Deformation-induced residual stress changes in SiC whisker-reinforced 6061 Al composites
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INTRODUCTION
R E S I D U A L stresses play an important role in the loadbearing process in composites. In the case of SiC/A1 composite materials, a recent study conducted by Arsenault and Taya m indicated that the yield stress of the composite under uniaxial compression is different from that under uniaxial tension. It was further indicated that this asymmetric response is a direct consequence of the average tensile thermal residual stresses in the matrix. Continuing effort has lead to an understanding that thermal residual stresses are responsible for the asymmetric constitutive relationship of discontinuously reinforced SiC/AI composites. [2'3] In a later study using the finite element method (FEM), S h i e t al. t4~ also reported that thermally induced plastic flow has a significant influence on the behavior of matrix plastic flow when the composite is subjected to external loading. Residual stresses may be generated from either thermal or mechanical treatment. However, despite the importance of the residual stresses in composites, current research has been primarily concerned with the thermally induced residual stresses in composites due to differences in the coefficients of thermal expansion ( A C T E ) between matrix and reinforcement. For composites with a crystalline matrix, diffraction is a powerful tool for determining bulk average elastic strains, from which the corresponding stress components can be deduced, tsl In addition, diffraction with neutrons offers advantages over X-rays. t6,71With X-rays, the measurement is typically taken from material just beneath the surface, whereas thermal neutrons are far more penetrating. For aluminum, the depths from which half the diffracted energy arise are about 2 0 / x m and 20 mm for X-rays and neutrons, respectively. There have been efforts to determine the matrix thermal residual stresses in SiC/A1 composites. Using neutron diffraction, Allen et al.[8] measured thermal residual stresses and the deformation-induced bulk stresses in a N. SHI, Graduate Student, and R.J. ARSENAULT, Professor, are with the Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115. A.D. KRAWlTZ, Professor, and L.F. SMITH, Graduate Student, are with the Department of Mechanical and Aerospace Engineering, University of MissouriColumbia, Columbia, MO 65211. Manuscript submitted December 16, 1991. METALLURGICAL TRANSACTIONS A
SiC/A1 composite. Withers e t a / . 191 determined the matrix lattice strains after cooling from different temperatures. Ledbetter and Austin tl~ and Ericsson et al. t11] measured thermal residual stresses in the same composite system using X-ray diffraction. They were able to show that the average normal residual stresses are tensile. Research has also been concerned with the modeling of the process of residual stress generation. Povirk et al. tlzl employed the FEM technique to model the generation of thermal residual stresses in SiC/A1 whiskerreinforced composites. Using a unit cell approach, they found that, with fi
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