Keywords: composites, delamination, damage resistance, probability, scattering, FEM.

Composite materials exhibit complex phenomena associated with damage onset and propagation. In order to take into account these complex phenomena, in the last years, different methodologies oriented to the prediction of the impact induced damage extent [1,2,3] have been developed. The development of damage resistance design methodology, especially for aircraft structures, has allowed a more efficient management of the damage. Impact damage resistance, which is the ability of a composite structure to withstand a damage induced by a low velocity impact can strongly depend on material properties. That is the reason why it is worth investigating the influence of material properties uncertainty (significant for composites) on the damage resistance of composite structures. In this context the Davies procedure [4] has been adopted and implemented to predict the threshold impact energy.

In this paper a probabilistic approach has been adopted to understand the impact of material properties scattering (longitudinal, transverse and shear moduli) on the damage resistance of a composite panel, evaluated at different positions over the panel. Starting from the statistical distributions of damage resistance at each position, a subsequent sensitivity analysis has shown that the influence of the transverse modulus scattering on the damage resistance can be neglected if compared with the impact of other random material properties on the same output variable. Also, the impact of these more significant variables on the scattering of the damage resistance is higher in correspondence of the stringers. Thus, to obtain a composite panel less affected by the material properties scattering, the manufacturing process of the stiffeners should be improved reducing the material properties scattering.

The other important goal of the work has been the comparison, based on a probabilistic approach, between the performance exhibited by two different panels which differ between each other in the number of the plies and in the width of the stringers. One of the panels has been considered better than the other one in terms of damage resistance performance. The damage resistance averaged over the panel, its spatial distribution over the panel and the probabilistic distribution at each selected point have shown a better structural response of the first analyzed panel compared with the second one.

Using this new approach it is demonstrated that a composite panel, which should perform better from a geometrical point of view, will perform poorer than some other panels, ranked worst when using the traditional deterministic design approach.

1

G.A.O. Davies, X. Zhang, "Impact Damage Prediction in Carbon Composite Structures", Int. Journal of Impact Engineering, 16(1), 149-170, 1995. doi:10.1016/0734-743X(94)00039-Y

2

J.F.M. Wiggenraad, E.S. Greenhalg and R. Olsson, "Design and Analysis of stiffened Composite Panels for Damage Resistance and Tolerance", NLR-report, NLR-TP-2002-193.

3

G. Dorey, "Impact Damage in Composites - development, consequences and prevention", Proc of ICCM 6 and ECCM 2 Elsevier Applied Science, 1987.

4

G.A.O. Davies, D. Hitchings, J. Wang, "Prediction of threshold impact energy for onset of delamination in quasi-isotropic carbon/epoxy composite laminates under low-velocity impact", Composite Science and Technology, 60, 1-7, 2000. doi:10.1016/S0266-3538(99)00092-5

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