Validity Check of an analytical Dimensioning Approach for potted Insert Load Introductions in Honeycomb Sandwich Panels

Kurzfassung

An easy to handle mechanical-analytical dimensioning approach for insert load introductions on sandwich panels with honeycomb cores is of great value, since remarkable weight reductions can be achieved if the diameters of all inserts in a structure are reduced to their inevitable minimum. This is of special interest for mass critical structures of e. g. satellites, airplanes or race cars. For these structures, a combination of core connected (i. e. potted), through-the-thickness inserts in sandwich panels with fiber reinforced face sheets and aluminum honeycomb core material is frequently in use, since it offers an outstand lightweight performance. Unfortunately, a straight forward, mechanical-analytical dimensioning approach, especially for this particular insert-sandwich combination, is still missing. An analytical-mechanical model, basing on the higher order sandwich theory, was developed by Ericksen in 1953 for inserts without a connection to the core (i. e. clamped between the face sheets). In 1981, Hertel modified Ericksen’s model in order to achieve more convenient to use solution equations. However, Hertel used his modified model also for strength predictions of core connected inserts, although the core shear stress progression, decisive for the strength of an insert, differs highly from clamped inserts. However, since Hertel did not validate his extension of the modified Ericksen model onto core-connected inserts in any way, the authors suspect a misunderstanding. Unfortunately, the model was used frequently for core connected inserts afterwards and is even cited in a standard reference published by ESA and ECSS. To address this uncertainty, the authors carried out a comparison of experimental results to the predictions of the modified Ericksen model for core connected inserts within this work. Therefore, initially a cumulated overview of the Ericksen model and Hertel’s modifications is provided, since this is not available in literature. Included are current approaches for the homogenization of the anisotropic FRP- and honeycomb material properties in order to receive moat precise results. The comparison of experimental and analytical results offers mostly a poor correspondence in terms of the strength of an core connected insert. This, on one hand, reveals that the modified Ericksen formulation is not applicable on core connected inserts without significant restrictions in contrast to Hertel’s assumption. Moreover, it also becomes apparent that the various state-of-the-art test data interpretations deliver widely diverging results for the strength of an insert load introduction. Consequently, the analytical dimensioning of core connected inserts remains unreliable. Yet, this investigation derives useful improvement measures on both, theoretical and practical side.