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The reliability of a composite structure depends on the materials strength variability. Unidirectional composites fail sequentially initiating from the very weakest fiber sites with matrix binder providing local redundancy by transferring load to neighboring fibers until cumulation and clustering of these sites lead to sever stress concentration and ultimate structure failure. As a consequence, the variability of the metal matrix structure is traceable to the strength variability of the constituent fiber, the metal matrix coating process, and the composite wire consolidation process. This report focuses on the partitioning of the first two sources of variability, identification and modeling of the dominant parameters, together with experimental measurement on a current graphite-aluminum composite. The statistical strength of several graphite spools are measured by testing single filament specimens at the beginning and at the end of the spools, thereby characterizing the statistical parameters associated with the strength variability among the spools and within each spool. The graphite-aluminum wire, produced from continuous liquid infiltration process are tested in tension. The metal matrix composite statistical strengths from differ spools are compared with the respective statistical strength of the parent fiber. The results suggest that, given proper interpretation, single filament fiber strength is a sensitive parameter for quality assurance of metal matrix composites.
statistical strength, dimensional scaling, size effect, testing methods, filament testing, metal matrix composites, Weibull distribution
Professor of Aeronautics, Naval Postgraduate School, Monterey, CA
Army Materials Technology Laboratory, Watertown, MA