Fracture Toughness Testing of Alumina

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Abstract
It is recognized that the plane strain brittle fracture of ceramics can be described in terms of a critical stress criterion of failure. Provided that fracture is induced by thermal or mechanical mismatch between anisotropic grains of equal or different phases, slip, or twinning, the microcracks propagate in an unstable manner when the local tensile stress ahead of a stress concentrator exceeds a critical value (σmc). If fracture loads are well below stresses leading to “intense microcracking,” nonlinearity is confined to the vicinity of the notch. The σmc-concept establishes a local criterion for unstable cleavage fracture which is related to microstructural features using the model of the critical elastic energy density within the process zone proposed by the author. Similar to the case of metals, the stress state of specimen shape also has a strong influence on measured KI-value of ceramics. For plane strain, the process zone is an autonomous region. The overall size depends on the grain/microcrack size distribution and the elastic interaction of the microcracks. For plane stress, the process zone is strongly related to the specimen thickness, hence, the grain size dependence also will change with specimen thickness.

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