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XCVTL Further Tests of Brittle Materials under Combined 

 Stress. By Walter A. Scoble, A.R.C.Sc, B.Sc, 

 Wliitworth Scholar *. 



Introduction. 



INHERE are three theories o£ elastic strength which are 

 recognized by engineers. — that a material fails under a 

 definite (a) Maximum principal stress, (b) Maximum shear 

 stress, (c) Maximum strain. A large number of tests on 

 ductile metals have been made, and these have shown that 

 the maximum shear stress approximately determines the 

 failure of such materials, and consequently that theory is 

 justified. Although tests in pure tension and torsion indi- 

 cate that ductile materials fail by shearing, whereas brittle 

 materials fracture across the plane of maximum principal 

 stress, it has not been usual to make any distinction between 

 the metals when the results of tests under combined stress 

 have been recorded/ or, at most, following the lead of Gruest, 

 the metals have been termed ductile. Brittle materials have 

 been almost entirely neglected. Yet there are two important 

 theories which have been disproved for ductile materials, 

 which lead to very similar results, and which are supported 

 by the forms of the fractures obtained with brittle materials. 

 Even without any new experimental evidence, it appears to 

 be very probable that brittle materials obey the maximum 

 principal stress law. 



Previous Tests. 



The author used cast iron bars for his original tests of a 

 brittle material under combined stress f. Fracture is the 

 most satisfactory criterion of strengh for a brittle material. 

 But it is doubtful whether cast iron follows Hooke's law 

 with sufficient closeness to allow the ordinary elastic theories 

 to be applied to it ; certainly it is not perfectly elastic to 

 fracture. Consequently it is not a satisfactory material from 

 the standpoint of an elastician, but it was adopted because 

 it is the brittle material which is most commonly used in 

 engineering practice. 



The results of the tests on cast iron showed that both the 

 maximum principal and shear stresses at fracture varied 

 considerably if they were calculated on the assumption that 

 the metal was elastic to rupture. But the planes of failure 

 approximated very closely to those of the maximum principal 

 stress. The metal certainly yielded somewhat, and it was 



* Communicated by the Physical Society : read April 22, 1910. 

 t Troc. Phys. Soc. London, vol, xx. 



