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I.C. Skrna-Jakl, D. H. Pahr:
"Influence of specific core reinforcement designs on the stiffness behaviour of hybrid core sandwich panels";
Vortrag: 90th Annual Meeting of the International Association of Applied Mathematics and Mechanics (GAMM 2019), Vienna; 18.02.2019 - 22.02.2019.

A well-known disadvantage of sandwich structures is their very low out-of-plane shear stiffness, which must not be neglected in the design phase. Hybrid core sandwich structures, consisting of a foam core reinforced with thin composite beams, overcome this issue and thus represent a very efficient lightweight construction concept. Current and future application areas of these kinds of structures are, in addition to aerospace structures, industrial fields such as wind energy, transportation systems, ship building and roof constructions.
The investigated hybrid core exhibits - due to its anisotropic 1D reinforcements oriented at 45° - an improved out-of-plane stiffness behaviour. Purpose of the present study is to numerically investigate the influence of specific core reinforcement designs on the out-of-plane as well as the inplane stiffness behaviour of hybrid core sandwich panels, taking into account the increase in weight due to the built-in stiffeners. Finite Element unit cell analyses are performed employing the homogenization software MEDTOOL [www.dr-pahr.at].
3D-solid FE-models are used to study the changes of the engineering constants due the following parameter variations:

. change of the reinforcement material from UD-composite to pure matrix material,
. the presence of matrix joints in the crossing region of adjacent stiffeners,
. interpenetration of stiffeners at the crossing point, and
. omission of the foam core.

The results show a significantly different behaviour for inplane and out-of-plane responses. On the one hand, the specific inplane engineering constants may decrease by up to 17%, in comparison to the data obtained from unit cell analyses of the unstiffened foam core sandwich, depending on the parameters selected. The reason for the decrease is that the inplane stiffnesses are mainly determined by the behaviour of the face layers and the core stiffeners cause predominantly an increase in weight. On the other hand the results show that the specific values of the out-of-plane pressure and shear stiffnesses can be increased up to 16-fold and 60-fold, respectively, compared to the corresponding results of a foam core sandwich without stiffeners. The distinct increase of these values is due to the orientation of the beams in the direction of principal stresses.
In addition FE-analyses of standardized sandwich compression tests are performed, employing "structural" rather than periodicity boundary conditions. The comparison of the effective outof-plane engineering constants obtained by the numerical test and unit cell analyses are used to evaluate experimental results.