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



45. Quantities tabulated. 



48. Results of the experiments — the tests on solid bars. 



47. The material and shape of tubes satisfactory. 



48. The elastic limit phenomenon. 



49. Yield-point stresses of the same type nearly constant throughout 



series on each tube. 



50. The variation of the maximum stress. 



51. The variation of the maximum principal strain. 



52. The maximum shearing-stress or slide nearly constant. 



53. Graphical presentation of variations. 



54. Conclusions probable for general type of sti'ess. 



55. Effect of a volumetric stress (steel). 



56. Convenient view of general type of stress. 



57. The copper and brass tests, and diagrams. 



58. Practical conclusions and note. 



1. J A CK of knowledge of the Laws of Strength. — From 



-*-' the point of view of both pure and applied science, 

 it would be of interest to know the complete laws of the 

 strength of materials ; but although a multitude of tests have 

 been made in certain simple modes, our knowledge of the 

 laws of strength has been extended by few experiments 

 exposing the material to two or more principal stresses, and 

 thns, except in the simpler cases, the elastician is without 

 experimental guidance as to what he should seek analytically 

 as determining the strength of the body under consideration. 



In the series of experiments herein described the materials 

 employed have been subjected to a certain variety of simul- 

 taneous principal stresses; and the results are recorded in the 

 hope that they will prove of service to elasticians and engineers. 



The simplest, and most primitive, method of ascertaining 

 the strength of a material is to subject a cylindrical specimen 

 of it to a direct tension or compression, and to increase the 

 force until the specimen breaks or collapses ; the breaking- 

 stress, thus found, being taken as the basis for calculations of 

 the strength of all pieces of that material used in structures or 

 machines. 



As material is frequently exposed to torsion, another fre- 

 quently employed test is to break a circular cylinder of 

 material by the application of a torque, the stress so found 

 being used in calculations for shafts, &c. 



2. Separation of the Isotropic Materials into Ductile and 

 Brittle. — A consideration of the form of the fractured surface 

 in the various types of test affords some suggestions. In the 

 case of ductile materials such as iron and steel (structural) 

 broken by tension, the material draws out at the point of 

 rupture and the surfaces of fracture, plane or conical, are 

 partly or entirely inclined to the axis of the specimen at 

 about 45°. W, however, cast iron be broken in tension, there 



