Direct Measurement of Fatigue Damage Accumulation in Ceramics and Elucidation of Its Mechanisms

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There have been "Crain-bridging degradation model" and "Grack resisting-reactivating model" (or "Wedging microcracking model" ) proposed to explain the fatigue crack growth behavior in ceramic materials. The significant difference between the two models is that in the former the crack propagates during loading only but in the latter it should do both during loading and unloading. Paying attention to this point, we tried to elucidate the mechanism of fatigue damage in ceramics.The basic data on fatigue crack growth in silicon nitride and alumina were measured so that the effects of microstructural factors (grain size and aspect ratio), stress ratio and crack closure on crack growth behavior were made clear. Unsteady crack growth was examined in relation to the microstructure. Afterwards, the crack extension during loading and unloading processes in silicon nitride was precisely measured by using the conductive film-potential method. As a result, it has been found that the crack growth occurs not only during loading process but also during unloading process and the fraction of the crack advancement during unloading increases with increase of frequency. Judging from these results, it was concluded that the "Crack resisting-reactivating" or "Wedging microcracking" mechanism should be dominantly operative in the cyclic fatigue in silicon nitride ceramics.