MICROSTRUCTURAL MECHANISMS OF CYCLIC FATIGUE-CRACK PROPAGATION IN GRAIN-BRIDGING CERAMICS

The microstructural basis of cyclic fatigue-crack propagation in grain -bridging ceramics is investigated for monolithic, hot-pressed alumina and silicon nitride. In both materials, rates of subcritical crack gr owth under cyclic loads are observed to be many orders of magnitude fa ster than corresponding growth rates under monotonic loading at equiva lent stress-intensity levels. This behaviour is attributed to diminish ed crack tip shielding under cyclic loads caused by a degradation of t he grain-bridging zone in the wake of the crack tip associated with fr ictional wear at the grain/matrix interface; the reduced shielding act s locally to enhance the crack-tip driving force in cyclic fatigue. Mi cromechanical modelling of this process is shown to be consistent with fractographic and in situ crack-profile analyses. The effect on crack -growth rates of microstructural and mechanical variables are examined in light of this mechanism. (C) 1997 Elsevier Science Limited and Tec hna S.r.l.