Fatigue and static crack propagation in yttria-stabilized tetragonal zirconia polycrystals: Crack growth micromechanisms and precracking effects

The influence of precracking techniques in the crack growth behavior of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) is investigated. Load-bridge and cyclic-compression precracking enhance subsequent tensile crack growth rates, in comparison to results that are found with precracks that are extended under four-point bending prior to testing, The actual influence of these precracking techniques in the near-threshold crack growth regime is remarkably different, Although load-bridge precracking produces a pattern of crack growth fluctuations for stress intensity factors, K, lower than the effective crack-growth threshold of the material, compression-fatigue precracks start to propagate under far-field tensile loads at very fast growth rates and for K values that are slightly higher than the effective threshold, Crack-tip shielding by tetragonal-to-monoclinic transformation develops gradually, influencing the crack growth behavior in Y-TZP, Proposed fatigue crack growth micromechanisms involve damage accumulation at the crack-tip region. For K-max > 3 MPa.m(1/2), fatigue crack growth rates are strongly affected by environmental interactions at the crack tip, and postulated fatigue micromechanisms include the cyclic degradation of crack-tip shielding.