MECHANICS OF FIBER FRAGMENTATION

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 tens ile load, either a lateral crack runs outwards into the resin, initiat ed by the break, or a debond (or equivalently a cylindrical crack in t he resin) propagates along the fibre. Debonding always occurs with thi n fibres. Strain energy release rates have now been calculated, analyt ical ly for long debonds and by FEA for short ones. The force to propa gate a debond is found to increase as the debond grows, reaching a fin al value, termed ''pull-out force'', that is higher for softer fibres. If this force exceeds the strength of the fibre, then the fibre break s again. This is the proposed mechanism of fibre fragmentation. For we akly-bonded, stiff fibres, the inferred minimum distance between break s, 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 rat io of fibre stiffness to resin block stiffness increases, in agreement with observation.