Adding brittle fibers to a brittle matrix can create a composite that is substantially tougher than the monolithic matrix by providing mechanisms for energy dissipation during crack propagation. A model based on probabilistic principles has been developed to calculate the increased energy absorption during fracture for a brittle matrix reinforced with very short, poorly bonded fibers. This model, previously developed for planar fiber orientations, is extended to consider the three-dimensional fiber orientations that may occur during composite fabrication. The fiber pull-out energy is assumed to dominate other fracture energy terms, and simple parametric studies are performed to demonstrate the effect of fiber orientation, fiber length, fiber diameter, and fiber-matrix interfacial shear stress. In particular, the fiber orientation effects may be grouped into an effective “orientation parameter.” The model predictions compare satisfactorily with the limited data available, and offer a conceptual framework for considering the effect of changing the physical variables on the fracture energy of the composite.

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