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This paper addresses issues of fracture for materials in which large grains, exist in a matrix of much smaller grains. Polycrystalline ZnS and ZnSe, used for optical components, frequently fall into this category. For material microstructures of this character failure can occur from flaws contained within the isolated large grains so that the governing fracture toughness is that of a single crystal of the material, rather than that of the polycrystalline matrix. I point out that there are currently limitations and uncertainties associated with the most popular experimental procedure available to determine the fracture toughness for small crystals. As an alternative to testing, I discuss the state of our ability to predict fracture toughness based solely on knowledge of lattice parameters and elastic properties. The historical background and examples of such a prediction technique are demonstrated. Second, the role of microstructure in governing the transition from single crystal to polycrystalline values of toughness is shown. I postulate that mixed mode fracture mechanics expressions can be used to explain the transition in fracture toughness.
ceramics, fracture toughness, grain size, polycrystalline toughness, single crystal toughness
National Institute of Standards and Technology, Gaithersburg, MD