Shear testing of honeycomb cores for inserts
Résumé
Sandwich structures offers exceptional benefits for aeronautical applications, this technology has proved to be a great solution for reduction mass problems. However, these panels are very sensitive to concentrated loads and therefore assembling them is not as simple as it may seem. To solve this inconvenience, in most of the cases a local densification of the core is made to rise the mechanical properties in the desired area where the joint will be installed. This is commonly referred as an insert and this technology is by far the most used for assembly sandwich panels for spatial or aeronautical applications. In this work, we will focus our research on potting made inserts on Nomex honeycomb cores.
The loading capacity of an insert may depend on several factors such as the geometry of the insert, the mechanical properties of the components (skins, core and potting compound), the loading direction, etc. However, in the available literature on insert design it is stated that the first damage observed during a pull-out test of an insert is directly related to the resistance of the core. Therefore, it can be said that the mechanical shear properties of the core are of primary importance to understand the failure scenario of an insert.
To determine these shear properties there are several test specimens that allows to measure the average shear properties of the honeycomb structure like the single, double and quadruple tensile plate shear tests, the three and four points beam tests, among others non-conventional tests. However, none of them consider the effect that the insert densification has on the adjacent cells, and this is very important because when the insert fails it does in the adjacent cells to the insert.
To inquire into this subject another approach is needed, several tested specimens of inserts submitted to pull out loads were cut to observe the damage scenario in the interior of the core. It was observed that the buckles in the pull-out test and in the double rail tests are different, also the folding shape of the cells is different. Thus, a nonlinear investigation of the shear buckling is required.
To understand the buckle evolution of the Nomex honeycomb core HRH-78 when is submitted to shear loads, several 3D models were developed in Abaqus and a benchmark of boundary conditions was made to study their influence on the formation of the buckles and subsequent folding of the cells.
In this work five double rail specimens made of Nomex HRH-78 core material were tested and analyzed. Also, three beam specimens with potting were tested and analyzed. In addition, different FE models whit equivalent boundary conditions of a rail test, three points beam whit and without potting were developed and the results are compared.
This work aims to contribute on the analysis of inserts in sandwich structures.