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

Abstract

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 (Δaf < 2 mm) cracks emanating from machined notches (root radius, ∼ 15–150 μm) were investigated, where Δaf is the extension of the fatigue crack from the notch. Growth rates (da/dN) were far higher at the same applied stress‐intensity range (ΔK) and fatigue thresholds, ΔKTH, 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, da/dN vs ΔK 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 da/dN > 10‐8 m/cycle, however, non‐steady‐state behavior was observed at all crack sizes, indicating that achieving steady state at each ΔK 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, ΔKeff These results provided (i) a lower threshold (in terms of ΔKeff) below which both long and short fatigue cracks should not propagate, and (ii) an estimate of the intrinsic toughness, K0, 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.