The Yield of Mild Steel with Particular Reference to the Effect of Size of Specimen

Abstract

The paper is a description of an investigation undertaken to determine, by tests under various conditions of uniform and non-uniform stress distribution, the criterion of yield in specimens of mild steel. Apparatus is described for heat-treating the material after machining in such a manner as to avoid surface decarburization, and for determining the stresses at yield of specimens tested in tension, compression, flexure, torsion, and combined tension and torsion. The results show that the material used is uniform and isotropic. No differences are found between the yield in tension and compression. Tension tests on thin tubes and on solid specimens of varying size indicate a progressive change from single-crystal stress-strain characteristics to the normal characteristics of polycrystalline material. Tests in combined tension and torsion give results which accord with the theory of yield at a critical value of the shear strain energy stored per unit volume of the material, but this is shown to be entirely due to the size of the specimens tested. In all cases of nonuniform stress distribution, the yielding is shown to depend on the specimen size, being delayed until a shear stress not less than the shear stress at yield under uniform stress is applied to a thickness of material of the order of a few crystal diameters.