Stabilized Zirconia As A Structural Ceramic: An Overview

Crack tip shielding phenomena, whereby the “effective crack-driving force” actually experienced at the crack tip is locally reduced, are examined with reference to fatigue crack propagation behavior in metals, composites and ceramics. Sources of shielding are briefly described in terms of mechanisms relying on the production of elastically constrained zones which envelop the crack (zone shielding), on the generation of wedging, bridging or sliding forces between the crack surfaces (contact shielding) and on crack path deflection and meandering. Examples are taken from the fatigue behavior of high strength lithium-containing aluminum alloys, aluminum alloy-aramid fiber-epoxy laminate composites, and zirconia ceramics. It is shown that, whereas crack tip shielding can provide a potent means of enhancing “resistance” to crack growth, such extrinsic toughening mechanisms can result in the apparently anomalous behavior of “small cracks” and to the susceptibility of brittle materials to fatigue failure.

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Mechanisms of fatigue crack propagation in metals, ceramics and composites: Role of crack tip shielding☆

Fatigue crack propagation in engineering materials has been the subject of considerable research, and extensive review articles have appeared over the past several years. Most of these investigations focused on the behaviour of ‘long’ fatigue cracks, even though the characteristics associated with the extension of small cracks in metals and alloys remain relatively unexplored, despite their unquestionable importance from an engineering standpoint. In this review, the mechanics and micromechanisms of the subcritical growth of short fatigue cracks are examined, and aspects of their propagation behaviour are contrasted with those of long cracks in terms of fracture mechanics, microstructure, and environment. Cracks are defined as being short (i) when their length is small compared to relevant microstructural dimensions (a continuum mechanics limitation), (ii) when their length is small compared to the scale of local plasticity (a linear elastic fracture mechanics limitation), or (iii) when they are s...

Propagation of short fatigue cracks

This is the second paper in a series of three devoted to the application of scanning tunneling microscopy (STM) to mechanics problems. In this paper, improvements to the digital image correlation method are outlined, a technique that compares digital images of a specimen surface before and after deformation to deduce its two-dimensional surface displacement field and strains. The necessity of using the framework of large deformation theory for accurately addressing rigid body rotations to reduce associated errors in the strain components is pointed out. In addition, the algorithm is extended to compute the three-dimensional surface displacement field from STM data; also, significant improvements are achieved in the rate as well as the robustness of the convergence. For (STM) topographs, the resolution yields 4.8 nm for the in-plane and 1.5 nm for the out-of-plane displacement components spanning an area of 10 μm×10μm.

Submicron deformation field measurements: Part 2. Improved digital image correlation

Thin films and multilayers comprised of different classes of material are often used for various functional requirements. As these become relatively large in section and geometrically more complex, thermomechanical integrity is a major concern. It influences performance, yield and reliability. A methodology for thermomechanical design is needed that complements procedures used for circuit design. This article elaborates the principles of fail-safe thermomechanical design, based on the damage mechanisms known to occur in these systems. Among the important mechanisms are delamination and crazing of brittle layers. thermomechanical fatigue of metallic constituents and interface decohesion. The damage mechanisms are generally activated by residual stress, both thermal and ‘intrinsic’. The origins of these stresses are discussed, as well as stress redistribution effects that arise because of bending, discontinuities, etc. Emphasis is given to measurement methods which provide those data needed for implementation of the fail-safe design methodology.

The thermomechanical integrity of thin films and multilayers

Residual surface stresses generated in ceramic materials by diamond grinding were studied. The stresses are characterized both by X-ray measurements of stress magnitudes and by line forces (product of stress and layer thickness) obtained from the bending of plates that were machined on one side. The line forces tend to increase with the hardness of the material but are insensitive to the rate of material removal during grinding (over a limited range of variation). Residual stress measurements are compared with measurements of strengths of the ground surfaces.

Residual Stresses in Machined Ceramic Surfaces

Rates of fatigue propagation of short Mode I surface cracks in Al 2219-T851 are measured as a function of crack length and of the location of the surface crack tips relative to the grain boundaries. The measured rates are then compared to values predicted from crack growth models. The crack growth rate is modeled with an underlying assumption that slip responsible for early propagation does not extend in significant amounts beyond the next grain boundary in the direction of crack propagation. Two models that contain this assumption are combined: 1) cessation of propagation into a new grain until a mature plastic zone is developed; 2) retardation of propagation by crack closure stress, with closure stress calculated from the location of a crack tip relative to the grain boundary. The transition from short to long crack growth behavior is also discussed.

Growth rate models for short surface cracks in AI 2219-T851

Optical observations and X-ray measurements were used to detect the martensitic transformation of tetragonal ZrO/sub 2/ to the monoclinic structure during room-temperature stressing of Mg-PSZ. The transformation occurs at stresses as low as approx. =200 MPa and is reversible. Additional observations of transformation around Vickers identations and cracks suggest that application of much higher stresses prevents the reverse transformation on unloading.

Reversible Stress‐Induced Martensitic Transformation in ZrO2

The influence of alloy grain size on growth rates of surface cracks 20 to 500 μm in length was studied in Al 7075-T6 specimens prepared in 12 and 130 μn grain sizes. Grain boundaries temporarily interrupt the propagation of cracks shorter than several grain diameters in length. Linear elastic fracture mechanics is inadequate to describe resulting average growth rates which must instead be characterized as a function of cyclic stress amplitude, σa, and alloy grain size as well as stress intensity range, σK. These observations are rationalized using two models, one that relates crack closure stress to alloy grain size, and a second that relates the development of microplasticity in a new grain in the crack path to grain size. In addition, growth rates were found to be faster in fully reversed loading than in tension-tension loading, especially in the large grained material. Evidence is presented to demonstrate that this is a consequence of the fatigue induced development of a compressive residual surface stress during tension-tension loading. These complex effects, and the role of grain size in determining short crack growth, are discussed.

The Effect of Grain Size on Fatigue Growth of Short Cracks

An X-ray diffraction technique was used to measure the residual surface stresses on ground surfaces of Al2O3/ZrO2 (tetragonal) composites. To accomplish this, it was necessary to use a radiation for which the penetration depth was small (large wavelength), e.g. CrKα radiation. It was found that these materials could have compressive stresses as high as 1 GPa on their surface, which is a result of the molar volume associated with the tetragonal-to-monoclinic ZrO2 phase transformation. This transformation was induced by the grinding stresses and a reasonable correlation between the amount of ZrO2 transformed and the magnitude of the residual stresses was established. Evidence was found that the transformation depth is controlled by the amount of stabilizing oxide added to the ZrO2 and by the ZrO2 grain size. In the materials studied, transformation depths were found to be typically <20 μm.

M. R. James

Papers

Residual Stresses in Machined Ceramic Surfaces

Growth rate models for short surface cracks in AI 2219-T851

Reversible Stress‐Induced Martensitic Transformation in ZrO2

The Effect of Grain Size on Fatigue Growth of Short Cracks

Factors Influencing Residual Surface Stresses due to a Stress-Induced Phase Bansformation