Hello!

I have a question about  sandwich core technique.
That is...

For example,
If I use  two set of  5mm nomex honeycomb core(10mm thick in total) to  make a sandwich panel,
is it  as strong as  a single 10 mm nomex honecomb core panel?
the  double 5mm sandwich has reasonable strength?
or the double 5mm sandwich is easy to break?


actually,
I want to build a curved sandwich panel.
I tried a 10mm thick  nomex honeycomb core at first,
but the 10mm thick core was too strong to bend.

 I wonder that if I use  a 5mm core,then laminate,bend  and add one more 5mm honeycomb core.
I mean,,  the sandwich is   CFRP>honeycomb core(5mm)>CFRP>honeycomb core(5mm)>CFRP.
If this idea is O.K. I will be able to build thick and strong curved sandwich panels.


Any advice,
I would be happy.

Thanks,







 

That should work.  It is essential to tie the two cores together as you have done with a thin layer of composite.  Otherwise the panel would just easily sheer along the join line.

Use a good structural adhesive/high strength resin on the join.  If done properly then the bond will be stronger than the honeycomb which is a good situation to be in.

The panel wont be much stiffer as a result of 2 cores. The physical extra material will add fractionally to the stiffness, but being in the dead centre, the loadings on it are more even.  

In terms of failure, logic would dictate a bent panel would sheer the core on the side under tensile force while the side under compressive force may have the core crush. That is assuming the outer skins don't fail first.  Without testing it would be hard to see the exact failure path.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

>> Warren san,

Your advice is very helpful and informative!  many thanks!


well...It seems that
Ｉ should be careful about a direction of loading.
loading on a bended CFRP>sandwich panel may cause  inside separation...
hum...
using bended sandwich panel is not that good idea, I feel.
If possible, I should look for a different choice.

I agree with Warren, just want to clear up the following:

>  I wonder that if I use  a 5mm core,then laminate,bend  and add one more 5mm honeycomb core.

>using bended sandwich panel is not that good idea, I feel.

> loading on a bended CFRP>sandwich panel may cause  inside separation...

Do you want to laminate an essentially flat glass or carbon outer skin, bond a honeycomb core to this laminate, and THEN bend it into the final shape and apply more glass or carbon (and then another honeycomb layer and the second outer skin) to lock everything in place? Don't do this! Unless the curvature is very small this will stress and distort the sandwich. It will be impossible anyway if the final part is curved in more than one direction.

That's not "the way it's usually done".

Using two honeycomb layers separated by a thin glass or carbon barrier will certainly work. You can do this, but you need to laminate everything  in the final shape. Either in a mold, or on a plug.
The middle glass or carbon layer connecting the two honeycomb cores doesn't have to be thick and strong, being in the middle of a sandwich it will not contribute much to the strength, it is pretty much dead weight but it is still required. If you have problems laying a bigger honeycomb into or onto your part because it is curved too much you can try to use several smaller pieces and fill the gaps with a resin-microballoon mixture.

Tell us more about your project and we can give you better advice.

As brainfart says, you want to avoid bending the outer layers to form the curved panel.  Making carbon panels with a curve in is fine from scratch on say a curve piece of metal as the mould.  A panel that is bent to shape will always try and atraighten and already being partially loaded and stessed will fail sooner.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

What grade nomex are you using ?

Are you aware of the OX/Flex core,s ?

Is the orientation critical on your core ? one plain will have a LOT more flex than the other, can you make that work for you ?

When you say cfrp are you talking wet lay or pre preg ?

Have done double cores a few times with pre preg and it gives you a very strong laminate but I use a inter laminate ply of carbon with a film glue either side between the cores

>brainfart san,

Thanks for your advice!

>Warren san,
Thanks for assisting!


>Do you want to laminate an essentially flat...

I am going to use  a curved mold then I put CF and core  to the mold.
so  I think there is no problem.  

>The middle glass or carbon layer connecting the two honeycomb cores doesn't have to be thick and strong,
being in the middle of a sandwich it will not contribute much to the strength, it is pretty much dead weight but it is still required.

I see. that is very important information!


>Tell us more about your project and we can give you better advice.

O.K.    its a little shameful though...lol
To be honest, I want to build some wheelchairs.I am a wheelchair user.
My goal is that  to build some unique and relatively reasonable (cost or price)wheelchairs.
for example, more compact folding wheelchair , or more  sporty(which has agility) everyday wheel chair....
I know there are some CFRP wheelchair products in the world.but they are too large (my body is smaller than western people)and too expensive for me.
I feel CFRP wheelchairs on the market are something like HIGH END goods for rich people.
In reality those wheelchairs are lightweight but NOT handy to me(I test-rided the high-end CFRP wheelchair).

I tried to talk to professional CFRP parts supplier in Japan that supplies CFRP parts for Moto GP.
They say Its possible to build CFRP wheelchairs.but also say it will be very costly to　develop the CFRP wheelchair from scratch.
So I gave up to ask that supplier to develop the wheelchair at this point.
I decided to build some wheelchairs as proto-type  by myself.then if these proto-type are good,I might step to collect money and ask the pro to develop CFRP wheelchair.


Anyway
the problem right now is that  how to build up my first wheelchair in real world!
Here is my first original wheelchair that I imagine.



The most important part is legs  and spacer/frame.
They need to be enough strong. I simulated these parts a little.
then I notified that so many CF layers are needed if I build thin legs like these.
But I want to build this type of wheelchair because this is simple , a little compact  and maybe a little stylish.

I tried 10mm core but it was too stiff to bend. so I am going to try 5mm and 5mm core for legs(like image above).
but  if  this idea is not practical, I would  build a different type of wheelchair.



thanks for reading such a long sentence.


 

>f1rob san,

Thanks for asking!

>what grade nomex are you using
commercial grade. maybe 1/8 inch cell 1.8 pcf( I might change the grade)

>Are you awaer of the OX/Flex core,s?
sorry I dont understand.

>Is the orientation critical on your core? one plain will have a LOT more flex than the other, can you make that work for you?
well...I have not purchased 5mm core yet. but I think the 5mm core is flexible so it will work well.

>when you say cfrp are you talking wet lay or pre preg?
I would use  wet lay and vacuum bagging technique.

> O.K.    its a little shameful though...lol
> To be honest, I want to build some wheelchairs.I am a wheelchair user.

Absolutely no reason to feel ashamed, this is a very useful application of strong and lightweight composite parts. Compared to the all the mostly decorative car parts seen here all the time, whose main purpose in life is aesthetics, looking pretty.

As a way to reduce cost may I suggest building all the parts during the experimental/development stage with glass fabrics and foam cores. Once the design is finalized and the molds are done switch over to the expensive materials.

The front and rear arm, for the wheels... not sure if carbon/honeycomb is the best choice here, I let others, more experienced users decide this. These parts have to absorb considerable shock loads, e.g. when rolling over a curb, so they need to be overbuild which results in a rock hard ride. What do commercial wheelchair manufacturers use here? May I suggest UD glass with a few layers of +/-45° for the beginning, this will provide more flex and some dampening. You can switch to carbon at a later date.
All the places where bolts go through the laminate will require strenghtening so you won't crush the sandwich, either make them solid laminate, high density foam, filling the honeycomb with flox, carbon fiber tubes... keep asking as you go along with your built.