Study on Carbon Fi bre Tube Inserts ESTEC / Contract No. 16822 / 02 / NL / PA J. Block, T. Brander, J. Lyytinen, K. Marjoniemi, R. Schütze, L. Syvänen Carbon fibre tube inserts were at first developed by the DLR Institute of Structural Mechanics in Braunschweig for a very specific application, namely for the structure of the ROSETTA Lander. The design requirements on this spacecraft structure (relatively thick sandwiches, a large number of inserts per unit area, many through-the-thickness connections) made it inevitable to look for a novel insert design with a better relation between insert weight and load-bearing capability than the conventional insert types use to have. The result was the concept of the carbon fibre tube insert, which was initially only qualified for the specific conditions and load cases on the ROSETTA Lander. The intention of the present study was now to achieve a more general qualification and to validate the potential of this novel insert design. The study was performed by the DLR Institute of Structural Mechanics together with two Finnish partners, namely the Helsinki University of Technology (HUT) and PATRIA Finavicomp Oy. The design principle of carbon fibre tube inserts in comparison with classical potted inserts is depicted below. The load introduction from the metallic insert part into the sandwich is not performed by a „pot“ of epoxy resin, but by means of an extremely stiff, thin-walled carbon fibre tube which fits exactly between the face sheets. For implementation, the tube is foldable and spreadable in radial direction. Once implemented, the tube is bonded to the honeycomb core by a thin adhesive layer. A small amount of evenly distributed epoxy resin is already sufficient to ensure good contact to the surrounding cell walls of the honeycomb core. Thus, a radius which is scarcely larger than the tube radius serves as equivalent of the “effective potting radius” in the classical case. On the other hand, the full length of the (extremely stiff) carbon fibre tube actively contributes to the shear load transfer into the (much softer) honeycomb core, because the tube always goes through the whole sandwich thickness. The form-locking contact under both face sheets makes the sandwich in the vicinity of the insert practically incompressible. Carbon fibre tube inserts exist in two basic versions: Type 1 (left picture) replaces the classical through- the-thickness insert, while Type 2 (right picture) replaces the potted insert. Both types were thoroughly investigated. Over a wide range of sandwich parameters, they offer a better relation between insert weight and load-bearing capability than conventional inserts. A basic requirement is that the ratio between insert tube diameter and cell size must be ≥ 2. Also the sandwich thickness should be significantly larger than the length of the metallic insert cap(s), because only the “hollow” portion of the carbon fibre tube contributes to an advantageous ratio of (out-of- plane) loading capability and weight. The achievable critical insert load, upon which the first failure or plastic deformation occurs, is rather high both in out-of-plane direction (tension, compression) and in-plane direction (shear). For the material combination selected for the present study (carbon fibre fabric face sheets and aluminium honeycomb core) these critical insert load values were determined both experimentally and analytically. For the analytical prediction of out-of-plane strength, the formulas given in the ESA Insert Design Handbook were modified. For in-plane shear loading formulas were developed. Carbon fibre tube inserts are not sensitive to fatigue loading of moderate amplitude and to exposure to a wide range of temperatures. They are also quite robust when torsion and bending moments are applied. Their implementation in sandwich plates is relatively easy and allows cost-effective assembly if they are used in large numbers.
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