Cyclic Fatigue from Frictional Degradation at Bridging Grains in Alumina

Abstract

Tension—tension cyclic loading tests have been conducted on a coarse‐grained alumina ceramic that exhibits toughnesscurve behavior by grain‐interlock bridging. Fatigue effects are observed in the regions of both short cracks, using indentation flaws, and long cracks, using compact‐tension specimens. A true mechanical fatigue effect is demonstrated by running the tests below the static fatigue limit. A custom‐made device for in situ observation of crack propagation in the scanning electron microscope enables us to identify bridge degradation as a cause of the fatigue process. “Wear” debris cumulates at the sliding intergranular frictional contact points, indicating a loss of traction at the junction. The basis of a fracture mechanics model describing the effect of this frictional degradation in reducing crack‐tip shielding is outlined and fitted to the data. It is suggested that the bridge degradation fatigue mechanism may be widespread in polycrystalline ceramics with pronounced toughness curves.