Honeycomb structure, adhesive contact
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Yes, there is a simple way to model honeycomb sandwich panels in ANSYS. Honeycomb behaves as an orthotropic material, which means the properties change with the direction through the material. You can create an orthotropic material model in ANSYS for the honeycomb core that looks like this.
Note that honeycomb core is stiffer in shear in the L direction than the W direction, so you have to use a coordinate system in ANSYS to orient the material the way you want to for your application.
The material for the aluminum face sheets would be like the aluminum that is included in the materials database, which is an isotropic elastic material.
In CAD, you create a solid body with the thickness of the honeycomb core and the outline of the shape of the panel you are using. The top and bottom face sheets that bond to the honeycomb core can be created as either solid bodies or as sheet bodies. This is a very lightweight representation of the honeycomb sandwich since you don't model the individual cells. Those are represented by the material property assigned to the solid body that encloses the volume of the honeycomb core.
If you create the face sheets as solid bodies, you will have three bodies for the sandwich structure. In DesignModeler, select those three parts and Form New Part to create a single part with three bodies. When you mesh in Mechanical, common nodes at the coincident surfaces will hold each face sheet to each side of the core thickness. These three bodies should be sweepable so you will have an efficient hex mesh to represent the sandwich (not a tet mesh). Make sure you have two or more elements through the thickness of each solid body.
Well made aluminum honeycomb sandwich should fail in the core itself, and not by the epoxy failing. Therefore you don't need to include the epoxy in the model as it is stronger than the aluminum honeycomb core. Vendors of honeycomb core publish the strength of the core. The table below shows the values of shear strength that can be compared with the shear stress computed in the ANSYS model to determine if a failure is predicted for that load.Â
Note that if you have a much lower stress than the strength shown above, but the stress cycles between zero and positive stress or negative and positive stress values, the core can exhibit a fatigue failure. Unfortunately, manufacturers rarely publish fatigue strength values or S-N fatigue curves, so you would have to do your own fatigue testing if cyclic loading is expected.
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Achilleas,
If you purchase honeycomb core from a manufacturer, they have done the experimental work to measure the shear modulus of the core in two directions: the ribbon direction (L) and transverse to the ribbon (W). They also measure the compressive modulus in the 20 mm thickness (T) direction. You can see those values in the table in the first post in this discussion.
Once you have those three modulus values, and also the strength values in those three directions, you are ready to use that data in an orthotropic material model. That means you create a solid model, 20 mm thick, and assign the orthotropic material to that solid. You have to make sure a coordinate system has its X, Y, Z directions pointing in directions that correspond to the thickness, ribbon and transverse directions of the modulus values entered in Engineering Data.
Now you also have to share topology with some facesheets that might have it's own orthotropic material constants if it is a composite layup, or it could be an isotropic material if it is aluminum facesheet.
It sounds like you have actual honeycomb foil geometry, like is shown in the image above. Does it have double thickness of foil as shown in that image? When core material is tested to obtain the shear modulus, it is bonded to 20 mm thick face plates. That is because the only flexing that is supposed to occur is in the core. Here is the test apparatus from ASTM C273 -11 Standard Test Method for Shear Properties of Sandwich Core Material.
If you want to simulate the experimental measurement of core shear modulus, you can simply make one face of the core (all the honeycomb edges) be a fixed support and the other face of the core (all the edges) have a remote force, with a Rigid behavior, to pull the core sideways.
Shell elements and beam elements can conduct and convect heat just as well as solid elements, so don't rule shell models out just because you want to do a thermal model.
As I said, I'm not an ACP user, so someone else can comment on that. I will say that I have made models that use carbon fiber sheets. I have them measured to obtain the orthotropic properties so my models flex the way the real part will flex, but I am not interested in interlaminar stresses.
 Regards,
Peter
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Dear Achilleas,
When I was building models of honeycomb sandwich, I built two models, one with shells bonded to a solid core, and one with three solids using shared topology.
They gave different deformations for the same load.
I don't recall what formulation of Bonded contact was used, but I did an experiment to measure the deflection under 2000 lb load during a quarter-point bending test and had a shell model where I adjusted the shear modulus of the core until the FEA model matched the experimental data.
Below is a plot of the experimental Force-Deflection data for three samples cut along the ribbon L direction, and the slope I fitted to the data. Another plot had three samples cut along the W direction, which had a lower slope.
Below is the FEA model, which used symmetry to model one half.
I used Parameter Set feature of Workbench to plot this curve to look up the experimental deflection under the 2000 lb load to see what the shear modulus should be for that material direction, L or W.
In this way, I knew my model accurately matched the experimental data.
Regards,
Peter
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