Stabilized Zirconia As A Structural Ceramic: An Overview

Direct microscopic evidence is presented in support of an explanation of R-curve behavior in monophase ceramics by grain-localized bridging across the newly formed crack interface. In situ observations are made of crack growth in tapered cantilever beam and indented flexure specimens of a coarsegrained alumina. The fractures are observed to be highly stable, typical of a material with a strongly increasing resistance characteristic, but are discontinuous at the microstructural level. Associated with this discontinuity is the appearance of overlapping segments in the surface fracture trace around bridging grains; the mean spacing of such “activity sites” along the trace is about 2 to 5 grain diameters. These segments link up with the primary crack beneath the specimen surface, and continue to evolve toward rupture of the bridge as fracture proceeds. The bridges remain active at large distances, of order 100 grain diameters or more, behind the crack tip. Scanning electron microscopy of some of the bridging sites demonstrates that secondary (interface-adjacent) microfracture and frictional tractions are important elements in the bridge separation process. Evidence is sought, but none found, for some of the more popular alternative models of toughening, notably frontal-zone microcracking and cracktip/internal-stress interaction. It is suggested that the crackinterface bridging mechanism may be a general phenomenon in nontransforming ceramics.

Crack-Interface Grain Bridging as a Fracture Resistance I, Mechanism in Ceramics: I, Experimental Study on Alumina

A mechanics model of microcrack toughening is presented. The model predicts the magnitude of microcrack toughening as well as the existence of R-curve effects. The toughening is predicated on both the elastic modulus diminution in the microcrack process zone and the dilatation induced by microcracking. The modulus effect is relatively small and process-zone-size-independent. The dilatational effect is potentially more substantial, as well as being the primary source of the R curve. The dilatational contribution is also zone-size-dependent. The analysis demonstrates that microcrack toughening is less potent than transformation toughening.

Crack‐Growth Resistance of Microcracking Brittle Materials

Crack tip shielding by micro-cracking in brittle solids

A fracture mechanics model is developed for nontransforming ceramics that show an increasing toughness with crack extension (R-curve, or T-curve, behavior). The model derives from the observations in Part I, treating the increased crack resistance as the cumulative effect of grain bridging restraints operating behind the advancing tip. An element of discreteness is incorporated into the formal distribution function for the crack-plane restraining stresses, to account for the primary discontinuities in the observed crack growth. A trial forceseparation function for the local bridge microrupture process is adopted, such that an expression for the microstructureassociated crack driving (or rather, crack closing) force may be obtained in analytical form. The description can be made to fit the main trends in the measured toughness curve for a coarse-grained alumina. Parametric evaluations from such fits conveniently quantify the degree and spatial extent of the toughening due to the bridging. These parameters could be useful in materials characterization and design. It is suggested that the mechanics formulation should be especially applicable to configurations with short cracks or flaws, as required in strength analysis.

Crack‐Interface Grain Bridging as a Fracture Resistance Mechanism in Ceramics: II, Theoretical Fracture Mechanics Model

Sub-critical crack extension can readily be observed in controlled fracture tests in fourpoint bending. A natural crack of any desired lengthc which exceeds the notch depthc0 by the amount Δc =c −c0 can be introduced into bend specimens by stable crack propagation. The stress intensity factor to achieve Δc increases considerably with increasing Δc. In pre-cracked specimens the stress intensity factorKI0 to start the crack and the critical valueKIC strongly depend on the natural crack length Δc whereasKI0 andKIC are independent ofc0 in solely notched specimens. From a quasi-continuous evaluation of the load-deflection curve recorded during controlled fracture, the “differential work of fracture” can be obtained as a function of the achieved crack length. It may be regarded as the crack extension resistanceR of the material because the balance between the energy release rateg1 andR is maintained throughout the experiment. By that, a formal analogy to theR-curve concept of fracture mechanics is given. The steady increase ofR is explained by multiple crack formation and by the interference of the fracture surfaces due to the angular development of the crack front.

W. Jillek

Papers

Sub-critical crack extension and crack resistance in polycrystalline alumina