Carbon/Glass relative strengths

Hi,

I am looking into the possibility of building a boat and in the process of just getting together some numbers and wanted to do a very quick comparason of the relative strengths of carbon and glass construction.

Based upon other boats made out of carbon of a similar design I would use 3 layers of 200gsm carbon either side of a 15mm 80Kg/m3 foam core, this gives me a rough price of carbon in order to make the hull etc. My question is, to get the same strength, how many layers of glass would I need?

I guess I am really looking to find a cost per square metre to compare the two materials in the panels to see if glass would be that much cheaper!

Thanks in advance.

 

Do you want the same strength, or the same stiffness? Are you basing this on high-modulus carbon, or low-modulus (cheap) carbon? There is a vast range of carbon material out there, and only one cheap glass (E-glass)
Steve

 

Strength/stiffness - a combination of both really! Stiff boats are extremly helpful when it comes to sailing, but pretty ueless if you can punch a hole through them!

This is based upon low modulus carbon (i.e. RC200 or XC400 if you are going from the SP product range) and E glass

I should also point out I am only looking for a very very rough idea, If I do go ahead I will be consulting a designer, just curious if thereis much difference in the price.

Cheers

 

Has anyone used glass otside and carbon inside? Will this give a balanced sandwich if you look at the local panel stiffness, but a more robust outer layer?

 

If you use Ampreg epoxy resin and vacuum the laminate you would need 520g/m2 of glass woven roving to achieve the same tensile strength of RC200. To match the compressive strength you need only 435g/m2. For the XC400 you would need twice as much glass. The carbon version would be approximately 1.5 times stiffer than the carbon version. If you use sandwich you could just increase the core thickness to increase panel stiffness. It's also possible to use carbon in the unidirectional stiffness tapes (beam cappings, bulkhead reinforcements). You get quite a good bang for your buck but you have to know what you are doing. Since the carbon is much stiffer it picks up most of the loads and the glass is not doing much. Just puutting a carbon patch onto glass structure for reinforcement doesn't work. The carbon picks up all the load, breaks and then the glass is overloaded and breaks too.

 

So say I had a layup of XC400 - UTC300 - 15mm 80Kg foam - UTC300 - XC400 (one layer of each, uni going down the length of the hull, biax at +/-45) I could replace it with XE450 - UTE250 - XE450 - UTE250 either side of a 15mm 80kg core and end up with a similar strength hull?

If I am reading your post correctly then that is exactly what I wanted to know, thanks very much

 

Almost. The factor is 2.6. So 400g/m2 carbon = 1040g/m2 glass. You've got only 900g/m2. For the UTC300 you would need 780g/m2 glass. You got only 500g/m2. Be carefull with the carbon unis. Do they cover the whole hull or are they stiffness tapes in the hull bottom? If they are stiffness tapes you obvieously have to match the stiffness and not the strength. Otherwise your boat is going to be a banana under rig tension. Also carbon boats usually are toughness critical in the outer skin. That means a few g/m2 of carbon are added to achieve a robust outer skin thickness. Glass skins are thicker and therefore you might get away with less than required to match the carbon strength. To increase the panel stiffness I would use 20mm core for the glass boat instead of 15mm core. You might be able to drop the core weight to 60kg because greater thickness results in smaller shear stresses and therefore you can use a lighter core. It gets rather complicated doesn't it?

 

What about carbon inside the ull and glass on the outside?

 

That is very interesting question!
I saw once the kevlar inner skin and e-glass at outer layer. Apparently kevlar/aramid don't have good properties to go on the outer skin.
What about that?

 

Facts are that aramid, when wet, loses it's catculated strength; Dufour made some 17 years ago the mistake to "energise" boats hulls by applying an aramide laminate between the layers of glass - lot's of delaminations were the result. Aramide - when not special treated - bonds badly using poly resins. Big mistake!

 

Hmm, so that's why... still, some builders use the aramid reinforcements and the e-glass protective layer. Using epoxyes?

 

Yes, correct. And also Dupont treats Kevlar with a toplayer so its bonding specs are better. But use in any case epoxy as a matrix material - that will give the best results. And keep it away from where it can get wet!

 

I did a technical paper some years ago that was published in Marine Technology of SNAME, Vol. 23, #2, April, 1986, pp. 165-174 called "Carbon Fiber Sailboat Hulls: How to Optimize the Use of an Expensive Material". It was also referenced by Larsson and Eliasson in Principles of Yacht Design. One of my references for this paper was another paper that I did before that for the Southeast Section of SNAME, Sept 1983, called "Fundamentals and Practicalities of Carbon Fiber Composites for Marine Applications." It became quite a bible for a lot of people in the marine industry.

I did physical tests and engineering analyses on 5 different laminates using unidirectional materials over foam core (4 samples) and one with balsa core (1 sample). In all the samples, the fibers were always running in the same direction, that is, I did not have any laminates with non-zero orientations.

#1: 3-ply S-2 glass, equal thickness skins both sides of 1/2" Airex core.

#2: 1 x S-2 glass + 2 x Carbon + 1 x S-2 glass, equal thickness skins both sides of 1/2" Airex core.

#3: 3-ply S-2 glass skins one side, 4-ply Carbon other side of 1/2" thick Airex core.

#4: 3-ply Carbon, equal thickness skins both sides of 1/2" Airex core.

#5: 3-ply Carbon, equal thickness skins both sides of 1/2" balsa core.

I studied:
a. computer predictions for strength and stiffness
b. test results for strength and stiffness
c. weight per unit area
d. cost per unit area
e. specific strength vs. specific stiffness (i.e. strength and stiffness divided by weight)
f. a desirability factor
g. Impact tests.

The results were:

Laminate #2 was the overall winner when considering all of the factors above. The all-carbon laminate scored very high for strength and stiffness, but at the time, a very high cost. The cost is still pretty high.

Laminate #3, where the S-2 glass is one side of the core, and carbon is the other side, was the worst laminate. It is not strong or stiff, and it wastes the mechanical advantages of the carbon, particularly considering its price.

In my practice, I always advocate for the balanced laminate. When mixing materials of different strengths and stiffnesses, it is always beneficial to make the skins mirror images of each other either side of the core. You get the most efficient use of the materials. This mirror image technique is quite fundamental to general composite design, and has been proven innumerble times in engineering analyses and tests.

I don't always mix materials, and every design seeks to satisfy different needs. As you are aware, the science is complex, so you have to pick and choose the best combinations for any particular design.

I hope that helps.

Eric