Layup strategy advice

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#1

Hi,

I’m building a carbon fiber flight case for a double bass and I would like to ask for your advice on the layup strategy I’m planning to use. I have to reach a good compromise between the weight ( Ideally under 15kg just for the shells) and the strength, as it has to withstand possibly a lot of beating during airplane/car/train transport.

Here’s the layup i was thinking about:

1 layer 0/90 200gsm twill carbon fabric
1 layer 0/45 200gsm twill carbon fabric
1 layer 0/90 245gsm twill carbon fabric
10mm pvc foam core (airex or divinycell)
1 layer 0/90 245gsm twill carbon fabric
1 layer 0/45 200gsm twill carbon fabric
1 layer 0/90 200gsm twill carbon fabric

All this done by vacuum bagging with 50% Vf

What do you think? Is it up to the job?

Thanks

#2

I suggest you to use 5 layers outside and just 1 inside.
Knock resistance is more important than bending.
Maybe 10mm core is too much. I would use 6 mm or two layers of 5 with 1 layer in the middle.

#3

I would use aramides or glass/aramide. No carbon, except if stiffness is a factor.

#4

Glass could be suitable, but heavier. All kevlar of course not, it’s too weak in compression.
Anyway, carbon is what guarantee the most shock resistance.
The purpose of the material, in this application, is not to absorb energy by plastic deformation or fracture, but to be able to keep the shape under load.

#5

I would use glass and Kevlar as well. Carbon performs pretty poorly when impacted by a sharp point.

6 layers of 200 gram glass would be lighter than 6 layers of 200 gram carbon. 6 layers of 200 gram kevlar would be the heaviest.

The structure would buckle before the compression strength of the kevlar could be reached. Also the kevlar would yield a greater laminate thickness which will increase the stiffness. Intermix some glass and kevlar and you can have a pretty durable structure that is the same weight as the carbon.

I had to make some storage cases that had to withstand an impact drop for a certain height containing a certain weight. I used glass and kevlar to meet the objective. The carbon tests would fail if the case landed on the corner.

#6

Get the cheap Chinese carbon/kevlar hybrid off Ebay for your outer layer… looks sweet.

#7

I agree on vou about the Kevlar on the puncture resistance.
But, based on the total final weight (fiber+resin) carbon is actually the best material to rely on.
He wrote that weight is a concern. Otherwise, I also know that I could make a 1cm solid laminate with cotton fibers to be able to withstand the loads.
About the laminate thickness, it is not a problem because we are talking about a sandwich laminate.

#8

One of my customers makes flight cases for expensive paintings. All glass/aramide. He did make some of carbon once, but these cracked on impact, and in airport handling, impact is received regularly.

#9

Evidently, he didn’t use enough layers, or used a wrong/poor resin.
I wouldn’t use glass or Kevlar on an F1 chassis…

#10

F1 chassis doesn’t have much impact resistance to cope with on a normal basis, does it? Carbon is too brittle. You’ll crack it. glass/aramid can handle much more abuse. You don’t need the stiffness of carbon, but the strength of aramid.

#11

You kidding, I hope.
In this case we need stiffness because we have to protect what’s inside the case with little clearance. Any deformation of the case would damage what’s inside it.
Aramid strenght is only in tension.
I’m not aware of any load bearing STRUCTURE that makes use of Kevlar.

#12

I’m not kidding. If you need stiffness, the yes, carbon is the best choice for it’s weight. In protection, its not. Maybe you should build some CF bulletproof vests then, and add some more if you don’t feel it’s strong enough :wink: Is the case maximum size? A bit of clearance would be nice.

So what I’m saying: you can build a lighter case out of aramid/glass then you would with CF. The Aramid one would’t be as stiff, but way better at impact resistance. The carbon one would be stiffer, but you’ll need more fibre to get it strong enough.

(load bearing structures: boat hulls.)

#13

I see… urban legends are hard to be beated…

#14

Roberto, you do mean that nice looking yellow carbon fiber, do you? :slight_smile:

#15

I like the silver one more :wink:

#16

We used to build safety xxxxpits for F1 power boats and always had a multi layer balanced aramid then carbon and then glass on 16mm balsa core layup with the glass at 45deg to the layer it was above designed to break apart eating energy but still stay in one piece saving ocupant, the iside was a heavy aramid to save injury from carbon slivers.
Had boat pilots survive high seed roll overs etc.
So include a arimid of sorts for puncture resistance.

#17

I suggest you do some destructive research :wink:

Take a hammer, and start beating. panel with core, and without, aramid one, carbon one. Use the pointy tip.

Urban legends aren’t that hard, try to beat physics, thats hard :wink:

#18

Last time I saw someone testing in such a way he had to go to hospital to get some stitches (hammer catapulted into his eyebrow…:))

#19

well, standing in the way of your tools is never a good idea. Or one of your colleages standing in the way of your tools…

(if you hit a aramid reinforced cored panel with force, and you stand in the way of the hammer, I don’t think you’ll get away with only stitches… )

#20

It’s all a tradeoff of weight, and strength. Kevlar will help with impact and wear resistance. No need to add tons. Glass helps with impact, but makes things very heavy. For a case, you can make the carbon 10 layers of 10oz, and you are set. However, it’ll be heavy. You have to see what ultimate strength you need for support, and then add kevlar on the outside to handle cracking, or maybe on the inside as well. There is no ONE way to do something like this.