﻿534 Mr. W. A. Scoble on the Strength and Behaviour 



but the following points should be noted : — Mr. Guest used 

 thin tubes, and although this course appears to be justified, 

 experiments on solid bars are desirable, the distribution of 

 stress being different. The tensions were applied either 

 directly or by internal pressure. Perhaps the most important 

 practical instance of combined stresses is that due to torsion 

 and bending. Due to bending the stress varies gradually 

 from a maximum tension to a maximum compression, the 

 shear stress due to torsion being zero at the centre and a 

 maximum at the surface. This is a further difference in the 

 distribution of the stress. Several tests with different loadings 

 were made on each of the tubes mentioned, and the results 

 show that in certain cases at least the properties of the 

 material changed appreciably, making accurate comparison 

 impossible, or at least difficult. It was impossible to allow 

 the specimen to yield very much, so that the value found for 

 the critical stress was open to error. The writer preferred 

 to use separate specimens, with possible slight differences in 

 properties, and to allow the yield to be quite definite and of 

 considerable magnitude. 



2. Separation of Metals into Ductile and Brittle. 



Before discussing the theories of elastic strength, it will be 

 well to separate the materials into two classes — ductile and 

 brittle. It is unnecessary to discuss these at length as the 

 difference in properties is sharply defined. Ductile materials 

 yield considerably before fracture, drawing out at and near 

 a particular section when under tension, whereas those of the 

 brittle class yield very little, or are very " short." The 

 planes of fracture vary with the two classes when tested 

 under the same conditions, and it is significant that with 

 ductile specimens the planes of fracture approximate to those 

 of greatest shear. This indicates that after yield a ductile 

 material behaves like a viscous fluid, the large yield support- 

 ing this view. 



3. The Behaviour of Brittle Materials should not be judged 

 by Tests on Ductile Specimens. 



Perhaps the most common examples of combined stresses 

 in practice occur in vessels under internal pressure and shafts 

 subjected to bending and twisting. The material used for 

 the first may be ductile or brittle, but with ductile the thick- 

 ness will be comparatively small and the maximum stress or 

 maximum shear theories will lead to little difference when 

 calculating the dimensions. From the distinction drawn 



