Second-order stresses and strains in heterogeneous steels: Self-consistent modeling and X-ray diffraction analysis
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Second-Order Stresses and Strains in Heterogeneous Steels: Self-Consistent Modeling and X-Ray Diffraction Analysis K. INAL, J.L. LEBRUN, and M. BELASSEL Theoretical and experimetal methods have been developed to characterize the effect of mechanical loading on the mesoscopic and macroscopic mechanical state of polycrystalline materials. Ferritic and austenitic single-phase materials were first analyzed, then phase interaction was studied in a multiductile phase material (austeno-ferritic duplex steel) and a natural reinforced composite (pearlitic steel). The theoretical method is based on the self-consistent approach in which elastic and plastic characteristics of the phases have been applied through the micromechanical behavior of single-crystal-using slip systems and microscopic hardening. The effects of a crystallographic texture and phase interaction during loading and after unloading were studied. The elastic and plastic anisotropy of the grains having the same crystallographic orientation were assessed by diffraction strain analysis. The simulation was compared with the experiments performed using the X-ray diffraction technique. In the considered duplex and pearlitic steels, it was observed that the ferrite stress state is much lower than the austenite and cementite ones. The results of diffraction strain distribution have showed the pertinence of the models and give valuable information, for example, for the yield stress and the hardening parameters of each phase in a two-phase material.
I. INTRODUCTION
MANY studies have been conducted to characterize the materials metallurgy and associated mechanical properties, considering polycrystalline materials as homogeneous or isotropic. However, in the case of heterogeneous materials, new approaches should be used in order to take into account each kind of heterogeneity and its effect on the material. Therefore, scientific developments should clearly establish the complete characteristics of the considered materials with respect to their elaboration process and use. This characterization step leads to a better understanding of the relation between mechanical properties and microstructure. From a mechanical point of view, the various observations at different scales in a single-phase material may be formalized into the stress orders that can be described as follows:[1,2] first-order stress, which is represented by the macroscopic scale; second-order (or mesoscopic) stress, which is present at the level of the crystallite; and third-order stress, in which the mechanical state varies over a few interatomic distances. This description is more complicated, since the local stresses depend on the dislocation density, distribution, and organization. In a multiphase material, a pseudo-macroscopic scale is introduced, according to Reference 3. In this article, the superscripts I, II and III will denote macroscopic (or pseudomacroscopic), mesoscopic, and microscopic mechanical states, respectively. To distinguish the macroscopic or pseudomacrosc
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