Talk Composites - The Forum for Advanced Composites

Using thin honeycomb panels as skins in a very thick honeycomb panel

http://www.talkcomposites.com/Topic38056.aspx

By konaMike - 10/1/2020 4:49:59 AM

Hi fellow members,

I am building a structural rectangular carbon box beam 2.5” thick x 6.0” wide and 60” long. The structural objective is to equally maximize the specific bending (in both the vertical and horizontal axis) and torsional stiffness of the beam using no more than 800 grams of cured material. In the application for the beam there are no significant point loads and load distribution can be considered to be uniform across the entire surface of the object . Other factors such as the ultimate strength, fatigue strength, surface finish, layup, sourcing material, etc can be ignored for the purposes of this inquiry. At this point in the process I am only concerned with identifying the form that will produce the stiffest and lightest beam. As my background is more in the area of materials engineering, I am seeking the advice of forum members with structural experience. Having said that, my sense is a closed edge sandwich panel with uni-carbon faceplates and a carbon honeycomb core may be the best approach to achieve this objective. Would anyone disagree or suggest another approach?

Also, if a sandwich panel construction is the best approach, owing to the 2.5” thickness of the panel, I am having doubts about whether the standard “thin face sheet” sandwich panel construction will produce the lifghtest and stiffest result as opposed to a “double sandwich” design in which the face sheets of the main panel are also sandwich (perhaps with nomex core) panels albeit very thin - say 0.25” thick. My theory is that since “face panels” will be 10x stiffer thin face skins and only marginally heavier, this approach will allow the cell size of the main 2.5” thick panel core to be much larger and therefore much lighter. For the 2.5” core I am considering using “end-grain” pre-cured laminate sheet so my second question is whether the cell shape should be a hexagon. My sense is that a set of circular cells (short tubes) bonded to adjacent cells along their tangents would be lighter, have better bucking strength and provide superior shear transfer?

Thanks in advance for your consideration and assistance.
By SleepingAwake - 10/2/2020 9:13:16 PM

 
konaMike - 10/2/2020 5:37:57 PM
 
f1rob - 10/2/2020 9:31:22 AM
how about starting with a foam core too,in its simplest form you can cut it along its lengh in both the vertical and horizontal plane
carbon wrap each off these smaller farmers before joining an then wrapping the whole assembly.
you now have a caton tube with 2 internal webs, if you leave the ends open you can remove the foam too lighten it even more.
you also have the option of cutting the foam into as many sections at the start and having a many webs as needed

I hear you and my sense is that honeycomb or pure foam may not be the lightest approach beyond a certain panel thickness. This is the open question. Perhaps the webs you have suggested could also be panels to increase their buckling strength? I have actually tested a similar approach with a urethane foam core. I moulded the foam plug using two part urethane foam. It came out at 550 grams, so a little heavier than 2 lb/cu.ft. But 550 grams for the core is 70% of my total target weight...so way too much. I then drilled out a few 1.5 inch diameter cores using a whole saw. Each core weighed 2.61 grams which is slightly heavier than the equivalent diameter carbon tube  of 1.5 in dia x 2.5 in len x 0.007 in thk wall. This is when I realized that by replacing the foam with carbon tubes, it may be possible to reduce the core weight by 50% or more and still increase its shear strength. The problem of course becomes that the out face skins must be stiffer to span across the large diameter tubular cells, so why not convert the skins to thin panels made with nomex honeycomb? 

well at a certain point you just have a tubular structure and not a sandwich panel anymore. top skin a thin sandwich with a rohacell or nomex core, bottom skin as a single skin (on traction side) and on the sides just a +/-45°single skin to transfer shear loading and give torsional rigidity. but in that case i would switch directly to BIM as a production method. And just like that you are back to incremental improvements that you didn't like in the bike industry BigGrin