Bauschinger effect and multiaxial yield behavior of stress-reversed mild steel
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I INTRODUCTION
FOR a metal subjected to prestrain, the Bauschinger effect (BE) will occur once it is re-strained in the direction opposite to prestrain. The BE has been observed in numerous materials, including not only single-crystal[1,2] and polycrystalline metals[3,4] and single-phase and multiphase alloys,[5,6] but also composite materials.[7,8] In order to explain the BE, a great number of models or theories have been developed. These models are mainly related to two dislocation mechanisms: long-range internal stress (often called the “backstress”) created by dislocation pileup at barriers, based upon Seeger’s work-hardening theory,[9,10] and a directional resistance of dislocation motion, suggested by Orowan.[11] The BE can exert a significant influence on the properties of materials, such as through reductions in flow and yield strengths during reverse straining as well as anisotropy, which are undesired in fabrication and application of materials or engineering components. It is evident that the BE is of practical importance for those processes which involve a reversal of plastic strain, for example, plastic processing, leveling, and fatigue (especially low-cycle fatigue). Concerning the reduction or removal of the BE, one of the common ways is through thermal treatment[12–18] of prestrained metals, e.g., strain aging and stress-relief annealing. However, the effectiveness of a thermal treatment in reducing the BE depends strongly on the temperature, and the BE is not eliminated completely until recrystallization occurs.[15] Moreover, when steels are annealed at about 600 8C, the work hardening induced by prestraining is reduced because of recrystallization,[18] accompanied by the removal of the BE. Warm working[19] has been proven to be effective ZHONGCHUN CHEN, Research Associate, SYUJI MAEKAWA, Graduate Student, and TAKENOBU TAKEDA, Professor, are with the Department of Mechanical Systems Engineering, Faculty of Engineering, Yamagata University, Yonezawa 992-8510, Japan. Manuscript submitted December 4, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
in preventing a loss of prestrain hardening. This method attempted to raise strength by dynamic strain aging[20] in the temperature range from 250 8C to 300 8C and reduce the BE simultaneously. But unfortunately, the working temperatures are within the range of blue brittleness of steels. In addition, the reduction in the BE by cyclic torsion[21] and removal of surface layers in single crystals[22,23] has also been studied. On the other hand, in the 1950s, a technique named “stress aging”[24] was proposed. Stress aging is a kind of thermomechanical treatment consisting of heating a material at suitable temperatures while the material is subjected to stress from an external load. This technique was originally used to promote strain aging with the help of the stress acting on materials during aging.[25,26,27] Elsesser et al.[28] studied the aging behavior under stress with the same direction as prestress, so as to reduce the BE of steels. Recently, Takeda et
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