To add fixings to composites you generally need a way to spread the load and prevent "rip through".  For small fixings a common simple method is to bond on what is known as "big head" fasteners which is a plate with a captive nut or bit of stud on it.  You can bond and laminate them into place.

Another solution if you are bolting through a solid laminate is to use a load spreading plate on the other side of the laminate with captive nuts or studs on it. 

For cored panels generally you need an insert of some kind, be it aluminium or other metal or resin potted into a cut out void to help spread the load and avoid the fixing crushing the core.

Specifically for cored or honeycomb panels have a look on Hexcel.com at some of thier technology guides. Specifically this one: Sandwich panel technology is great for showing how to put fixings in cored panels as well as joining panels etc.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

wow that panel is certainly impresive!

i wonder how in practical terms you might mount something load bearing onto it? like if i wanted to attach a rope eye onto it and have the stress go through that? would it be best to just glue it on? Is there some way of working out how far you would have to spread the load? like how big the base of the eye would have to be ?

Both Nomex and Kevlar are DuPont registered trademarks for slightly different versions of "Para-Aramide" fibres.

In some instances the manufacturers of the honeycomb call it "Kevlar-Honeycomb" in other "Nomex-Honeycomb" but they are basically all made from sheets of "aramide fiber paper"

 --- relatively short, randomly oriented fibres in some kind of phenolic resin binding agent.

So... yes... they're basically the same thing...

Aramide fibres are fire-proof ... hence the use of "Nomex" in fire-retardant racing clothing or gloves.


My testing setup was very simple ... (sorry never took any pictures, can't think why though)

I put the cored panel sample on a large piece of ground flat stainless steel, on the floor... then I
put a short piece cut of off round titanium bar (it was what was there at the right size  ) 7 mm in diameter on top of the sample, then another smaller piece of flat stainless. then added four pieces of soft foam rubber around for balancing and stepped up on the top stainless steel flat... That's rather close to 75 kg on that ~1 sq cm Ti bar...   

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

Is that not nomex honeycomb? if not how does it compare to nomex?

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

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

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

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

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?

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