Surface energy measurements from Griffith cracks in boron fibres

Journal of Materials Science -     

The initial stages of growth and the origin of proximate voids in boron fibres

The formation of so-called proximate voids (i.e. voids near the core-mantle interface) in boron fibres has been studied experimentally. The mantle growth was interrupted in very early stages of nucleation and growth of boron nodules on the tungsten wire substrate. Scanning electron microscope investigations of surface topography and cross-sections revealed a morphology of preferential nucleation of boron nodules along axial die-mark ridges on the tungsten wire surface, where tight parallel axial rows of growing nodules are formed which, particularly around deep wire surface grooves, may hedge in axially extended voids of a morphology earlier found to be typical of proximate voids. The physical conditions of this mechanism are studied with respect to geometry, surface energy and diffusion parameters.     

Radial cracks in boron fibres

The morphology of so-called radial cracks in boron fibres is studied in transverse and axial cross-sections, as well as in fracture surfaces. A formation mechanism is proposed in terms of the transverse residual stress pattern. Small pre-existing voids, (so-called proximate voids) are found to act as stress concentrators which contribute to the opening up of radial cracks in quantitative agreement with the Griffith criterion for brittle fracture.