Application of the Griffith criterion to fracture of boron fibres

Tensile tests were performed on boron fibres and the resulting stresses sigmasub(f) recorded. The fracture surfaces were investigated by SEM, with emphasis on the type of flaw nucleating the fracture. It was found the pre-existing, so-called proximate voids, in the boron mantle near the core-mantle interface, have transverse irregularities, large enough to create stress concentrations. The radial extensions, 2c, of fracture initiating proximate voids in mirror zones zones were measured from the micrographs. The obtained 2c-sigmasub(f) relation was found to be in good agreement with the Griffith criterion for brittle fracture. In addition, the experimental log 2c-log sigmasub(f) plot gives some information on the value of the surface energy, γ, of boron. (author)