A strength based criterion for the prediction of stable fibre crack-bridging

The likelihood that a fibre will form a stable bridge across a propagating crack has been investigated by means of experimental measurements and the theoretical consideration of the balance between interfacial strength, fibre strength, the physical properties of the component materials and the stress transfer characteristics of bonded and debonded interfaces. A simple strength based criterion has been developed from traditional stress transfer models to predict if a fibre will form a stable bridge across a propagating matrix crack. The first step in the analysis takes into consideration a range of stress transfer parameters to predict whether the build up of stress caused by the matrix crack is sufficient for the fibre to fracture or if it will cause debonding from the surrounding matrix. This is then extended to include the subsequent build up of stress in the case where the fibre debonds from the matrix. The criterion is compared to experimental data for a range of fibre/matrix systems. Raman spectroscopy was used to measure the point to point distribution of fibre strain along Twaron, Zylon, T50u and M5 fibres embedded in an epoxy resin matrix and lying perpendicular to a matrix crack. The experimental data showed that while some fibres debonded partly from the matrix and formed a stable bridge, others fractured with only a small amount of debonding.