Strain distribution in copper tensile specimens prestrained in rolling
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I.
INTRODUCTION
WHEN the strain path is changed during the plastic deformation of metals, the flow behavior diverges from that typical of a monotonic deformation. A transient in the work hardening occurs after the change of strain path. Results from previous research concerning the mechanical behavior after reloading have been summarized as follows:[1–4] (1) a low initial flow stress (compared with the stress at the same equivalent strain in the monotonic path) is followed by a relatively high work-hardening rate; and (2) a high initial flow stress is followed by a lower workhardening rate. The first case corresponds mainly to strain path changes associated with Bauschinger experiments (inversion of loading conditions). However, most behavior in complex strain paths concerns the second case, where the active slip systems change partially or totally after reloading. For single-phase materials, the parameters expected to influence reloading behavior are the morphology of the grains, the crystallographic texture, and the dislocation microstructure. The importance of the morphology of the grains is not well studied; but it seems reasonable to support the idea that at moderate prestrain values (up to about 0.30), the change of the grain shape is not significant. As far as the evolution of the crystallographic texture is concerned, it is known that during deformation, it evolves slowly toward a ‘‘stable’’ orientation, which depends on the material
J.V. FERNANDES, Associate Professor, is with the Mechanics Department, Faculty of Science and Technology, Polo II of the University of Coimbra, 3030 Coimbra, Portugal. M.F. VIEIRA, Assistant Professor, is with the Metallurgical Department, Faculty of Engineering, University of Oporto, 4099 Porto Codex, Portugal. Manuscript submitted March 28, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
and strain path. Generally, its influence in plastic behavior can be neglected at low strains.[5,6] The effects of the existing microstructure on the behavior after reloading can be understood in terms of latent hardening and Bauschinger effect experiments. The first case corresponds to the activation of slip systems not activated during prestrain: the critical resolved shear stress of these systems is compared with that of systems operating during prestraining (the latent hardening ratio (LHR) is defined as the ratio of the critical resolved shear stresses in secondary and primary systems[7]). The LHR is lower for coplanar systems than for the intersecting systems. In the coplanar system, its value is quite independent of the prestrain amount and close to unity (at about 1.1 for copper[8]). For intersecting systems, the LHR value is clearly higher than unity, decreasing with increasing deformation (for copper, during stage II of deformation, values close to 1.3 have been found[8]). The latent hardening phenomena can be expressed in terms of dislocation-dislocation interactions. The change of slip system is associated with an increase in the forest dislocation density and a lack of mobil
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