When a round bar is subjected to torsion, there take place not only pure shearing strains crosswise to the axis, but also some axial and radial deformations, the amount of which can not be neglected especially when the problem of plasticity is treated. This phenomenon has attracted so little attention of investigators that, as far as the author is aware, no paper has yet been published on this subject. The author conducted, as the first step, a series of rather qualitative experiments, in which observations were made upon the secondary strains as well as the modes of fractures set up in bars under torsion. Tests were made upon round bars, solid & hollow, of mild steel and cast iron. The conclusions derived from the results of experiments are : - (1) The amount of axial deformation due to torsion is considerably different according as the test specimen is solid or hollow. (2) The radial deformations differ not only in magnitude according as the specimen is solid or hollow, but also in sense as the material is cast iron or mild steel. (3) The state of stresses in bars under torsion is much more complicated than has hitherto been generally supposed. (4) The angle of fracture of cast iron bar is remarkably different as the specimen is solid or hollow. In solid specimens the angle of fracture measured with reference to the circumference ranges from 30 to 35 degrees, and in follow specimens it is nearly constant being equal to 45 degrees. Regarding the causes for inducing the secondary strains and their effects upon the stress distribution and the fracture, author's explanations, in some extent, are proposed. In conjunction with these, Prof. Sunatani's theory "On Laws of Failures of Solid Bodies due to Stress" is briefly discussed, and some corrections to his theory are suggested. The author found out that no torsion testing machines hitherto constructed is perfect in applying pure torsion on the test specimen, and hence he undertook the design of a new testing machine under the guidance of the late Prof. A. Inokuty. This machine has been used with a fair degree of success throughout the author's experiments, and is described in details in the paper.
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