Insert sizing of sandwich panels by means of a failure mode map
Résumé
Sandwich structures are widely used in many industrial applications like acclimated transportation, aeronautics, aerospace, naval, civil engineering, fluid storage, electronics, etc. These structures offer an exceptional benefit when they are used in aeronautics, the incorporation of this technology into the aircraft structures has proved to be a great solution for mass reduction problems due to their important bending stiffness and low weight, which allows to design lightweight parts.
Nevertheless, sandwich panels are only used for secondary structures of airplanes, like gear doors or control surfaces. The reason is because their design is complex (against impacts or loads introductions for instance) and difficult to ensure the quality requirements.
In this context, sandwich panels joints through inserts is a problem that is still investigated today and is the main subject of this research.
The insert sizing is usually made using the methods proposed in the “Insert design handbook” of the ESA [1] and the “Military handbook 23A” [2]. These analytical methods based in the research carried by Ericksen in 1953 could lead to important errors of the pull-out allowable load prediction for some cases [3].
This research is about the study of an insert installed into a sandwich panel for aeronautical applications with Nomex honeycomb core and CFRP skins. The objective is to develop a model of the insert behavior (including the different failure mechanisms) when it is pulled out.
This includes a detailed study of the Nomex honeycomb core shear damage behavior [4], and a modelling strategy that allows to incorporate the nonlinear shear behavior at a very low computational cost [5]. Also, a study of the breaking of the potting is made to better understand this failure mode of inserts. Then, a F.E. model of the insert is developed and validated by comparison against experimental results showing good agreement
Finally, this F.E. model it is used extensively to perform virtual tests and to trace an insert failure mode maps that allows to estimate the insert pull-out strength as function of the properties of the inserts, like size and their material properties.