The effects of reinforcement additions and heat treatment on the evolution of the poisson ratio during straining of disc
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INTRODUCTION
VARIOUS authors have studied micmstmcture/mechanical property relationships for discontinuously reinforced aluminum (DRA) [14] and other composite materials, tg,l~ A simplistic rule of mixtures is often used to explain the trends in the elastic properties of the composites, t91 whereas other more precise models have been proposed which account for the size, shape, and distribution of particulates on the elastic properties.rio-20] While DRA materials possess higher stiffness than the base matrix alloys, the elastic modulus of a material indicates the relative change in dimension along the loading direction in the elastic regime. In contrast, the Poisson ratio of the material indicates the absolute ratio of dimensional changes in the transverse direction to that of the changes in the longitudinal direction. In an isotropic metal, there is a single-valued elastic Poisson ratio that is characteristic of the material, while the Poisson ratio in the plastic regime approaches the theoretical value of 0.5 in the absence of damage (e.g., voids). While significant attention has been devoted to studying the effects of changes in the reinforcement size, shape, and volume fraction on the elastic modulus of DRA materials, relatively little work has investigated similar effects on the evolution of the Poisson ratio with increasing strain in these materials. Experimental work on cermet systemstx61has shown that the Poisson ratio of the material depends upon the volume fraction and the aspect ratio of the reinforcement. One implication is that in composites where the distribution of reinforcement is not homogeneous, the local Poisson ratio of
PREET M. SINGH, Senior Research Associate, and JOHN J. LEWANDOWSKI, Professor, are with the Department of Materials Science and Engineering, The Case School of Engineering, Case Western Reserve University, Cleveland, OH 44106. Manuscript submitted December 14, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
the material will be different as a result of different volume fractions of reinforcements. In regions of higher reinforcement clustering, the elastic modulus of the material will be higher, while the Poisson ratios will be lower. This will induce shear stresses in the regions with different material properties. For fiber-reinforced composites loaded longitudinally, Ebert and Wrightu91 have reported that the mechanical interaction between the fibers and matrix is primarily dependent upon the differences in the Poisson ratio of the components and, to a lesser extent, on the differences in Young's modulus. Johnson et al.[2~ have studied elastic properties of SiC-fiber-reinforced Ti-based composites under load control testing conditions and have shown that the direction of loading with respect to the fiber alignment affects the Poisson ratio and elastic modulus of these composites. Banno u61 developed a modified-cube model to predict the elastic properties for ceramics with porosity and has shown this model to predict the properties for the CoWC systems and for sodium boro
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