Influence of time-dependent phase transformations on bending tests

PSZ-zirconia shows the effects of stress-induced phase transformations with the tetragonal phase changing to the monoclinic modification. This transformation occurs spontaneously when the local stresses reach a critical value. In addition, the time dependent contributions of the transformation are quoted in the literature. The phase transformation behaviour exhibit the well-known consequences on the fracture mechanical properties (R-curves) determined by macroscopic cracks. In case of uncracked specimens (i.e. specimens with natural flaw populations) the phase transformations will influence the stress state and will affect the strengths and lifetimes. This will be significant in bending loading, where spontaneous and time-dependent transformations will change the stress distribution. Therefore, the investigation deals with uncracked bending specimens. The numerical treatment is discussed and an example is described in detail with reference to data measured by Liu and Chen, Fatigue deformation mechanisms of zirconia ceramics, J. Amer. Ceram. Soc., 75 (1992) 1191–1204, and Finlayson et al., Creep of Mg-PSZ at room temperature, J. Amer. Ceram. Soc., 77 (1994) 617–24.