Stress-State dependence of strain-hardening behavior in 2014 Al/15 vol Pct Al 2 O 3 composite
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INTRODUCTION
FOR metallic materials, generally the strainhardening curves are assumed to be given by a unique effective stress-strain function independent of the stress state. For isotropic materials, the effective stress and strain are assumed to follow von Mises criterion (i.e., the effective yield stresses under uniaxial tension, equibiaxial tension, and compression are the same). It has been found that the effective stress-strain (o~-e) curves in aluminum alloys, copper, and brass under these states of stress are not significantly different. 1~'2,3~However, it is questionable if the above argument is true for a material reinforced with a high-volume fraction of nondeformable particles, particularly because nonuniform deformation can occur around the particles. For applications in metal forming and forging, it is essential to determine if there is a variation in the stress-strain relationships as a function of stress state. It is clear that the reinforcements can disturb the plastic flow in the matrix. For instance, the flow pattern could be a lensoid distortion of the slip and substructure development around the particles during rolling, 14] as shown in Figure 1. Although the microstructure in this figure has been recrystallized and recovered due to heat treatment, it is well documented that the particle does disturb the matrix flow pattern and result in two different microstructures during deformation. It is possible that the flow pattern around the particles could also change when the applied states of stress on the composite are varied. This could affect the effective flow stress in the matrix differently under uniaxial tension, equibiaxial tension, and compression. D.-G.C. SYU, formerly Graduate Student, the University of Michigan, is Senior Engineer, Taipei Municipal Government, Hsintein, Taipei, Taiwan, Republic of China. A.K. GHOSH, Professor of Materials Science and Engineering, is with the University of Michigan, Ann Arbor, MI 48109-2136. Manuscript submitted August 11, 1992. METALLURGICALAND MATERIALSTRANSACTIONSA
Zahl and McMeeking tS] used the finite element method (FEM) to study the effect of residual stresses on the yielding behavior of composites comprised of elastic particles well bonded to a ductile matrix. They found that for a composite with a uniform distribution of matrix material around the particles, i.e., spheres or unit cylinders with no clustering, a transient softening of the composite was predicted due to residual stresses around the particles. This transient softening was comparable under tension and compression. With an increase in the volume fraction and/or an increase in thermal straining, which results in an increase in the yielded area, the composite would have a lower effective yield stress under compression than under uniaxial tension. However, the maximum stress limit of the composite was not influenced by the residual stress. They also concluded that the effect of softening was more pronounced with angular particles than with spherical particles. In addition to the lo
