The effect of fibre diameter and interfibre spacing on the tensile properties of copper-tungsten composites

Abstract

Stress-strain tests have been carried out on continuous fibre-reinforced copper-tungsten composites at a strain sensitivity of 10−6. The material was prepared by the combination of filament winding, electrodeposition and hot-pressing using fibres between 10 and 50 μm diameter and volume fraction in the range 0·1-0·6.The composite elastic modulus, microyield stress, 0·1% proof stress, stress at 1% total strain and fracture stress, all exceeded the values expected from the rule of mixtures, based on the properties of the tungsten wires and unreinforced copper prepared by a similar route to the composites.The difference between the experimental and rule of mixtures modulus values is an order of magnitude greater than predicted by elastic constraint theory, but no satisfactory explanation could be found for this effect.All the remaining parameters which involved matrix plasticity (and fibre plasticity at high strains), showed positive deviations from the rule of mixtures which have been interpreted in terms of matrix strengthening. This strengthening arises from effects due to the matrix grain size, and also to the abnormally high dislocation densities in the matrix as a result of differential thermal contraction on cooling from the pressing temperature. These effects are only likely to be observed in composites containing small diameter fibres, where the interfibre spacing falls below about 50 μm.