CYCLIC FATIGUE LIFE AND CRACK-GROWTH BEHAVIOR OF MICROSTRUCTURALLY SMALL CRACKS IN MAGNESIA-PARTIALLY-STABILIZED ZIRCONIA CERAMICS

Cyclic fatigue stress/life (S/N) and crack-growth properties are investigated in magnesia-partially-stabilized zirconia (Mg-PSZ), with particular reference to the role of crack size. The material studied is subeutectoid aged to vary the steady-state fracture toughness, K(c), from approximately 3 to 16 MPa.m1/2. S/N data from unnotched specimens show markedly lower lives under tension-compression compared with tension-tension loading; "fatigue limits" (at 10(8) cycles) for the former case approach 50% of the tensile strength. Under tension-tension loading, cyclic crack-growth rates of "long" (> 3 mm) cracks are found to be power-law dependent on the stress-intensity range, DELTA-K, with a fatigue threshold, DELTA-K(TH), of order 50% of K(c). Conversely, naturally occurring "small" (1 to 100-mu-m) surface cracks are observed to grow at DELTA-K levels 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 restricted role of crack-tip shielding (in PSZ from transformation toughening) with cracks of limited wake, analogous to the reduced role of crack closure with small fatigue cracks in metals. The implications of such data for structural design with ceramics are briefly discussed.