My supplier has something called kevlar honeycomb, as far as I know it is made out of kevlar paper(?) pre-impregnated with phenol resin. It is realyyyy expensive 70euro exl.vat for 2mm x 2550mmx1250mm and 48kg/m^2, he claims that it is the lightest sandwich in the world. I don't have money to buy it, but when I have some $ I will make some reviev of that, this is how it looks like :

You can see this type of material a lot on different expo's and so on. They use it for aero-projects and racecar-seats. mostly as a core-material to add some stiffness but keep te lightweight. 

As a raw core, it certainly isnt the lightest in kg/m3  Some aluminium honeycombs with a large cell size are far lighter.

however what they probably meant is its the lightest core for its relative performance and that may well be accurate.

It is used a lot in the RC model aircraft scene too.  You can make some really light panels with a single layer of light woven glass either side of the core to give a nice stiff bulkhead panel or stiffening panel.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Beginner question: Let's say I make two flat panels. They are the same size, roughly as big as a regular computer monitor.

The best way to compare is with like laminate to start with.

This image from the FSAE website shows the relative performance:



As you can see its quite easy to roughly work out the relative difference between having a core and not.

Cored panels arent always necessary though so it depends on your situation.  Sometimes the added thickness can be a disadvantage, also core materials all have thier own way of being processed into the laminate and often the facilities available to you can limit that. eg you can easily resin infuse Lantor Soric and PVC closed cell foam cores, but you cannot infuse a honeycomb panel as the cells will fill with resin.

With honeycombs, the film adhesives needed for the minimum weight possible are expensive and hard to process, so manually making panels with bonding glue by hand can be heavier than expected.

So its all dependant on what is best for your situation.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Aren't foam core composites weaker in shear? Let's say the top CF panel moves left, and the bottom panel moves right. That core can't be too strong that way, can it?

Hi,

I have done a few small testing parts using both Alu-honeycomb (small cell EC stuff) and similar cell size kevlar HC..

The Kevlar HC is much easier to work with, since the it absorbs a small amount of the normal laminating resin at the ends, ant thus bonds much stronger to the outer CF layers in the sandwich.

The Alu HC material seems to be a tad stiffer, but the outer layers shear or tears away from the core much easier than with the Kevlar HC.

(even when bonded with Permabond special HC adhesive)

Also, the Kevlar HC sandwich, seems to handle a lot more compression distortion, before showning a permanent deformation in the surface layer of the sandwich.

On the small cell Alu HC sandwich, a point pressure of approx. 75 kg/ sq-centimetre caused a 1 mm deep permanent deformation in the surface layer.. This did not happen to the Kevlar test sample. It seemed to deform a bit more than the Alu HC, but returned to a flat surface after the pressure was removed.

For the two small parts I made, there was only a very small difference in weight, with the Kevlar being maybe 10% heavier.

No doubt the kevlar HC is an excellent material, but its use is rather specialized with the prices you pay for it.

(I do not build a lot of ultra-light RC planes or F1 Racing car "tubs"  :rolleyes

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Florin, it all depends on your specific application.  Such severe shear loading would probably not be typical of many common uses of a cored panel.

Dravis, did you do quite a lot of tests on your composite panels??? 75kg/cm2  is quite impressive amount of load. 

I did a crude test of a couple of cored panels for a personal project of mine.  Being insane, i prefer to run things over with cars!!

This is my test panel using 25mm aluminium honeycomb 3.2mm cell and 2mm carbon skins:



and a 10mm foam cored panel with 1mm skins:

Albeit under a normal car!!!   I will be destroying said test panel under my 4x4 later in the week!


Although crude tests (and that was the point for me really), it demonstrates how composite panels can be very strong for certain applications and that a cored panel is a great way of doing that.

The particular reason for those tests was to help determine a initial design laminate for a carbon chassis.  Typical steel car monocoque body shells will likely have steel no thicker than 3mm in structural areas and certainly thinner on a lot of the floor pan areas and such similar steel panels will buckle with ease with a fraction of the load used above.

Sure its broadbrush rough engineering but it proves the point i set out and gives me a base laminate profile i can use in the CAD drawings and simulations and work from there.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

The 1mm skin was a layer of 200gsm 2/2 twill and a layer of 650gsm 2/2 twill resin infused.

The 2mm was an old sheet of 1 layer 200gsm plus 3 layers of 450gsmand a final 200gsm.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant