Effects of Large Grains on Transgranular Fracture Toughness
Stephen W. Freiman
This paper addresses issues of fracture for materials in which large grains, exist in a matrix of much smaller grains. Materials used for optical components, such as ZnS and ZnSe frequently fall into this category. For materials of this sort 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. It is pointed 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, I discuss the state of the science in 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. It is postulated that mixed mode fracture mechanics expressions can be used to explain the transition in fracture toughness.
Proceeding ASTM Special Tech. Publication
November 1, 2000
American Society for Testing and Materials
ceramics, fracture toughness, grain size, polycrystalline toughness, single crystal toughness