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Honeycomb Core Poisson Ratio

Honeycomb Core Poisson Ratio

(OP)
Hi, I would like to ask if I use lamination for calculating sandwich properties, what Poisson ratio should I use for the aluminium core?

I usually assume 10 for nomex... But I'm not sure... Thanks!

RE: Honeycomb Core Poisson Ratio

The core properties should not affect sandwich panel laminate properties, except the thru thickness normal and shear stiffnesses. Just use 0. for Poisson's ratio.

RE: Honeycomb Core Poisson Ratio

(OP)

I use my lamination excel tool to calculate the equivalent sandwich E or Bending Stiffness...for Aluminum facesheet and core but it seems not getting same value as the formula below...

For the formula below, is it correct that the E is the facesheet E? Thanks!

RE: Honeycomb Core Poisson Ratio

Yes, both E and nu are for the facesheets.

The bending stiffness formula is wrong. Instead of hc should use (hc + hp). Ie distance between midplanes of facesheets.

RE: Honeycomb Core Poisson Ratio

well, yes, except it should be (h+t) given the labelling in the sketch.
and, yes, it is the facing properties, not the core, that are relevant to these calcs.

RE: Honeycomb Core Poisson Ratio

(OP)
Thanks both SWComposites and rb1957!

So basically, the properties of honeycomb depends highly on the facesheet for in-plane properties... but bending would depend on the height of the core.

I was trying to see if Al sandwich can replace Al solid in term of light weight and more stiffer... but based on the calculation, although the weight can be saved about 20%, the stiffness is smaller except bending stiffness could be improved... unless otherwise using prepreg facesheet ..

RE: Honeycomb Core Poisson Ratio

"the properties of honeycomb" ... the properties of sandwich panel.

yes, a sandwich will have slightly lower bending stiffness than a solid plate, but much lower weight.
To retain the same bending stiffness, the sandwich panel would need to be slightly thicker.

To use "preg" or composite (Carbon, or E-glass) faces in lieu or Al would also restore stiffness, at the expense of material (and handling) costs.

RE: Honeycomb Core Poisson Ratio

Also... a 'trick of the trade'... wherever a stress concentration occurs in a honeycomb panel or component...

The core is filled partially/discretely, or completely, with thermosetting potting compound. This is typically epoxy resin filled with glass or phenolic microballons or 'other low density/rigid fillers/reinforcement materials'... or are injection-filled with foaming [self expanding] epoxy/etc potting mixtures.

These compounds reinforce the filled honeycomb cell [added crush resistance] and rigidize the foil walls [from buckling/shear loads] and increase adhesion contact area and greatly increase resistance to moisture/fluids intrusion [filling voids and increased cell sealing].

Potting is invariably used at fastener [hole] penetrations with or without an embedded insert... and for areas that are machined 'thin/flimsy'.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Honeycomb Core Poisson Ratio

(OP)
Another question about using core such as nomex, aluminum, foam. The stiffness is increasing for Alu >Nomex > foam but in calculation, we assume modulus is very small or negligible in pcomp or 2D analysis. How does the alu core stiffness being captured in the 2D FEA compared to foam core? Thanks!

RE: Honeycomb Core Poisson Ratio

Core properties are a function of the core density. So you can’t just state al > nomex > foam.

The only core stiffness properties that typically matter are the thru thickness shear moduli.

RE: Honeycomb Core Poisson Ratio

(OP)
Hi @SWComposites, does the assumption we use for modeling honeycomb core (E11 ~=0)also applied to foam?

I notice from some literature directly use foam modulus into FEM...

RE: Honeycomb Core Poisson Ratio

Probably depends on the density of the foam core. I’m mostly familiar with honeycomb type cores.

RE: Honeycomb Core Poisson Ratio

(OP)
I am using ansys not nastran to model composite honeycomb. Both facesheet and core are aluminum, and I use lamination method to calculate the effective E, G and poisson ratio. Can I use these data with isotropic material model or I have to use orthotropic material model?

When I tried to use orthotropic material model, I could not get a reasonable results as I set all thru thickness properties to 1E-6. I get the result similar to isotropic model when I use same shear modulus in three axes...

RE: Honeycomb Core Poisson Ratio

sarclee... FYI only...

Aluminum-skin + aluminum HC core + aluminum-skin is generally NOT referred-to as 'composite honeycomb [sandwich Assy]'... it is an aluminum honeycomb sandwich Assy.

To be a composite/hybrid honeycomb sandwich Assembly, at least one layer... one or both face-sheets and/or HC-core... must be non-metallic [fiber + resin].

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Honeycomb Core Poisson Ratio

sarclee - are you modelling the sandwich with a single shell element? Why not model facesheets with shells and core with solid element?

RE: Honeycomb Core Poisson Ratio

why would you do that, instead of using a CQUAD4 with a laminate property ?

RE: Honeycomb Core Poisson Ratio

(OP)
@WKTaylor, thanks for pointing it out. I do agree with you. will be careful of the terms.

@SWComposites, yes. I am modeling the sandwich with a single shell element as I can quickly update the model for sizing.

But I notice a problem in ANSYS - thru thickness issue.
I make assumption for the core properties (set the modulus, shear modulus and poisson ratio to small value in three directions) and then I model the stack up Al facesheet + Al HC core + Al facesheet in Ansys but I notice that there is buckling mode appear in modal analysis which I do not expect it happens.

Same issue happen if I assume the Alu sandwich as orthotropic material with assuming very small value in thru thickness properties for G13, G23.

Note: I obtain E11, E22, V12 and G12 from lamination calculation and I use it to model sandwich in single shell by assuming it orthotropic.

RE: Honeycomb Core Poisson Ratio

Ok, please post the exact properties that you are entering to Ansys, and the sandwich panel materials, material properties and thicknesses of the facesheets and core.

RE: Honeycomb Core Poisson Ratio

And the only thru thickness properties a shell element should need are G13 and G23.

RE: Honeycomb Core Poisson Ratio

(OP)
[UPDATE]
The plate shear modulus unit was wrong. The result looks reasonable after I fix it. Appreciate everyone for your kindness!

0.5 mm facesheet (Al 2024) + 9 mm HC + 0.5 mm facesheet (Al2024)

Based on the lamination calculation, E11 = 7240 Mpa, G12 = 2758 Mpa and V12 = 0.33

It seems the honeycomb core thru thickness properties might be wrong. I am checking the properties ...

RE: Honeycomb Core Poisson Ratio

FYI my practical experience with aluminum honeycomb panels/part-Assys.

I usually see panels/parts designed to withstand ultimate design + a-lot... then typically fail suddenly by localized crushing/crippling/delaminating/core-shearing.

This design practice is simply because of practical material thickness [face-sheets, honeycomb foil/hex-size], cutting/machining, adhesive edging/close-out and a variety of 'penetrations requirements' [thru fasteners, cutouts, etc],etc, etc. The difference between a perfect min-weight design and a practical slightly heavier design is a few percent... while added 'robustness' helps endure 'wild-card' long-term environmental effects [way too many possibilities to discuss here].

Be aware of: (a) damage at/along edges/closeouts, and a other mountings [an amazing variety of reasons]; Or (b) moisture intrusion [or other fluids such as oil, fuel, etc] ... leading to internal corrosion and/or lowered adhesive glass transition temps; or (c) extreme point [puncture] loads or added mass; (d) electrical bonding/grounding requirements; (e) extreme temperature variations/requirements [Example: my jet was designed of -65F to +160F... but has endured actual conditions of -100F to +190F].

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Honeycomb Core Poisson Ratio

I would watch your units ... mixing meters and mm, Pa and MPa.

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