Microbuckle tunnelling in fibre composites

The propagation of compressive failure in multi-directional composite lamin ates is modelled by the tunnelling of a microbuckle within the load-bearing axial plies, with concomitant delamination of the neighbouring off-axis pl ies. The microbuckle tunnels at its tip in a crack-like mode III manner, an d the steady state tunnelling stress is estimated by calculating the energy difference between the upstream unbuckled stale and the downstream buckled state. The downstream state is analysed in detail using a plane strain ana lysis of a microbuckle with delaminations from its tips. In the downstream 2D problem, microbuckling of the axial plies is represented by the generati on of an inclined mode II crack, with an associated microbuckling tip tough ness and a constant sliding stress across its flanks. The delaminations at the interface between the axial and adjacent off-axis plies are idealised a s traction-free mixed-mode interfacial cracks. Predictions of the steady-st ate tunnelling stress are obtained for an isotropic solid by solving an int egral equation and by the finite element method; finite element techniques are then used to solve the tunnelling problem for an orthotropic solid and for a cross-ply laminate. For each case, the tunnelling stress and the dela mination crack length are obtained as functions of the ratio of delaminatio n to microbuckle toughness, and of the inclination of the microbuckle band. The tunnelling stress provides a useful lower bound for the compressive st rength of a thick laminated structure. (C) 2000 Elsevier Science Ltd. All r ights reserved.