Fiber fracture during processing of continuous fiber, metal-matrix composites using the foil/fiber/foil technique
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INTRODUCTION
T H E fracture of fibers during the manufacture of continuous fiber, metal-matrix composites (MMC's) has been one of the major obstacles hindering the development and application of this emerging class of materials. In service, the presence of broken fibers causes stress concentrations in adjacent, unbroken reinforcements and in the matrix between the broken and unbroken fibers, as well as a reduction in the load-carrying ability at the ends of the broken fibers. I~l This results in composite failure at stresses lower than those expected. Fiber fracture during processing may occur in one of three predominant modes: tensile, bending, or torsional (Figure 1). Possible sources of such loads include the following: (a) externally applied stresses during consolidation; (b) stresses arising from differences in the thermal expansion coefficients of the matrix and the fibers; (c) stresses induced by matrix phase transformation during heat-up or cool-down periods; and (d) bending stresses induced by the cross-weave wires used to align the fibers in mats used in the manufacture of F / F / F composites. Although the phenomenon of fiber fracture is regarded as an important consideration in the design of continuous fiber MMCs, its occurrence has received limited attention. To our knowledge, the only previous systematic study of fiber fracture appears to be that of Elzey et al.,12.31 who investigated fracture during consolidation P.D. NICOLAOU, formerly Graduate Student, Department of Materials Science and Engineering, Carnegie Mellon University, is Research Visiting Scientist, Metals and Ceramics Division, Materials Directorate, Wright Laboratory. H.R. PIEHLER, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. S.L. SEMIAT1N, Senior Scientist, is with the Metals and Ceramics Division, Materials Directorate, Wright Laboratory, WL/MLLN, Wright-Patterson AFB, Dayton, OH 45433-7817. Manuscript submitted July 6, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
of plasma-sprayed monotapes. In their model, nonuniform matrix flow was assumed to occur during consolidation because of the presence of asperities on one of the surfaces of the monotape. The asperities cause the fibers to deflect in a bending mode. However, as the deflection increases, the load exerted on the asperities (and hence their deformation) increases as well. Accordingly, it was hypothesized that fiber fracture is minimized by imposing conditions for which the asperities deform more rapidly than the macroscopic densification rate. Because of the large deformations undergone by asperities and major differences in geometry, the Elzey et al. model is not applicable to the prediction of fiber fracture for other MMC processing approaches. The objective of the present work was to develop a model to describe the key geometric, material, and process variables that lead to fiber fracture in MMCs consolidated from foil/fiber/foil ( F / F / F ) layups. The failure mechanism and basis for th
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