MECHANISMS OF CYCLIC FATIGUE‐CRACK PROPAGATION IN A FINE‐GRAINED ALUMINA CERAMIC: THE ROLE OF CRACK CLOSURE

— Cyclic fatigue‐crack growth and resistance‐curve behavior have been studied in a fine‐grained (∼ 1 μm), high‐purity alumina. Specific emphasis is given to the mechanisms associated with crack growth that are controlled by the maximum ( K max ) and the alternating (Δ K ), stress intensities and to the role of crack‐face interference (crack closure), which is known to be an important crack‐tip shielding mechanism in metal fatigue. Significant levels of subcritical crack growth were detected above a threshold stress intensity of ∼60% of the fracture toughness ( K c ) in the alumina, with growth rates displaying a far larger dependence on K max compared to Δ K. The role of crack closure was examined using constant‐ K max experiments, where the minimum stress intensity ( K min ) was maintained either above or below the stress intensity for crack closure ( K cl ). Where K min < K cl , growth rates were found to exhibit a lower dependence on Δ K , which was rationalized in terms of the frictional wear model for crack growth in grain‐bridging ceramics. It is concluded that crack closure, as conventionally defined, has little relevance as a crack‐tip shielding mechanism during fatigue‐crack growth in grain‐bridging ceramics, due to the low dependence of growth rates on Δ K compared to K max .