Keywords: composites, laminated, low velocity impact, critical impact force, delaminations, FEM.

The use of composite materials in aircraft structural components has allowed the design of lighter structures with higher corrosion resistance and better fatigue behaviour with respect to the metal structures. However the composite structures are more brittle and more vulnerable to damage induced during service. In particular the low velocity impact damage in laminated composites can be considered very debilitating for certain loading conditions. In fact this kind of impacts are able to produce, matrix cracking and delaminations that can considerably reduce the structural load carrying capabilities especially under compressive load. In order to take into account these complicated phenomena, in the last ten years, different methodologies, oriented both to the prediction the impact induced damage extent and to the prediction of the residual strength after impact [1,2,3,4,5], have been developed. The main objective was to design panels with no induced delaminations after low velocity impacts or panels with induced delaminations still able to sustain the design loads.

In this paper the onset of impact induced delaminations in stiffened composite panels has been analysed. The approach developed in [1], based on the determination of a threshold critical impact force for the impact induced delamination onset, has been adopted. This approach, already adopted in [2] to calculate the damage resistance of composite panels for optimisation purposes, has been applied to a geometrically non linear Finite Element Analysis in order to investigate the influence of the loading conditions on the impact damage resistance of composite panels.

An application to a stiffened composite panel is presented in order to underline the sensitivity of the impact damage resistance to the level of load and to the impact location. The theoretical basis of the implemented approach is briefly introduced and the results of numerical analyses are shown. First of all, the results of a linearised buckling analysis in terms of buckling load and deformed shapes are presented. These preliminary results are compared with the ones coming from the more accurate non-linear analysis, able to simulate the complete compressive behaviour before and after the buckling up to the final failure of the stiffened composite panel under consideration. Non-linear Strain curves and load-displacement curves are presented to show the effect of compression on the overall structural behaviour of the stiffened panel.

The damage resistance of the panels has been assessed by means of the aforementioned approach. The influence of the applied compressive load on the capability of the panel to absorb the energy due to low velocity impacts has been extensively investigated by performing multiple non-linear buckling analyses. Finally, indications on a novel approach, based on the one proposed in [1,2] and oriented to a more detailed definition of the threshold critical impact force for the impact induced delamination onset in composite stiffened panels, are given.

1

G.A.O. Davies, X. Zhang "Impact Damage Prediction in Carbon Composite Structures", Int. Journal of Impact Engineering, Vol 16 N0 1, pp. 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

Z.L. Gu and C.T. Sun, "Prediction of Impact Damage Region in SMC Composites", Composite Structures, vol. 7, pp 179-190, 1987. doi:10.1016/0263-8223(87)90028-6

5

A. Riccio, P. Perugini, F. Scaramuzzino, "Development of an Effective Numerical Tool for Predicting The Compressive Behaviour of Delaminated Composite Panels, "Advances in Computational Structural Mechanics, Civil-Comp Press, Edinburgh, pp 163-174, 1998. doi:10.4203/ccp.55.9.2

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