Carbon vs Fiberglass test

[schicks]

I am making these rod holder fishing belts for a buddy. He really wanted full carbon, i said ok, figured it would be a good opportunity to do some testing. Here are the details.

both parts infused using epoxy resin.
both parts used quadaxial 24 oz material in glass and in carbon.
both parts got 4 layers.

weight difference:
Glass 183 grams
Carbon 181 grams

there is a noticeable difference in rigidity between the two. Carbon part is more rigid, where the glass has a little flex.

When it comes to jet skis, fiberglass is the material to use in my opinion (might change). That little bit of flex goes a long way when you're sending a ski 10ft into the air. Ive seen a lot of cracked carbon hulls in my day, not so many cracked fiberglass hulls.

Im going to make more test samples and put them on a 3 point bend test to failure. I think i know what the result will be, but im just curious what failure load and mode will be.

 

[Storbeck]

Good stuff.

But, the difference in stiffness was fully expected, the difference in strength would be the interesting thing to me.

Always figured that carbon was more prone to cracking because people take advantage of (or try to take advantage of) the higher strength of carbon to use less of it, and are more likely to push the minimum weight with carbon.

Build a carbon hull at the same weight as a glass hull, if one assumes same quality, and the carbon would be much more durable, no?

 

[schicks]

Im not sure, thats what i want to find out. Everyone talks about how carbon is lighter, but i dont think thats entirely true based on my initial A/B test. So maybe manufactureres are reducing layup to remove weight, but technically equal strength.

 

[Storbeck]

The "24oz" material is literally weight per square yard, and the resin, properly done, is usually a percentage of the weight of the cloth. I'm not sure if carbon and glass use the same percentage there, but based on the results of your layup they did.

So for one thing, nice work I think this indicates you are doing a really good job with your layup (and it shows, those parts look great).

I think this all ends up with the two parts weighing identical because they should (and it's always nice when stuff works out the way it is supposed to).

All of that is theory though, actual testing is always best, I think you are taking the correct approach.

 

[kevbo]

The only reason to use carbon is for weight reduction. This weight saving is expensive, nearly $100 per pound. For a given dimension, carbon will weigh less, be stiffer, and "should" be a little stronger than glass. A hull doesn't stress the fiber nearly as much as it does the resins ability to stick to it. Do all your testing in bending and don't worry about tension or compression.

 

[te4250]

Carbon fibers have higher Young’s modulus, tensile strength and lower density than glass fibers, it’s not a conspiracy. The performance of a particular composite part has way more to do with the manufacture methods, layup schedule, appropriateness for the given loading than carbon vs glass. The mass efficiency of a fiberglass part purely in bending, with fibers mostly oriented along the bending axis, and 1” of aluminum hex core separating the upper and lower layers, will be way way higher than that of a carbon fiber part with a more isotropic layup and no core. Specific strength of a similar layup schedule in a similar loading scenario can easily vary more with the amount of epoxy used than what type cloth was used, even how much moisture the composite part has absorbed between manufacture and testing. Iirc carbon parts would take something like a 30% hit in strength after becoming saturated sitting in normal humid air for a while.

 

[te4250]

Not to say the parts don’t look good and you shouldn’t do some testing on them, just saying composites is a big can of worms haha

 

[tipafew]

Tem called my ski “the ice breaker” when he built it and it was still only like 63lbs. Only one way to find out for sure which one is stronger. Bumper car time.

 

[Z.Bx2]

The glass lay up is heavier due to the fiberglass being able to saturate greater than the carbon weave. More resin in the layup= more weight . Its also important to understand carbon layups and weave directions. Bi axial, uni directional are my commonly used at work, uni direction is only used as a strut in the direction where reinforcement is needed. When i do laminations i used as much vacuum as possible, 21 pounds or so if i can. The strength of all carbon products is in the carbon weave and its intended use, not exactly the resin used.

 

[Wetbuffalo]

The glass lay up is heavier due to the fiberglass being able to saturate greater than the carbon weave. More resin in the layup= more weight . Its also important to understand carbon layups and weave directions. Bi axial, uni directional are my commonly used at work, uni direction is only used as a strut in the direction where reinforcement is needed. When i do laminations i used as much vacuum as possible, 21 pounds or so if i can. The strength of all carbon products is in the carbon weave and its intended use, not exactly the resin used.

I see on some of the RC model plane sites that a lot of the guys that are doing hand layups with vacuum bagging have moved to using infusion epoxy. This is due to the fact that they feel they can get a good wet-out with less resin because of the viscosity difference. This translates to lighter parts like you stated because less resin is used.

 

[CBiDrive]

Carbon parts weigh less because it only takes a fraction of the material thickness to achieve the same strength as a fiberglass layup. If you use the same weight material you will end up at nearly the same weight part. This is why you don't really see much fiberglass in the standard carbon 5oz weight. It would just take to many ply's to get the strength required.

The only place on my ski I used anything heavier was the tray floor and the nose cap handlepole mount (5 plys 20oz carbon fiber yielded over a 1/4 in thick for handlepole mount and 2 plys 20oz and 3 plys 5oz for tray). If you clock your fabric correctly with carbon you can get amazing strength out of it at half the thickness of fiberglass. I prefer vacuum bagging myself as it does remove all excess resin and compacts the fabric to eliminate voids. Infusion is nice but expensive to set up.

All I did was chase wake surfers around to launch off of and never saw a crack appear anywhere after 7yrs.

 

[Quinc]

@schicks how did the testing go?

 

[Carbon Blaster]

This is a super tricky conversation as there are so many variables, I'll give you some examples, (all are made infusing with epoxy resin):
A 200gm carbon cloth infused with 18% resin weighs exactly the same as a 200gm fibreglass cloth infused with 18% resin.
The carbon will feel stiffer than the fibreglass if it's just flat plate.
The carbon will feel much stiffer than fibreglass if it has curves in the shape.
The fibreglass will flex much more than the carbon.
The carbon will snap before the fibreglass will crack.
Now to complicate the crap out of things:
2mm solid carbon is many times stiffer & stronger than fibreglass.
2mm carbon is stronger than mild steel plate of the same weight (not thickness & this greatly changes as you go thicker with carbon).
After 2mm of solid carbon, the strength is exponentially increased compared to the strength through the flex of fibreglass.
The type of resin used will make an enormous difference to tensile strength. You can use epoxy that has a tensile rating of 30 mpa, we use epoxy tested to 98 mpa, so epoxy has a huge effect on the strength of carbon.
The carbon layup will have an exponential effect on strength & flex compared to the identical layup in fibreglass. (Eg: 200 uni-directional is much stronger/stiffer than 200 uni fibreglass in combination of layup).
All the carbon hulls I've built will have different types of carbon, laid in different directions & at very different thicknesses depending on the placement of the hull, hood, etc.
So you can see everyone can be correct with their personal opinion or finding, based on what they've experienced themselves, but you can make carbon lighter, stronger, with the right amount of flex than a fibreglass product.
When I was designing the layup for the 20 plus carbon parts in the carbon blasters, we had over 50 test layup samples made in many carbon types, thicknesses, etc & had them tested at Boeing, then designed the layup based on the test results.
The hulls were many times stronger than fibreglass & just shy of 38kg lighter.