Carbon and kevlar cloth

Why are people using with carbon and Kevlar woven together?

Carbon is very stiff and Kevlar is like nylon (anchor rode) with lots of stretch.
What am I missing?

 

The blend of the two - carbon and Kevlar gives both properties so it is stiff and tough. OK neither one is maximised, but gives a useful blend. There are also 3 way weaves wher Diolen is added and probably others, maybe 4 way weaves. Also you will find differing and custom orientation of fibres for certain or specific applications.

 

Your presumption is wrong. Kevlar is NOT like Nylon. I have no idea where that notion came from. Kevlar is actually quite stiff--that is, it has a high modulus of elasticity, roughly between S-glass and Carbon fiber. Kevlar is also quite strong, particularly in tension, again between S-glass and carbon fiber. Where Kevlar falls down is in its compression strength--it is very low. It is below the compression strength of S-glass and even E-glass (E-glass is the common boatbuilding fiberglass, and S-glass is a higher grade of fiberglass that is sometimes used, particularly in military applications. In fact, S-glass is the military-grade version, and any non-military grade is called S-2 glass.)

Kevlar was once used for sailboat standing rigging because of its strength and stiffness. But as carbon fiber technology developed, it surpassed Kevlar as a rigging material. Kevlar is also highly susceptible to ultraviolet light degradation. Carbon fiber also replaced Kevlar in the making of sailboat sails for these very same reasons.

So, why mix carbon fiber and kevlar in woven weaves? The reason is to get the toughness of Kevlar into a laminate without sacrificing strength, at least by too much, anyway. As we all know from bullet-proof vests, Kevlar is, literally, bullet-proof. So some mixes of carbon fiber and Kevlar may be a good combination if you want a high-strength and stiff hull that is bullet-proof. Where Kevlar falls down in compression strength, carbon fiber can take up the slack. The overall laminate will not be as stiff as an all-carbon laminate, but it will be more impenetrable.

Personally, I have never really liked Kevlar in boat hulls because of its low compression strength. If you were going to use a hybrid material, a mix of carbon fiber and S-glass would make a lot more sense--there are positives all around with such a mix. In boat structures, there are plenty of areas in hull panels and structural joints were compression forces are very high, and you don't want Kevlar in those areas. Also, Kevlar is a devil to repair. If it is too close to the hull surface where it can experience abrasion, good luck in trying to make a fair repair. Kevlar is extremely hard to cut (you need special cutting shears) and once abraded, it is nearly impossible to get the abraded edges sanded down and laminated over. Most people who try to incorporate Kevlar into a laminate usually get its location and placement wrong so that it does not work effectively.

I hope that clears things up a bit.

Eric

 

Thanks Eric, I do use Kevlar but as a separate layer, I am concerned about the interwoven properties.

 

Do you mean properties as to the weave of a Kevlar-only fabric, as opposed to any other weave? That would correspond to what happens with any other fabric. Unidirectional fabrics are stronger/stiffer than stitched or knitted fabrics, which in turn are stronger/stiffer than woven fabrics. I am not aware of any other concerns with respect to Kevlar in this regard--Kevlar is Kevlar, and all the properties of the fibers translate correspondingly into the fabrics. Remember, Kevlar is very weak in compression, which can be its Achilles heel.

If you mean why use Kevlar woven layers alternating with carbon fiber woven layers, it may depend on what the designer or builder is trying to achieve with respect to strength and toughness. The Kevlar will add to the overall laminate's toughness against impacts, while the carbon fiber layers will provide the necessary strength in all directions, depending on the orientations of the weaves.

As with any laminate with a mix of fibers and weaves, it is important for overall stability of the finished structure that the laminate be a mirror image of itself through its central plane. This applies, too, with cored laminates--basically the laminate on one side of the core should be the mirror image of the laminate on the opposite side. This assures that stresses and deflections will be more or less uniform and minimize twisting deflections and stress risers when the laminate is under load.

Eric

 

Jim, I think what you are trying to ask is "Is it possible to mix and match two different types of fabric/cloth together?" If so, where do I place the layer? On top, at the bottom, or in the middle?

 

I have seen 2x2 carbon twill infused cloth with the carbon fibers broken resulting in a floppy hull.
That's what started my thinking.

 

Well, there could be lots of reasons for that. Under what circumstances did the carbon fibers break--during infusion, or after the laminate was put into service? Infusing carbon fiber is not as easy as infusing fiberglass, but it can be done. The hard part is knowing how much resin is in the laminate. Although carbon fiber wets out easily, one has a very difficult time telling where the resin is in the laminate as it is infused--you can't see the advancing resin very easily, so it's hard to tell if you have enough.

Also, was the carbon fiber 2x2 twill the appropriate fabric for the job? Would a 5-harness satin have been better, or perhaps would a plain 0/90 cloth have been better? All are different kinds of weaves with different laminating and handling characteristics. Or, would a knitted carbon fabric have been better, in which all the fibers lay down straight in the cloth and are stitched together, not woven? Finally, was there not enough carbon fiber in the laminate? Thickness is important--you don't want a laminate so thin that it will buckle on its own. Should there have been more layers?

And how does that experience tie into your question about mixing Kevlar and carbon fiber together? Would that have solved the problem? Or, would another layer of carbon fiber simply have solved the problem?

My other comments above are still valid. We don't have enough information from this example to comment further.

Eric

 

I would have guessed that if you know the forces will be say downward, then the bottom should be strong in tension and the top strong in compression.

Mixing fibers with different tensile moduli in the same layer/direction seems like a way to get the advantages of neither.

 

I think the answer to the original question, is that many exotic materials are used in laminates for marketing purposes. Telling the difference between good engineering and sales hype is difficult.

 

Seconded Eric, and completely agree with your other comments. Very often, but less so in recent years, some people think that because carbon is stiff you can reduce the thickness. Big mistake - stiffness is (rule of thumb) cubed with section height. So in reality you still need say thickness X hull but you can reduce the skins from Y (in glass) to Z (in carbon) but you still need the core. That is where the weight saving comes from.

Too many products have in the past shown insufficient return flanges, core thickness, lack of picture framing, etc just because the builder has thought carbon is a wonder material. It is a good material but needs to be understood properly to get the benefit.

Twill weave is intended for complex curvature only, personally I'd probably use plain instead if it could create the shape - there is some movement. Twill will pull enough when forming complex shapes that it can be stretched pretty thin - too thin perhaps?, so an extra layer may be needed. It is important to balance the layup too so say 1 way with stripe to right, one with stripe to left than any further layer back to one with stripe to right.

 

Sorry, I wasn't clear.
My example was a comparison between a hydroplane model with a layup of 2 layers of 2x2 carbon twill x 2 at 90 for 1 and 45 for the second. The second boat had the same lay up in carbon/Kevlar. Both were infused with the same process the all carbon was at 38% and mixed fiber was at 42% I assume because Kevlar holds more resin as has been documented elsewhere.
The mixed failed in service under the same loads, but the interesting part is that under 30x magnification I could see evidence of the Kevlar wearing on the carbon fibers at the break.
It looks like it failed first on the compression side first from the break patterns, but that is not diffident.
The all carbon boats have not had any problems.

 

You could be very right in your observations and conclusions. And as I said before, I never really have liked Kevlar in a boat laminate. As someone else said above, sometimes it's just the hype--and I have seen a lot of that over the years--people will use the materials for the hype and not for sound engineering principles.

Eric

 

Why are people using with carbon and Kevlar woven together?

Because it looks pretty.

 

Kevlar is a specialty fabric and its greatest strength is "toughness", the ability to hang on after a crack has developed. Shear modulus is a property of "toughness". Kevlar is very hard to cut with ordinary scissors or with a knife.

Where to place it? Let us use the shear formula to analize. The Vertical shear formula is =3/2 V/(bh) with V as the vertical shear.

In a uniformly loaded beam, simply supported, the greatest vertical shear occurs at the base of the support (edges). Like a hole being punched out with a die, shear takes place at the edge of the male/female die. So Kevlar reinforcement should be at the base of the panel where it is supported by frames/stiffeners.

A vertical shear is always accompanied by a counterbalancing force (a couple), known as the horizontal shear. For the same load model, the same formula applies and the greatest horizontal shear occurs in the middle of the laminate, the neutral axis, or the centroid of the laminate, where the fibers slide past each other.

Care should taken on the choice of fabric. Uni will not work as it splits easily and its splitting property is worse than carbon. Woven (fabric) works well as at least half of the fabric is at near 45degree angle, presenting a good profile for shear.

With a carbon/Kevlar/carbon laminate, the carbon takes up the tension/compresion stresses and the Kevlar takes up the shear load as it is placed in the middle.

Problem arises when the situation is, say carbon/Kevlar/Kevlar/carbon layup. The N.A. will then be at the middle of the Kevlar/Kevlar interphase which is the bondline. This bondline is pure resin and is the weakest part of the carbon/Kevlar, resin mixture. The Kevlar won't be of any use and delamination failure is most likely.