CYCLIC FATIGUE-CRACK GROWTH IN A SIC-WHISKER-REINFORCED ALUMINA CERAMIC COMPOSITE - LONG-CRACK AND SMALL-CRACK BEHAVIOR

The ambient-temperature subcritical growth behavior of both long and microstructurally small cracks is investigated during cyclic-fatigue loading in a SiC-whisker-reinforced alumina (Al2O3-SiC(w)) ceramic composite (fracture toughness, K(c) approximately 4.5 MPa.m1/2). Based on long-crack experiments using compact C(T) specimens, cyclic fatigue-crack growth rates (over the range 10(-11) to 10(-5) m/cycle) are found to be sensitive to the applied stress-intensity range and load ratio, and to show evidence of fatigue crack closure. Similar to other ceramic materials, under tension-tension loading the "long" (> 3 mm) crack fatigue threshold, DELTA-K(TH), was found to be on the order of 60% of K(c). Conversely, "small" (1 to 300-mu-m) cracks grown from micro-indents on the surface of cantilever-beam specimens were observed to grow at applied DELTA-K levels some 2 to 3 times smaller than DELTA-K(TH), similar to behavior widely reported for metallic materials. The observed small-crack behavior is rationalized in terms of the residual stress field associated with the indent, and the restricted role of crack-tip shielding (from mechanisms such as crack bridging, closure and deflection) with cracks of limited wake, analogous to closure effects with small fatigue cracks in metals. Consistent with the lack of zone-shielding mechanisms in Al2O3-SiC(w), under variable-amplitude cyclic loading crack-growth rates do not exhibit the marked transient response following block overload sequences as do transformation-toughened ceramics or ductile metallic materials. Possible mechanisms for cyclic crack advance in reinforced ceramic-matrix composite materials are discussed.