WHAT HAPPENS AFTER A FIBER BREAKS - PULL-OUT OR RESIN CRACKING

We consider tensile fracture of a specimen consisting of a single rigid fibre embedded in a cylindrical block of a linearly-elastic resin. When the fibre breaks, two possible modes of failure can occur. A circular crack may propagate outwards into the resin, leading to fracture of the specimen. Alternatively, a cylindrical crack can propagate along the fibre-matrix interface, starting from the break in the fibre, leading to fibre pull-out. The question is: which mode of failure will occur in practice? Finite-element analysis is used here to calculate the pull-out force and the force causing growth of a circular crack outwards into the resin, for samples containing fibres of different radius. A general criterion is obtained to predict the mode of failure. Even for samples with perfect adhesion between resin and fibre, pull-out of the fibre is expected when the fibre radius is less than about one-fifth of the sample radius. For fibres of larger radius, either pull-out or resin cracking can take place, depending on the relative levels of interfacial fracture energy G(a) and resin fracture energy G(c).