Description
A method for determining worst-case cyclic fatigue thresholds in gain-bridging ceramics by quantifying the role of bridging is demonstrated for a model alumina. Crack-growth properties for both long and short (<2 mm) cracks emanating from machined notches (root radii, r ~ 15 - 150 mm) were investigated. When compared as a function of the applied stress-intensity range (DK), growth rates (da/dN) were far higher and fatigue thresholds DK_TH were markedly lower with short cracks, with growth being observable at the lowest driving forces for short cracks emanating from razor micronotches (r ≈ 15 mm). For growth rates < 10^-8 m/cycle, da/dN vs. DK data for short cracks merged with the steady-state data for long cracks after ~2 mm of extension. This value corresponds well to the measured crack-bridging zone length for long fatigue cracks grown near DK_TH. The crack-tip shielding contribution due to bridging was quantified using multi-cutting compliance and crack-opening profile techniques. Bridging stress-intensity factors were computed and subtracted from applied stress intensities to estimate an effective, near-tip driving force, DK_eff. These results, in terms of DK_eff, provided a lower threshold below which both long and short cracks are not observed to propagate and an estimate of the intrinsic toughness for the state of the R-curve. Such results quantitatively affirm that the reduction role of grain bridging is a primary source of the transient behavior of short cracks in grain-briding alumina-based ceramics under cyclic loading. These methods are additionally applied to estimate the worst-case fatigue threshold and intrinsic toughness of a self-reinforced SiC based ceramic, for which direct testing at small crack sizes has been prohibitively difficult.