This review compiles knowledge about the mechanical and structural performance of brittle matrix composites. The overall philosophy recognizes the need for models that allow efficient interpolation between experimental results, as the constituents and the fibre architecture are varied. This approach is necessary because empirical methods are prohibitively expensive. Moreover, the field is not yet mature, though evolving rapidly. Consequently, an attempt is made to provide a framework into which models could be inserted, and then validated by means of an efficient experimental matrix. The most comprehensive available models and the status of experimental assessments are reviewed. The phenomena given emphasis include: the stress/strain behaviour in tension and shear, the ultimate tensile strength and notch sensitivity, fatigue, stress corrosion and creep.

The physics and mechanics of fibre-reinforced brittle matrix composites

An irreversible cohesive zone model for interface fatigue crack growth simulation

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

A new family of high-temperature, oxidation-resistant, ceramic composites, with (La)-monazite (LaPO{sub 4}) serving as a weakly bonded interphase, is proposed. Monazite is stable and phase-compatible with alumina at temperatures at least as high as 1,750 C in air. Especially important for use in high-toughness composites, the monazite-alumina interface is shown to be sufficiently weak that interfacial debonding prevents cracks from growing from monazite into alumina. Observations of fracture responses of fibrous and laminar reinforcements are presented.

Ceramic Composites of Monazite and Alumina

Laminated metal composites (LMCs) are a unique form of composite material in which alternating metal or metal containing layers are bonded together with discrete interfaces. The mechanical properties of these materials are reviewed. The tensile properties at low and high temperatures are described. At low temperature, very high tensile strengths can be achieved in deposition processed laminates and very high tensile ductilities can be achieved in roll bonded laminates by layer thickness refinement. At high temperature, superplasticity has been observed and agrees with predictions from constitutive creep relations. Damage critical properties (such as fracture toughness, fatigue, and impact behaviour) and damping can be superior to those exhibited by the component materials. The damage critical properties are strongly influenced by local delaminations at layer interfaces. Mechanisms responsible for many of the unique properties of LMCs have been proposed. The influence of processing, laminate archit...

Mechanical behaviour of laminated metal composites

Investigations of cracking in multilayered ceramic/metal composites are presented. Two aspects are considered: crack renucleation across intact single metal layers and subsequent crack extension. Crack renucleation criteria are determined and compared with predictions. High-resolution strain-mapping techniques are employed to determine the surface strain fields surrounding cracks. Good agreement is found between these experimental measurements and the predictions of a small-scale yielding model. Subsequent crack progression occurs either by the extension of a dominant, nearly planar crack or by the formation of a zone of periodically spaced cracks. Both patterns are analyzed. The dominant cracking behavior is found to depend on the volume fraction and yield strength of the metal.

Cracking and damage mechanisms in ceramic/metal multilayers

Layered materials comprised of one brittle and one ductile constituent exhibit crack growth characteristics that depend on the sequential renucleation of cracks in each brittle layer. An analysis of the problem is presented with two different interface responses. One for a well-bonded, slipping interface and the other for an interface that debonds. It is shown that either slip or debonding enhance the fracture resistance, with debonding being the more effective. The analysis is compared with experimental results for several layered systems.

The mechanics of crack growth in layered materials

The role of the interface in redistributing stress around cracks in multilayered ceramic/metal composites is investigated. The emphasis is on the different effects of interfacial debonding or of plastic slip in the metal phase adjacent to strongly bonded interfaces. The experiments are conducted on alumina/aluminum multilayered composites. Monotonic loading precracked test pieces causes plastic shear deformation within the aluminum layer at the tip of the notch without debonding. However, interfacial debonding can be induced by cyclic loading, in accordance with a classical fatigue mechanism. Measurements of the stress around the crack demonstrate that debonding is much more effective than slip at reducing the stress ahead of the crack.

Fatigue crack growth and stress redistribution at interfaces

Multilayer systems comprising either Al or Cu bonded to sapphire have been used to evaluate stress redistribution effects around cracks, caused by slip in the metal. Single precracks have been introduced into the outermost sapphire layer and strain distributions measured around those cracks upon loading. Both moire interferometry and (Cr3+) fluorescence piezospectroscopy have been used for that purpose. Finite element calculations of the plastic zone size, Ls, the crack opening displacement, δ, and the strains have been made, with the yield strengths of the metals, σo, regarded as unknowns. Comparison of Ls or δ with the measurements establishes σo. Once the relevant yield strengths have been determined, the calculated stresses and strains (as redistributed by slip in the metal layer) are found to be in reasonable correspondence with the values measured by either moire interferometry or piezospectroscopy. Some small discrepancies remain, which have yet to be explained.

M.C. Shaw

Papers

Cracking and damage mechanisms in ceramic/metal multilayers

The mechanics of crack growth in layered materials

Fatigue crack growth and stress redistribution at interfaces

Stress redistribution in ceramic/metal multilayers containing cracks