A. Kelly

Tensile properties of fibre-reinforced metals: Copper/tungsten and copper/molybdenum

Abstract T ensile tests at a variety of temperatures have been carried out on composites consisting of tungsten or molybdenum wires, uniaxially aligned in a copper matrix. Both continuous and discontinuous wires have been used, and both brittle and ductile tungsten wires. It is found that the breaking strength is a linear function of the wire content. A simple theory to explain this is developed and auxiliary experiments to check the theory are described. Some simple predictions about the behaviour of fibre reinforced metals are made from the results.

Comprehensive composite materials

A multi-reference source spanning the whole composites science field, this text covers such topics as: fibre reinforcements and general theory of composites; polymer matrix composites; metal matrix composites; test methods, nondestructive evaluation and smart composites; and design and application.

Theory of multiple fracture of fibrous composites

The theoretical stress-strain behaviour of a composite with a brittle matrix in which the fibre-matrix bond remains intact after the matrix has cracked, is described. From a consideration of the maximum shear stress at the fibre-matrix interface, the extent of fibre debonding and the crack spacing in a partially debonded composite are derived. The energetics of cracking and the conditions leading to an enhanced matrix failure strain are then discussed and, finally, the crack spacing expected in composites containing fibres isotropically arranged in two or in three dimensions is derived for the case of very thin and hence very flexible fibres.

Ductile and brittle crystals

Abstract An attempt is made to assess whether a crystal can break in a fully brittle manner or whether some plastic flow must accompany fracture. The criterion proposed is that, if the ratio of the largest tensile stress to the largest shear stress close to the tip of an equilibrium crack in the crystal is greater than the ratio of the ideal cleavage stress to the ideal shear stress, then a fully brittle fracture is possible. If the converse is the case, the crystal must always break with some plastic flow. Calculations are presented for sodium chloride, a Lennard-Jbnes solid, diamond, copper, silver, gold, nickel, iron and tungsten. The criterion appears to give results in broad qualitative agreement with experiment.

Independent slip systems in crystals

Abstract The slip systems observed in a number of crystal structures common amongst metals and simple ceramic materials are examined to see whether they allow the crystal to undergo an arbitrary strain without change of volume. For most materials, other than f.c.c. and b.c.c. metals, there are insufficient independent slip systems. The condition under which cross slip can give rise to extra independent systems is stated. The results explain in a natural way recent experimental findings on the ductility of polycrystals with the sodium chloride structure.

Interface effects and the work of fracture of a fibrous composite

The stresses near the tip of a crack which lies normal to a set of alined fibres is discussed when the elastic properties of the composite are appropriate to those of a carbon reinforced epoxy resin. Splitting parallel to the fibres is expected to occur before fibre fracture only if an interface parallel to the fibres of one fiftieth the strength of the composite parallel to the fibres is present. The two mechanisms of energy dissipation which have been suggested to occur in a fibre composite of brittle fibres in a brittle matrix, namely pull out and debonding, are discussed. Experiments to measure the latter are described. The work of debonding (< 104 J m-1) is usually less than the work of pull out. The theory of pull out is described and experiments to support it noted. An important result is that the work of pull out increases linearly with fibre diameter, and is likely to be inconveniently small for fibres of diameter ≾ 10 μm. The relative advantages and disadvantages of fibres of various diameters are discussed. Fibres of diameter ≿ 25 μm lead to large works of pull out and are stable and easily handled. Thinner fibres (< 10 μm diameter) give rise to constraint effects which are important in metallic matrices.

Tensile properties of fibre-reinforced metals: Fracture mechanics

Abstract Experiments have been designed to study how a material reinforced with aligned fibres fails at the root of a notch. Ductile and brittle tungsten wires and silica fibres have been introduced into a copper matrix made either by casting or by electrodeposition. A completely notch-insensitive composite can be produced provided splitting is tolerated parallel to the fibres. In these experiments splitting appears to occur in shear. If splitting does not occur then it is shown experimentally for thin sheets that fracture is governed by the established rules of fracture mechanics. The contribution of the matrix to the work of fracture is assessed. An important result is that the work of fracture varies linearly with the fibre diameter and in a copper matrix at room temperature is less than 10 7 ergs. cm −2 for a fibre diameter of 20 μ.

Electron microscope observations of deformed magnesium oxide

Abstract Observations have been made by electron transmission microscopy of thin flakes of deformed magnesium oxide. The ease of cross slip of screw dislocations in this material has been directly verified. Cross slip leads to the formations of many markedly elongated prismatic loops of dislocation. These are observed Profusely within slip bands, produced at room temperature. They are unstable at higher temperatures and break up into numbers of smaller prismatic loops. From these observations a mechanism for the widening of slip bands, observed under the optical microscope, is proposed. Certain dislocation interactions have been observed and analysed and preliminary observations made of slip band intersections.

Creep of discontinuous fibre composites II. Theory for the steady-state

A simple theory is developed to explain the results of paper I, for the steady-state creep of a fibre composite containing alined discontinuous fibres. Rigid and creeping fibres are considered and also the effect of sliding at the interface. The physical basis of the theory is that due to the presence of the fibres the rate of shearing of the matrix is increased. An assumption is made that this increase is inversely proportional to the fibre separation. It is shown that the assumption is not critical for large values of the exponent relating strain rate and stress of the matrix. The theory accounts for the experimental results in paper I, predicts how the strain rate sensitivity of the composite will depend on those of fibre and matrix and reduces to a known form for a Newtonian fluid.

Stress-strain curve of a fibre-reinforced composite

Abstract Careful measurements have been made of the stress-strain curves of tungsten wires of 10 μ and 20 μ. diameter and of aligned composites of the same wires in copper. At strains greater than the yield strain of the copper the apparent stresses in the matrix, evaluated from the law of mixtures, are very high. The effect appears to be due to the mutual constraint of the two phases caused by their different lateral contractions, since it disappears when the fibres yield and hence the transverse contractions become the same. The experimental results are compared with those predicted for a completely plastic matrix and the stresses are found to be higher than calculated. This is interpreted to mean that the yield of the copper is gradual, and that during stage II of the stress-strain curve of the composites an appreciable portion of the copper continues to deform elastically.