Study on the Influence of Strain History on Creep of Polycrystalline Metallic Materials at Elevated Temperatures

Abstract

The present study is the sequel of the authors' previous papers on the influence of strain history on metallic creep at elevated temperatures, with further examination of the materials and detailed discussion on them in a series of studies on the laws of plasticity in creep of polycrystalline metals. From the present analytical and experimental studies on low carbon steel at 450°C, the following conclusions have been made.
(1) In regard to the experimental examination of initial isotropy of the material tested, the determination of anisotropic parameter A in the yield function of Eqs. (1) and (2) is simpler and more reasonable than that of usual Hill's parameter.
(2) No remarkable development of anisotropy of the material crept under fixed principal stress axes has been observed.
(3) In general loadings, in which the principal stress axes are rotated and the level of the equivalent stress is varied during the creep, the creep curve under the condition fairly deviates from the analytical curves based on the strain-hardening hypothesis. It is considered that in such loadings the development of anisotropy of the material crept must be more remarkable than that under the fixed principal stress axes, and that it is therefore dependent on strain history.
(4) In the creep tests unber combined constant axial tension and repeated torsion reversals, it does not necessarily follow that the transient increase in shearing strain rate drops immediately after the stress reversal with repetition of the reversal. It is considered that the influence of Bauschinger effect on metallic creep is remarkable when the stress reversal is imposed on the material of which anisotropy is heavily developed.