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The basic objective of this study has been to relate the observed behavior of metal matrix eomposites to the micromechanical behavior of each component, as surveyed at the fracture surface. The principal method of investigation has been scanning electron microscopy. Scanning electron microscope examination of fracture surfaces of metal matrix composites has been found to be an extremely useful tool in interpreting micromechanical behavior. Application of the scanning microscope to the boron filament-aluminum alloy composite system has allowed detailed examination of the components and their interaction in situ. The boron filaments are observed to undergo substantial breakup in the compositing and loading sequence. Incomplete bonding at diffusion bond planes in the matrix metal is found to be a major problem in cross-ply aluminum-boron composites. Debonding of diffusion bond planes, observed at fracture surfaces, has been observed on a large scale in both cross-ply and unidirectionally reinforced composites. It is not yet clear whether such debonding cracks are a contributory cause or an effect of the final fracture process. Potential fracture initiation sites are discussed, and the effect of a triaxial stress state on failure of the matrix material between fibers is presented graphically. Observations made of fracture surfaces containing large amounts of incomplete bonding or debonding at diffusion bond planes lead to the conclusion that further optimization of fabrication parameters is needed to improve current state-of-the-art aluminum-boron materials.
fractography, fractures (materials), boron, aluminum, reinforced metals, mechanical properties, composite materials, metal matrix, evaluation, tests
Jones, R. C.
Associate professor of civil engineeringPersonal member ASTM, Massachusetts Institute of Technology, Cambridge, Mass.