Impact and Crash Modelling of Composite Structures: A Challenge for Damage Mechanics

The paper describes recent progress on the materials modelling and numerical simulation of the crash and impact response of fibre reinforced composite structures. Composite materials are now being used in primary aircraft structures, particularly in helicopters, light aircraft, commuter planes and sailplanes, because of numerous advantages including low weight, high static and fatigue strength and the possibility to manufacture large integral shell structures. Materials such as carbon fibre/epoxy are inherently brittle and usually exhibit a linear elastic response up to failure with little or no plasticity. Thus composite structures are vulnerable to impact damage and have to satisfy certification procedures for high velocity impact from runway debris or bird strike. Conventional metallic structures absorb impact and crash energy through plastic deformation and folding. Modern explicit FE codes such as PAM-CRASH are able to model these effects and are being successfully applied to simulate the collapse of metallic aircraft and automotive structures. This paper is concerned with the further development and improvement of such codes for modelling the response of composite structures under crash and impact loads. This topic is being studied in some detail within a CEC funded research project on ‘High velocity impact of composite aircraft structures’ HICAS [1]. This project includes an extensive composites materials and structures test programme, composites modelling developments, FE code implementation and impact simulations. Two important aspects of impact modelling are delamination, which is important in lower energy impacts and in failure initiation, and in-plane ply failure which controls ultimate failure and penetration in the structure. This paper summarises some of the modelling developments being carried out within the HICAS project on in-plane damage models for both unidirectional (UD) and fabric reinforced composite plies. Emphasis is given to composite materials models suitable for implementation into explicit FE codes, which can adequately characterise the nonlinear damage progression and different failure modes that occur in composites. A continuum damage mechanics model for composite plies under in-plane loads is presented. It is based on methods developed for UD ply materials in [2], which are generalised to fabric reinforcements. This model has a number of features not included in the existing ‘bi-phase’ model. It allows damage parameters for in-plane and through-thickness shear failure modes, as well as failures along and transverse to the fibre directions. Delamination models and strain rate dependence may also be incorporated in the damage mechanics framework. The model contains elastic damage in the fibre directions, with an elastic-plastic model for inelastic shear effects. A novel approach is being developed for delamination modelling based on laminates modelled numerically as stacked plies, with a new sliding interface whose failure properties are consistent with the fracture mechanics of composite delamination. The models are currently being implemented into PAM-CRASH and preliminary results are presented on simulations of impacted composite plates and progressive delamination. REFERENCES [1] HICAS High Velocity Impact of Composite Aircraft Structures, CEC DG XII BRITEEURAM Project BE 96-4238 (1998). [2] P. Ladeveze, E. Le Dantec, Damage modelling of the elementary ply for laminated composites, Composites Science and Technology, 43, 257-267 (1992).