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How to evaluate sandwich panel FWC performance thro FEM?

How to evaluate sandwich panel FWC performance thro FEM?

How to evaluate sandwich panel FWC performance thro FEM?

(OP)
I am analyzing a cowling of sandwich construction (carbon skins and honey comb core). It is idealized in FEA as shell (pcomp) mesh, presuressied by air loads. How should I evlauate the compressive strength of the core for the pressure loads, through the f06 element forces (Fx, FY, Fxy, Mx, My, Mxy, Qx and Qy) from nastran runs?

RE: How to evaluate sandwich panel FWC performance thro FEM?

i'd've thought that pressure would apply shear to the core, and bending loads onto the faces (tension on one, compression on the other).

RE: How to evaluate sandwich panel FWC performance thro FEM?

You can't do it with shell elements - there are no thru-thickness normal results.  You can either make a model with the core modelled as solids (but in most cases this is gross overkill) or take the shell loads and do a curved beam analysis to determine the thru-thickness normal stresses.  But if you have a non-uniform stress field, such as at the top of a core ramp, you are going to have to get some sandwich panel test data with ramps that fail in that mode in order to get a correlation to simple FWC test specimen data (or just be very conservative).
 

RE: How to evaluate sandwich panel FWC performance thro FEM?

doesn't he get thru thickness shear from the PCOMP card ?  i can remembering entering core as one ply in a sandwich panel ...

RE: How to evaluate sandwich panel FWC performance thro FEM?

yes, you get thru-thickness shear forces from the shell elements, through they are often not very accurate, but you do not get thru-thickness normal (sigma_z) forces/stresses.

RE: How to evaluate sandwich panel FWC performance thro FEM?

yes, but how much out-of-plane normal stress is in the core ? i thought the core is usually shear effective only, and the faces are in-plane effecive only ?

RE: How to evaluate sandwich panel FWC performance thro FEM?

rb - for a flat panel, yes there is typically very litter out of plane stress. But in a curved panel with bending moments, there are crush/pulloff thru-thickness normal forces, and when there are "kinks" in a facesheet, such as at the top of a core ramp, there are also crush/pulloff thru-thickness normal forces resulting from the change in direction of the in-plane facesheet loads.  Now whether the OP has either of these cases I have no idea.

RE: How to evaluate sandwich panel FWC performance thro FEM?

(OP)
Thanks all for the reply.  I have quick question. why flat sandich panel does not show out of plane stress, if uniform pressure applied normal to flat surface?  

RE: How to evaluate sandwich panel FWC performance thro FEM?

"why flat sandwich panel does not show out of plane stress, if uniform pressure applied normal to flat surface?" > it does, it is usually just so low as to be negligible; even 3# Nomex core as a flatwise compression strength above 100 psi.

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