Fracture Mechanics of High-Toughness Magnesia-Partially-Stabilized Zirconia

Crack growth resistance in MgO-partially-stabilized ZrO₂ (Mg-PSZ) has been studied using notched double-cantilever beams (DCB's). High-toughness Mg-PSZ exhibited nonlinear mechanical behavior in the form of residual displacements, related to the transformation of tetragonal ( t ) ZrO₂ precipitates to monoclinic ( m ) symmetry. The influence of this residual displacement on crack resistance behavior (" R -curve" behavior) was analyzed using several different fracture mechanics approaches. Specifically, the "global" resistance W _R (Δ a ), a J -integral type parameter W _J (Δ a ), were determined as a function of crack extension (Δ a ). Some of these parameters displayed a geometry dependence; their form depended on the initial notch depth and the size of the unbroken ligament. The early stages of crack growth were best described by W _J (δ a ). The residual strains building up in the wake during crack growth and their effect on specimen displacement made the W _R curves (and to some extent the W _J curves) dependent on the ratio between initial notch depth and crack extension. The only curves independent of geometry were the G (δ a ) curves, but only in a restricted range of geometry. However, the material resistance of Mg-PSZ is clearly under-estimated with such a linear elastic approach.