Keywords: sandwich panel, foldcore, finite element modelling, material model, aramid paper, honeycomb.

Sandwich structures with fibre-reinforced faces and a cellular core exhibit an excellent bending stiffness combined with a high stiffness-to-weight ratio. In the case of aircraft design, existing sandwich cores such as closed-cell foam and honeycomb are limited to secondary structures. This is due to drawbacks such as the accumulation of humidity, complex manufacturing and vulnerability to impact loads. New folded composite structural cores solve the problem of humidity accumulation and can be manufactured cost-effectively using a continuous process as depicted in references [1,2].

Analysis methods are being studied in the EU-project CELPACT [3] for new sandwich structures to support the design and certification of future aircraft components. One focus of the project is the folded composite core, which is made from sheets of phenolic resin impregnated paper with randomly oriented aramid fibres similarly to the material used in honeycomb cores.

As various different possible folded core geometries exist, there is a requirement to substitute the time consuming experimental determination of the nonlinear mechanical properties of the sandwich core materials with virtual testing using finite element simulation. Recently, this virtual testing approach has been applied to folded core structures by Heimbs et al. [4]. However, the development of qualified meso-models poses further challenges, as these models are sensitive to numerical factors and the chosen material constitutive laws

In this work a brief overview of material properties of aramid paper and the modelling of foldcore structures is presented. Because of the inhomogeneous composition through the thickness of the pre-impregnated aramid paper, the material is modelled using layered shell elements with four plies together with an elastic-damage failure model. The model is particularly adapted to reproduce the buckling modes of the aramid paper faces and the sharp kinking of the buckle peaks.

The dynamic test simulations with the explicit solver PAM-CRASH are validated by comparison with tensile test samples and experimental compression of folded core structures made of phenolic resin-impregnated aramid paper. Benchmarks for the comparison are the representation of cell wall buckling, folding and failure phenomena of the aramid paper model in foldcore structures for large deformations. The numerical results obtained indicate favourable agreement with experiments for the initiation of buckling and the folding process.

1

D. Hachenberg, C. Mudra, M. Nguyen, "Folded structures - An alternative sandwich core material for future aircraft concepts", Deutscher Luft- und Raumfahrt Kongress, München, 2003.

2

R. Kehrle, M. Kolax, "Sandwich structures for advanced next generation fuselage concepts", SAMPE Europe Technical Conference, Toulouse, September 13-14, pp. 11-16, 2006.

3

CELPACT, "Cellular structures for impact performance", EU Research Project, FP6-031038, 2006-2009.

4

S. Heimbs, P. Middendorf, S. Kilchert, A.F. Johnson, M. Maier, "Experimental and numerical analysis of composite folded sandwich core structures under compression", Applied Composite Materials, 14, 5-6, 2007. doi:10.1007/s10443-008-9051-9

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