Carbon Fibre/Epoxy vs fibreglass/polyester

[carbongr]

We are building small boats (5,50m) using the usual method (hand lay up), CSM fibreglass and polyester resin.
We lay up 5 layers of 450gsm CSM and 6kg of Polyester resin per sqm in total. The thickness of the finished laminate is about 6mm. After this step, many reinforcements (wood panels) are bonded to the laminate with CSM and polyester.
That is for the Hull. The deck is slightly lighter but the idea is the same.

We are looking into building the same boat using Carbon fibre, epoxy resin (Hexion RIM235) and infusion process. Our engineer claims that 2 layers of high strength 2x2 1000gsm C/F placed at 0 and 90 degrees will have the same (or even better) results as the 5 layers of the 450gsm F/G CSM and polyester resin.
That is for the skin of the boat only. The rest reinforcements will be the same.
According to his calculations the total thickness of the skin will be 2mm and the weight about 3,20kg/sqm (when the current weight is about 8,25kg/sqm, and 6mm thick).
The available time for the infusion can be more than 6 hours and the cure will take place in a large oven at 60 degrees Celsius. (we can heat it up more, but we are concerned about our moulds which are made with polyester resin).

Is this feasible? I mean can the 2 layers of 2x2 1000gsm C/F infused with epoxy resin have the same or better characteristics that the 5 layers of CSM fibreglass wetted by hand with polyester resin?

Generally speaking, is it a good idea to use carbon fibre for the hull's skin? Or due to the fact that C/F is brittle we may have serious problems?

Finally, what method do you recommend to use in order to check every laminate after de moulding it, in order to make sure that there are no imperfections (such as dry spots)?

Thank you very much in advance.

 

[RPstress]

Quite a few people make speedboats and similar using carbon for the hull. Gurit's website has a fair bit of advice, and Eric Greens's book is very good at

http://www.marinecomposites.com/

.

Two layers of 1000 gsm carbon poxy should be about 2 mm thick at 50–60% fibre volume fraction. 2x2 twill will have a modulus of about 60,000 MPa at 0 and 90 and about 11,500 MPa at 45°.

A likely undamaged strength is hard to estimate. At 0° and 90° it might get to a 1000 MPa or even a bit more at rom temperature, but a more sensible allowable would be maybe 250 MPa. Allowables are highly dependent on industry practice. In the 45° direction it might be more like 90 MPa.

The glass CSM is probably going to have a modulus of about 7200 MPa at 35% fibre volume fraction. It's strength is likely to be about 100 MPa. Again, it's very hard to quote an allowable. A sensible allowable might be about 40 MPa.

Stiffness-wise, then, you can be 7200/60000 = 0.12 times as thick for 0 and 90 directions, but only 7200/11500 = 0.63 times as thick in the 45° direction. Putting in a ply of carbon at 45° woulkd make a big difference to the properties in that direction.

Strength-wise you've got something like 40/250 =~ 0.2 times in the 0/90 directions and 40/90 = 0.44 times at 45°.

Some aspects of design depend more on the bending properties than the in-plane ones. Because the glass is so much thicker than the 2000 gsm or carbon it will be quite a lot better off if there's significant bending of the laminate. Bending stiffness goes with thickness cubed and strength with thickness squared.

Based on in-plane properties, then, having the woven carbon at one third the thickness of CSM glass isn't a mile out.

Bear in mind the proposed layup is orthotropic (low 45° properties) and the CSM should be roughly isotropic. Details of the design will dictate whether the thinner skin is acceptable. Also, the thinner carbon will be more easily damaged than the glass when the boat hits things.

 

[Qshake]

Agree with RPstress in that given the limitations of what we know of this application, the in-plane loading will likely not be an issue but bending or out-of-plane loading might for such a thin make up.

Serviceability issues might arise and force the laminate to thicker arrangement.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[mattyduk]

Just out of interest what sort of boat is it? Power sail etc. I only ask as the application has a massive impact on if CF is suitable?

 

[berkshire]

There are two articles in this months Professional Boat builder (

http://www.proboat.com

) one on page 32 and another on page 58 that address some of your questions.
B.E.

 

[gwolf2]

Adding a central third 45 degree carbon layer would be a wise precaution to get some strength in all directions.

I agree with RPstress' seemingly pessimistic overall strength values, it's all about the real world imperfections which reduce the theoretical strength.

It won't be as tough in bending, especially for local impacts because it is much thinner.

If there are any badly designed joints/intersections Like T's or L's the low number of ply carbon design may find them whereas the more crude, thicker glass design gets away with it.

If you go for carbon consider adding doubler plies at load points and intersections. Also consider adding raised flanges (stringers in aerospace parlance) to large panels where the reduction in thickness will reduce the bending stiffness.

gwolf.

 

[allcomposites]

Note that infusing a 50% Vf carbon laminate over a long distance is kind of tricky (if you don’t have good processing experience). Moreover due to the small thickness of the laminate and resin shrinkage, you will observe important shape distortions in the hull.

I will suggest using a perforated foam core (such as NidaCore), which will give you the flexural stiffness and geometrical stability. For such part, a 3/16” thick foam core will be enough.

You can also add a thin glass layer on the external surface to absorb small impacts and scratches which may damage the carbon fabric.

allcomposites

 

[carbongr]

Thank you all for your advices.
The addition of an extra 45°/45° layers should definitely make things better. However, it will increase the cost of the raw materials. But we will definitely consider it.

On the other hand and as allcomposites suggested, we will try to infuse a core between the two layers of C/F. A layer of Soric XF 2mm mat for example should work well as an infusion medium, too.
What do you think?

 

[allcomposites]

Lantor Soric is a very good option too. It is flexible and simplifies making non-planar surfaces such as a boat hull (much easier than NidaCore or Rohacell). However mechanical properties will be lower and you may observe important thickness variations on bag side, but this may be ok with your product.

You can also request small samples and make a couple of flat panels to compare different core materials (they use to send such samples for free).

allcomposites