CYCLIC FATIGUE-CRACK PROPAGATION BEHAVIOR OF 9CE-TZP CERAMICS WITH DIFFERENT GRAIN-SIZE

Cyclic fatigue crack growth behavior has been investigated in 9 mol% C e-TZP ceramics with grain sizes varying from 1.1 to 3.0 mum. To ascert ain the interaction between crack resistance curve behavior and cyclic fatigue crack growth, cyclic fatigue tests were conducted with short double-cantilever-beam specimens in two conditions: (a) with a sharp p recrack without preexisting t-m transformation and (b) with a sharp cr ack after R-curve measurements, i.e., with preformed t-m transformatio n in the crack region. Fatigue crack propagation occurs at applied str ess intensity factor values as low as about 40% of the K(I,infinity) v alues measured in the R-curves. The size and shape of the t-m transfor mation zones are found to be different for specimens obtained in monot onic loading R-curve measurements and in cyclic fatigue tests. For the specimens without preexisting t-m transformation the overall crack gr owth behavior can be described by the Paris power law relation: da/dN = ADELTAK(I)m with m values of 15 for the 1.1-mum grain size and betwe en 8 and 9 for the material with larger grain sizes. For the specimens with the preformed transformation zone, a ''V'' shape da/dN versus DE LTAK(I) relation is obtained. Explanations for these different results in the two conditions are discussed in terms of crack tip shielding e ffects.