XX Congresso Nazionale AIDAA – Milano, 2009 ANALYSES OF DELAMINATION IN COMPOSITE LAMINATES IN STANDARD TESTS AND LOW ENERGY IMPACTS A. Airoldi 1 , A. Baldi 1 , M. Daleffe 1 , G. Sala 1 , M. Basaglia 2 1. Dipartimento di Ingegneria Aerospaziale - Politecnico di Milano, Via La Masa 34, 20156 - Milano baldi@aero.polimi.it 2. Alenia Aermacchi S.p.A., Via Ing. P. Foresio 1, 21040 - Venegono Superiore (VA), Italy Abstract A numerical approach to model the nucleation and the progressive propagation of interlaminar damage in composite laminates below the barely visible impact level is proposed. The approach is presented and validated considering interlaminar fracture characterisation tests to establish Mode I (DCB test) and Mode II (ENF test) interlaminar fracture toughness of carbon fabric (FB) and unidirectional (UD) composites. The capability of the approach to model steady and unsteady-state delamination growth has been performed by means of the simulation of a four-points bending tests, on a curved beam specimen. It is subsequently applied to impact on composite laminates consisting of layers of unidirectional fibres. The numerical results have been compared with experimental data both for bending and for low energy impact tests. These ones represent a fundamental benchmark to investigate the damage tolerance of composite materials [1]. The possibility to numerically analyse the onset and the development of interlaminar damage in such tests represent a very important issue to develop efficient damage tolerant composite structure thus reducing the great amount of experimental tests usually required. The modelling approach presented in this paper uses cohesive material models [2] to represent interlaminar layers between the plies of a composite laminate. A particular modelling technique is adopted to overcome the difficulties relevant to the high penalty stiffness required for very small thickness cohesive elements used to connect the layers [3]. In explicit computations such penalty stiffness greatly influences the computational time and introduces high frequency oscillations in the laminate response. The adopted technique is based on a structural idealisation of the laminate consisting of lumped normal stress carrying areas, modelled by membrane elements, connected to elements capable to transmit the shear stress that correctly restores the equilibrium conditions in the layers within the laminate. The adoption of this technique leads to exploit the physical out-of-plane stiffness of the composite to characterise the connector elements and to reduce the computational time. A B D C Figure 1 - DCB test (A), numerical model of DCB test (B), numerical-experimental correlation of DCB test of a fabric specimen (C) and numerical experimental correlation of ENF test of an UD specimen (D).