Recent advances to assess mono- and multi-delaminations behaviour of aerospace composites

During the last 10 years, significant research efforts have been conducted in the aerospace field to develop a predictive fracture mechanics approach to assess the behaviour resulting from localised delamination defects. The main results of this paper will be presented in two parts. The first part addresses: • Development of specific experimental methods to monitor post-buckling delamination behaviour. • Validation of mixed mode propagation criteria and development of experimental protocol to determine material fracture mechanics parameters. • A robust numerical F.E. computation scheme (modified virtual crack extension) to assess energy released rate and mode partition along the delamination tip. Illustrations of the method will be presented through two typical configurations: • Local delamination near the surface in a monolithic composite. • Debonding between skin and honeycomb core of a sandwich structures. The second part will detail extension of the methodology proposed previously to assess failure mechanisms during static compression after impact of a monolithic composite plate. The damage morphology of an impacted composite plate is characterised by multi-delaminations with a typical helix geometry. The ruin mechanism is regarded as an elastic non-linear behaviour in the damaged area due to a progressive local buckling of delaminated plies until a coupled bending-compression failure. Damage evolution is predicted by introduction of delaminations in a 3D F.E. model with non-penetration contact conditions and elastic properties degradation of the damaged plies. Techniques to manage a mesh with multi-delaminations will be detailed.