres', 



W/TH/N 



Elastic Limit. 



represents the centre of a bar under a central bending load. 

 AOA' is the section midway between the supports. The 

 tensile or compressive stress (at a point in the section), due 

 to the bending, is set out perpendicularly to AA X . The com- 

 pressive stress at A is represented by AB. When the material 

 has not been stressed at A or A' beyond its elastic limit, 

 BOB' indicates the stress distribution over the section. If 

 the material has yielded at the outside, the inner layers are 

 more fully stressed, and a line like BCO replaces the straight 

 line BO. A small yield causes BOO, which represents the 

 true compressive stress at any point, to follow BDO more 

 closoly than OB. But BD represents a uniform stress over 

 the section, and this condition was therefore assumed. The 

 assumption that the shear stress due to torsion was uniform 

 may be similarly justified. 



The Specimens for the Present Tests. 



The behaviour of cast iron, therefore, appeared to support 

 the maximum stress theory, but it was necessary to assume 

 that the known yield caused a redistribution of stress. This 

 was not entirely satisfactory, and further to test this law it 

 was decided to use a material which was elastic to fracture, 

 or was more truly brittle, and which therefore obeyed the 

 usual laws of stress distribution at rupture. Specimens of 

 brittle alloys were obtained, but were condemned after a 

 careful examination, without being tested under combined 

 stress. The metal finally selected was Vickers, Maxim 

 "Cast 7" steel, which contained 0*7 per cent, of carbon. 

 The total length of a bar was 36 inches, and it was finch 

 diameter, with J inch squared ends. In order to obtain a 



