The radial strength of boron fibres and fibre splitting in boron-aluminium composites

Abstract

A technique has been developed with which to measure the radial strength of large diameter elastic fibres and it has been applied to the boron family of fibres. The fibres are compressed across a diameter and an analysis shows that the longitudinal splitting which occurs is due to tensile stresses normal to this diameter. The radial strengths of B 4C surface coated, SiC surface coated and uncoated boron fibres have been determined over a range of fibre lengths and at all stages of metal matrix composite fabrication. The radial strength of the fibres is limited at lengths greater than about 5 mm by surface irregularities and at shorter lengths probably by internal defects. The strength of the fibre is increased by surface smoothing during composite manufacture. Minimum radial strengths have been calculated for the different fibres as well as associated defect sizes using the Griffith fracture theory. A study of fibre splitting in a cross-plied Boron B 4C-6061 aluminium composite shows the cause to be directional loading during composite manufacture. This was due to fibres in different layers touching and a lack of matrix at the surface permitting the outermost layer of fibres to come into contact with the press.