Measurements of work hardening, anisotropy and the Bauschinger effect in mild steel by off-axis torsion tests

When metallic materials are plastically deformed, the physical effects which are listed as work hardening, the development of anisotropy and the Bauschinger effect occur. In order to measure these effects off-axis torsion test by the combined loading is developed. In this test the principal shear stress direction φ can be changed from 0° to 90° while the ratio of maximum and minimum principal stresses is kept-1. Experiments were carried out on tubular specimens of fully annealed and torsional-prestrained mild steels, and the yield loci and strain behavior were examined at three offset levels. The yield stress of the prestrained steel decreases with increasing angle φ. The difference between the directions of the principal shear stress and principal shear strain increment rises to a maximum value and then decreases. The occurrence of this maximum difference corresponds to the angle where the slope of the tangent to the yield locus is steepest. These behaviors strongly depend on the offset value used in defining the yield stress, and severe anisotropy appears when the small offset strain value is used. It is shown that the proposed off-axis torsion test is a very useful method for analyzing the anisotropic hardening behavior of materials.