Load sharing of the phases in 1080 steel during low-cycle fatigue
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I.
INTRODUCTION
MOST engineering materials are inhomogeneous, and the macroscopic mechanical behavior will be a composite of the behaviors of the material's components and their interactions. These components can be different phases, regions with different compositions, or even grains with different orientations. While the overall composite behavior is useful for many purposes, a more complete understanding of the material's properties requires a knowledge of the behaviors and interactions of the different components of the material. Stress measurements with diffraction provide a unique probe to study the mechanical behavior of the individual phases in a multiphase material. Different diffraction peaks arise from the different phases and only samplethat volume in the material containing the phase from which a peak arises. The diffraction peaks thus provide information on the individual phases in multiphase or composite materials. In this study, we have, for the first time, used the diffraction peak shifts in a plain carbon steel to measure the individual stress responses of the matrix and carbide phases in a 1080 steel subjected to cyclic plastic strain. Residual stresses are known to play a significant role in fatigue failure. Changes in residual stresses have been measured often in steel but usually only in the ferrite phase. The role of stresses in the carbide phase has been largely ignored. Since steel and many other materials are multiphase, it is therefore important to know the residual stress state of all of the phases and their relation to the fatigue process. R.A. WINHOLTZ, formerly Research Assistant, Department of Materials Science and Engineering, McCormick School of Engineering, Northwestern University, is Assistant Professor, Department of Mechanical Engineering, University of Missouri, Columbia, MO 65201. J. B. COHEN, Frank C. Engelhart Professor of Materials Science, Department of Materials Science and Engineering, and Dean, McCormick School of Engineering, is with Northwestern University, Evanston, IL 60208. Manuscript submitted November 21, 1990. METALLURGICAL TRANSACTIONS A
Steel was chosen for this study because of its historical and technological importance. The mechanical properties of steel are strongly affected by the amount and morphology of the carbides present which may be controlled by heat treatment. It is of interest to know how the individual phases interact to produce the bulk mechanical response of the material. With a proper understanding of the macrostresses and microstresses present in a material (from X-ray diffraction stress measurements), we may separate the stress response of the phases and study the effects of carbide morphology on the lowcycle fatigue properties of a steel. Wilson and Konnan tl] and Hanabusa e t a l . [2] have studied the interaction stresses between the ferrite and cementite phases of steel during tensile deformation and reported that large tensile interaction stresses develop in the cementite. In both of these works, the stresses in the cementite were
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