Honeycomb Failure 1

[silvershadow]

Hello to everyone,
i need to assess the strength of a sandwich construction and i'm doing some simulations using MSC/Nastran. I would like to know how can i check the core for wrinkling, crimping, core shear and so on.
Can somebody of you help me out?

I'm using carbon fibre skins with an aluminum core.

Thanks a lot

 

[SWComposites]

- wrinkling is a facesheet failure mode (under compression stress), not a core failure mode; there's lots of wrinkling papers out there. It is a more or less a facesheet on an elastic foundation stability mode. Most methods are semi-emperical.

- core crimping is a short wavelength stability failure mode; can be predicted using a plate buckling solution that includes the core thru-thickness shear flexibility.

- for core shear, you need to obtain the strength properties of the core, as a function of thickness, from test data. There is some vendor data available on the web for some cores.

couple of NASA refs to start with:
NASA-CR-1457 Manuel for Structural Stability Analysis of Sandwich Plates and Shells
NASA-CR-1999-208994 Facesheet Wrinkling in Sandwich Construction

lots more references out there; do a search at
scholar.google.com

 

[silvershadow]

Hello SWComposites

first of all thanks a lot for the answer and the references you gave me.
Relative to the core properties i've been able to find, experimentally and through the vendor all the data i need.
I'm checking core shear and crimping as well and this, having a PSHELL/PCOMP model is a pretty easy job.

It is a more demanding task to try to consider wrinkling since using a MAT8 formulation i cannot specify any normal stiffness( core compression stiffness) and so i cannot check this.

Any idea how to do it? (other than of course switching to a 3d modelling technique?)

Thanks a lot

Silver

 

[RPstress]

For basic honeycomb sandwich stressing see

http://www.hexcel.com/NR/rdonlyres/...7D8B547CA/0/HexwebAttributesandProperties.pdf

Local sandwich instability is usually done by hand stressing, often with force data from the FE.

Thanks for that 1999 ref., SW.

 

[PanelGuy]

All the info is right on. There are a couple of ggod books, the most basic is Tom Bitzer's book on honeycomb. The Hexcel Technical bulletins on honeycomb, Design and construction are the Cliff's notes, but Bitzer includes how to set up your lab (did it) what fixtures to buy (got em) and other good stuff.

As to what I am going to guess is the inquiry to the effect wrinkling or crimping of the core in manufacture and what it does to your design? It depends on how sensitive your system is to core defects. On many designs these core defects have an effect but it is not critical. Generally you are looking at situations where the skins are carrying the majority of the loading so they tend to have the most effect.

My thoughts anyway, stuff not already covered.

In almost every case where I balanced the skins and cores to fail simultaneously (FUN!)we had load/deflection criteria to pass. In those cases you are generally designing a panel that is very stiff with little deflection and ultimate failure is high enough that minor core ddefects don't matter.