F106A

My Feedback: (2)

Hi,
I remember reading somewhere on RCU that Kevlar is better to use in re-inforcing a fuse than carbon fiber. Carbon fiber is great when used during the layup process but Kevlar is better as reinforcement once the fuse is cured.
Just wondering if anyone can confirm this.
Thanks,
Jon

SJN

Use both at the same time

wojtek

My Feedback: (73)

when carbon gives out, it just breaks/snaps .. Kevlar on theother had at its breaking point will crack/bend however will not as easily come apart completely. Much better in holding up under torsion loads .. then again, carbon tends to be stiffer once cured. I agree with SJN, a blend works best .. also, cutting thick kevlar is a PITA ! , good shears are a must

~V~

rcjets_63

My Feedback: (5)

What it comes down to are what we engineers call "Material Properties" specifically ultimate strength, stiffness, and facture toughness.

Carbon fibre has a high ultimate strength (the force required to break it) and a very high stiffness (the amount it stretches/bends when force is applied to it). Fiberglass, Kevlar, and carbon fiber have all got approximatley the same strength and are stronger than aluminum or steel. Carbon fiber is far stiffer than aluminum, steel, fiberglass, and Kevlar. This makes carbon fiber ideal for making a strong, light, stiff structure and is why we use it.

The downside of carbon fibre is that it is brittle (has low fracture toughness). You can easily cut thin sheets or cloth with scissors. Carbon fibre cracks easilly particularly in cases where the force is quickly applied (during impact loading such as a crash, it can shatter). Furthermore, once it cracks a bit, the crack can easily grow longer and a cracked structure (wings, fuselage) is not a good thing.

Kevlar on the other hand, has a particularly high fracture toughness. It is much harder to crack, very difficult to cut cleanly even with very sharp scissors, and stands up well to impact loads. That's why it is used for bullet-proof vests.

So, when choosing a composite material to use as a reinforcement, you look at what type of loads the area being reinforced will like see. Kevlar is a great choice as a reinforcement in the upper wingskin over the main retrct in a Bandit for example. If you land hard enough to break the flex plates, the Kevlar has a better chance of stopping the aluminum frame of the retract unit from cutting through the upper wingskin. Caron fibre, with its high strength and stiffness is a good choice for a wing spar since you want a strong and stiff wing. You can also use it in the fuselage or structure to add strength or stiffness but it would not be a good choice in corners of cutouts since it can crack.

So Jon, strictly speaking, the generalization that it is better to use Kevlar than carbon fibre is not true.

Hope that helps,

Jim

roger.alli

Good information. Thanks for that Jim.

Roger

Craig B.

Great explanation based on education!

Craig.

Edgar Perez

My Feedback: (13)

Any online source for that?

Thanks

BaldEagel

Sorry to be perdantic, but Carbon fibre and Kevlar fibre are both formed into a string material that is weaved together to form a cloth, neither will crack when bent or subjected to a load, they may unravell if subjected to tension, but that is about the only negative both materials have.

The resin used to laminate these cloth's is what can crack, the thicker the resin layer the easier it will crack, with the correct amount of resin both will remain pliable in torsion or compression, both will take about the same amount of shear as well, in a single lay up that is not much, there is no doubt that Kevlar cloth is the stronger of the two, but if not correctly laid up the extra strength will never be obtained, when and if the resin cracks its the skin of the resin at the top or bottom of the crack that will subject the cloth to a tension load by trying to flex this will put a tearing load onto the cloth, (imagine trying to open a door the wrong way) the carbon cloth will fail before kevlar, Kevlar has a much tighter weave and the strands are longer therefore it is able to spread the load over a greater area.

The Polymer chain difference's between a Polyester resin (shrinks and is not flexable) and Epoxy resin (does not shrink and remains flexable) make the Polyester unsitable for any load bearing structure, unfortunatly it is still a fact that Polyester resins (cheaper) are used by us Aeronautical Engineers to the detriment of our airframes.

Mike

Wing spars need to flex longitudinally, but be torsional stiff to prevent turn over and failier, wings themselves need to have torsional resistance to prevent twisting under aerodynamic loads.

Bill Hatcher

My Feedback: (2)

Edgar, Here is a place I use for all my carbon and glass needs.
www.shopmaninc.com

SJN

Great quality products from R&G

http://www.r-g.de/en/

Carbon/kevlar
http://shop.r-g.de/en/Carbon/Aramid/

SCALECRAFT

My Feedback: (13)

Note, carbon creates radio problems.

Steve

Robrow

You can also make the structure as rigid or flexible as you like by increasing or reducing the flexible resin %.

Acrylic resin can also be used in a ratio of 80% rigid and 20% flexible to give a very robust layup that has good compliance properties without fracture.

Rob.

invertmast

My Feedback: (23)

I was always told if you want stiffness or strength, use Carbon. If you want impact resistance to prevent cracking and damage, use Kevlar.

Any truth to this?

rcjets_63

My Feedback: (5)

Mike,

I realize that we are trying not to make this too technical such that it is readable and can be more easily understood by the general population of this forum (as opposed to engineers only), but I think that (as written) you've got a few things wrong.

Composite materials are available in a variety of "raw" building-blocks, including yarns, uni-directional, weaves, braids, and several others. Not all are weaved together to form cloths. I thought it was pretty obvious that when refering about carbon fiber cracking, it means the carbon/resin cracking not the carbon filament only in it's raw form. Incidentally, carbon filaments will crack/break if you try to bend them tight enough). Also, the cloth isn't in any danger of unravelling under a tensile load once the resin has cured. Unidirectional materials aren't in any danger of unravelling under tension, torsion, or shear.

I am trying to see the point(s) of your second paragraph but am having difficulty. Perhaps because it's a collosal run-on sentence, has questionable grammer, as well as technical inaccuracies.

As for paragraph 4, wing spars do not need to flex longitudinally; high longitudinal stiffness is a good thing. As for the spar's torsional properties, the torsional stiffness of the wing comes primarily from the skin itself, not the spars.

Regards,

Jim

rcjets_63

My Feedback: (5)

In the interest of keeping it simple...

If you want stiffness, use carbon fiber
If you want strength, either can be used. Both are good though Kevlar has a slightly higher specific strength (strength to weight ratio)
If you want impact resistance, use Kevlar.

Regards,

Jim

rhklenke

My Feedback: (24)

I knew Jim was going to have the right reply to the inaccuracies in Mike's statements. Even in raw form (i.e., just the fibers without the resin) the two materials have very different characteristics. For example, Kevlar is used in sailboats in both the sails and in the running rigging. The reason being that Kevlar (and Carbon) are very low stretch and that is important in keeping the optimum shape in a sail for maximum efficiency. Carbon, on the other hand, is *sometimes* used in sails, but never in the running rigging - the reason being that the carbon fibers themselves are too brittle and if the sail is bent, or the rope is pulled tight around a small radius, the carbon fibers will break and the rope or sail will part.

This characteristic is evident in composites of both materials in that, as Jim said, a carbon composite will fracture much more easily than a Kevlar one - because as the resin cracks, the crack propagates to the fiber and carbon, being so brittle, will fracture much more easily right along with the resin. Kevlar, on the other hand, resists this fracturing better, and thus Kevlar composites are more fracture resistant. This also applies to cutting the composite. Cut a carbon composite with a Dremel wheel and you'll get a nice clean edge as the carbon cuts (fractures) right along with the resin, but do the same with a Kevlar composite, and you get an edge with lots of Kevlar "frays" (that are actually a pain to deal with) because the Kevlar thread does not fracture easily or cleanly...

Also, as Jim mentioned, both materials are available in unidirectional (or even bi- or tri-directional) fabrics that are *not* woven or twisted to avoid any issues with the fiber "straightening" under load. This is done in Kevlar ropes too, as the core is unidirectional, non-woven Kevlar strands for maximum stretch resistance, but it is covered with a woven nylon sheath for ease of handling and wear resistance.

Bob

rcjets_63

My Feedback: (5)

Bob,

Well written and totally accurate. Bravo!

See you at KY Jets,

Jim

mr_matt

My Feedback: (10)

Jim what charge number are you using while preparing these engineering white paper responses?

rcjets_63

My Feedback: (5)

LOL, I'm charging it to overhead. If you have any questions call 1-800-3825-968. [X(]

Cheers,

Jim

mr_matt

My Feedback: (10)

OUCH Just kidding, I am back in PMO, it is just a reflex reaction!!

rcjets_63

My Feedback: (5)

LOL. Welcome back to the the dark side and extra points to you for decoding!!!! All the best,

Jim

BaldEagel

.

rhklenke

My Feedback: (24)

Jim and I were talking and I mentioned that carbon composites are used in high-performance sailboat hulls because they are so light and strong, but the down-side is, when they fail, the failure is catastrophic:

http://www.youtube.com/watch?v=LA-REPv-ReY

That's OK on a day-sailing America's Cup boat where the tenders follow you around the course, but it can be darned inconvenient in a true off-shore racer. Most off-shore boats use Kevlar or a Kevlar/carbon mix, and some stick with S-glass, a higher tensile strength fiberglass. The offshore environment can be a bit rough:

http://www.youtube.com/watch?v=mnqUY...eature=related
http://www.youtube.com/watch?v=Kr-HQ...feature=relmfu
http://www.youtube.com/watch?v=Kybm_...eature=related

Bob

rcjets_63

My Feedback: (5)

It was great talking to you, Bob, and hearing about these high performance boats. Thanks for posting the videos; they're pretty cool and now I want to watch the upcoming race in October. However, I'm not getting into a body of water any larger than my bathtub when the wind is blowing like that.

By the way, Bob teaches engineering at a college in Virginia. There's some serious horsepower going on underneath that cranium!!!!

Back in the day when I was in graduate school, my composites prof had an R&D relationship with a leading sporting goods manufacturer. Wayne Gretzky has his aluminum hockey stick (sort of the "silver bullet" of hockey) but carbon fibre was becoming more readily available and had garnered a certain sex appeal with the public. As such, there was a big effort to incorporate it into sports equipment (hockey sticks, skiis, tennis raquests, etc) and a variety of prototypes were developed. The manufacturer used their sponsorships with NHL hockey players to perform the testing which was pretty amusing.

Along the line of thought that "if a little carbon fibre is good, a lot of carbon fibre is better", a couple of sticks had blades made out of only carbon fibre. One shattered on the first slap shot, the other ended up with a really neat rectangular hole through it (the exact size of the side of a hockey puck). Another problem was that increase in stiffness caused the players to consistantly shoot wide and they were getting pi$$ed. They'd been usingd wooden sticks their entire life and had learned to compensate for the grreater deflection. They weren't impressed with the sticks expecially when our group of "geeks" attempted to tell the players that the sticks were actually more accurate and the players' compensation was the problem.

I thought the funniest thing was watching the players attempt talk to the R&D guys. Professional hockey players have incredible physical talent but mentally most aren't exactly in danger of winning a Nobel Prize. There was quite the communication barrier as the players would talk about the "feel" and "performance" of the stick and the R&D guys would ask them what exactly they meant. The problem was eventually solved by the prof who developed a sort of translation glossary. My personal favorite was...when he says "feel" the player means "frequency response".


Regards,

Jim

SCALECRAFT

My Feedback: (13)

So, after all this, where exactly did you need reinforcement?

Steve