Mechanics of fibre fragmentation

Abstract

A fragmentation specimen consists of a single fibre embedded along the axis of a long narrow resin block. When the fibre is broken by a tensile load, either a lateral crack runs outwards into the resin, initiated by the break, or a debond (or equivalently a cylindrical crack in the resin) propagates along the fibre. Debonding always occurs with thin fibres. Strain energy release rates have now been calculated, analytically for long debonds and by FEA for short ones. The force to propagate a debond is found to increase as the debond grows, reaching a final value, termed “pull-out force”, that is higher for softer fibres. If this force exceeds the strength of the fibre, then the fibre breaks again. This is the proposed mechanism of fibre fragmentation. For weakly-bonded, stiff fibres, the inferred minimum distance between breaks, i.e. the critical fragment length, is deduced to be of the order of the geometric mean of the radii of fibre and resin block, about 0.1–0.5 mm for typical fragmentation specimens, and it increases as the ratio of fibre stiffness to resin block stiffness increases, in agreement with observation.