120 Mr. J. J. Guest on the Strength of 



49. Yield-point Stresses of same type nearly constant 

 throughout series on each tube. — It is to be noted that through- 

 out a series o£ tests on any tube the yield-point stress never 

 having been much exceeded, it has nearly the same value in 

 tests of the same type, though a series of similar tests tend to 

 cause it to rise. 



50. The Variation of the Maximum Stress. — Proceeding" 

 now to a consideration of the variations of the maximum 

 principal stress, of the maximum principal strain, and of the 

 maximum shearing- stress at the yield-points of the different 

 tests, we find that the first-mentioned varies very largely, its 

 least value being found in the pure torsion tests, while in the 

 tests by tension and internal pressure its value is practically 

 constant, and is the same whether the greater of the principal 

 stresses is axial or circumferential. The experiments on 

 Tubes VII. and VIII. show the latter emphatically. We 

 hence conclude, firstly, that in the case of two tensions (such 

 as in a boiler) the lesser tension does not appreciably affect 

 the yield-point; and, secondly, by the similarity of the yield- 

 point stress, whether the greater tension be axial or circum- 

 ferential, that the material is practically isotropic in respect 

 to ihe occurrence of the yield-point,- — -at any rate as regards 

 the tangent-plane stresses, with which we are most concerned. 



51. Variation of the Maximum Principal Strain. — The 

 maximum strain at the yield-point varies throughout 

 the experiments : it is greatest in the simple tension 

 experiments, and least either in the torsion tests or when the 

 axial and circumferential tensions are equal. Both the strain 

 as measured (where the axial stress is greatest) and as calcu- 

 lated from the stresses and elastic constants (and so not including 

 the elastic-limit effect) are tabulated and can be compared ; 

 the calculated maximum strain is sometimes radial and 

 negative. The variations of the maximum strain from con- 

 stancy are considerable for all the materials experimented 

 upon. 



52. The Maximum Shearing- Stress or Slide nearly constant. 

 ■ — The maximum shearing-stress developed, and the corre- 

 sponding maximum shearing-strain or slide (which are quan- 

 tities directly proportional and independent of the intermediate 

 principal stress within the elastic limit) are comparatively 

 constant, although the amount is highest in the case of pure 

 torsion and diminishes towards the tension case. It will be 

 remembered that the plane of the maximum shear changes 

 from being a tangential plane of the specimen in the torsion- 

 tension cases to a plane intersecting the axis of the specimen 

 in the tension-internal pressure tests. 



