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The character and occurrence of mist, hackle, and crack-branching features on ceramic fractures are reviewed and possible mechanisms for causing these features are discussed. Besides glass and polycrystalline fractures, substantial attention is paid to fractures of ceramic crystals, for which new data are presented and similarities shown to fractures of brittle metal crystals. Distinct geometrical effects exhibited by crystal fracture features are illustrated and discussed, as are the variable distances from fracture origin to the onset of features such as mist.
Crack branching of ceramics, which is suggested to be the merging of hackle, is shown to follow the same type of relations as for the onset of mist and hackle. If there is rebranching, it tends to occur at multiples of the original branching distance. Substantial data (again much of it new) show that branch angles are similar for a wide variety of ceramics as well as for brittle fracture of metals. Branch directions are shown to depend on the nature of the test specimen (or crystal directions), and biaxial testing is shown to increase branch angles over those for uniaxial flexure of tension testing. Observations, mechanisms, and effects of intergranular versus transgranular fracture are discussed. The determination and character of flaws causing fracture are summarized, and questions concerning their behavior are raised (for example, effects of complex flaws and of mixed-mode failure).
fracture, fractography, ceramics, fracture mode, fracture origins, fracture mechanism, fracture surfaces
Head, Ceramics and Glass Branch, Naval Research Laboratory, Washington, D.C.