Micromechanical modeling of reinforcement fracture in particle-reinforced metal-matrix composites
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INTRODUCTION
M E T A L S reinforced with particles or short fibers of ceramics exhibit a variety of failure modes during the application of monotonic or cyclic loads: (a) fracture of the reinforcing ceramic, (b) ductile failure by the nucleation, growth, and coalescence of voids within the metallic matrix, and (c) delamination and crack growth along the interface between the matrix and the reinforcement. An understanding of the micromechanics of these failure processes is essential for improving the mechanical performance of metal-matrix composites that are currently being used or intended for use in a number of engineering applications. It is now experimentally well documented that fracture of the reinforcing particle is one of the principal failure mechanisms in metal-matrix composites. A number of independent research studies ttr-251 have identified the following general trends associated with particle fracture. (1) The propensity for particle fracture increases with increasing reinforcement concentration. (2) The propensity for particle fracture increases with increasing overall plastic strain. (3) In the same tensile test specimen, larger particles fracture more easily than smaller ones. (4) Regions of the composite with clustered reinforced particles exhibit a greater degree of particle fracture than regions where the local concentration of the particles is more dilute. (5) Cracks within the reinforcement are usually oriented M. FINOT and Y.-L. SHEN, formerly Graduate Research Assistants, Brown University, are Graduate Research Assistant and Postdoctoral Research Associate, respectively, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. A. NEEDLEMAN, Professor of Engineering, is with the Division of Engineering, Brown University, Providence, RI 02912. S. SURESH, Richard P. Simmons Professor of Materials Science and Engineering and Professor of Mechanical Engineering, is with the Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted November 15, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
normal to the loading axis for uniaxial tension, and parallel to the loading axis for uniaxial compression. (6) The tendency for reinforcement failure depends on such factors as the reinforcement geometry and shape, matrix and reinforcement composition, interface properties, and thermomechanical processing techniques (such as extrusion). (7) Defects that are introduced to the reinforcing phase during processing may serve as preferential nucleation sites for failure during subsequent mechanical loading. (8) The damage introduced in the composite as a consequence of particle fracture can also trigger or influence other failure modes. For example, sharp microcracks that develop as a result of particle fracture can enhance localized ductile plastic flow within the matrix, thereby promoting such additional failure mechanisms as ductile separation by void growth or shear banding. (9) Experiments show that particle fracture (a) decreases
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