Crack‐Size Effects on Cyclic and Monotonic Crack Growth in Polycrystalline Alumina: Quantification of the Role of Grain Bridging

The role of grain bridging in affecting the initial rising portion of the R ‐curve and the transient, non‐steady‐state behavior of short cracks during (cyclic) fatigue‐crack propagation has been quantitatively examined in a 99.5% pure alumina. Fatigue‐crack growth properties for both long and short (Δ a f < 2 mm) cracks emanating from machined notches (root radius, ∼ 15–150 μm) were investigated, where Δ a f is the extension of the fatigue crack from the notch. Growth rates (d a /d N ) were far higher at the same applied stress‐intensity range (Δ K ) and fatigue thresholds, Δ K TH , were markedly lower for short cracks than for corresponding long cracks. Crack extension was measured at the lowest driving forces for short cracks emanating from razor micronotches with ∼ 15 μm. For growth rates <10 ‐8 m/cycle, d a /d N vs Δ K curves for short cracks merged with the demonstrably steady‐state curve for long cracks after ∼2 mm of crack extension. This length corresponds well to the extent of the measured crack‐bridging zone for a near‐threshold steady‐state fatigue crack. For d a /d N > 10 ‐8 m/cycle, however, non‐steady‐state behavior was observed at all crack sizes, indicating that achieving steady state at each Δ K level is difficult. The crack‐tip shielding contribution due to such grain bridging was determined using both direct compliance and the more accurate multi‐cutting/crack‐opening profile techniques. Bridging stress‐intensity factors were computed and subtracted from the applied stress intensities to estimate an effective (near‐tip) driving force, Δ K eff These results provided (i) a lower threshold (in terms of Δ K eff ) below which both long and short fatigue cracks should not propagate, and (ii) an estimate of the intrinsic toughness, K 0 , for the start of the R ‐curve. Such results quantitatively affirm that the reduced role of grain bridging is a primary source of the transient behavior of short cracks in grain‐bridging alumina‐based ceramics under cyclic loading.