General question mixing kevlar carbon

I've built a kevlar daysailerand am thinking of trying something bigger. I am studying the engineering aspects of composite construction and will probably use a designer for the final specs however there is one general principle I'm trying to understand. What is the purpose of a kevlar/carbon laminates or mixing them as skins in a cored construction? It seems that combining the two would lead to either a weaker or heavier laminate. Due to their different elasticity, under the right loading conditions wouldn't the carbon fail while the kevlar remained intact? I can see using kevlar as a reenforcement where the loads were primarily in tension but other than that isn't all carbon the way to go?

 

Sparrot,

There are a lot of misconceptions about Kevlar and carbon fiber, and fiberglass for that matter, and this may be a good place to put some things straight. All fibers used in composite laminates have varying strengths and stiffnesses, depending on what they are made of and how they are woven, stitched, or otherwise formed into fabrics. The more working and weaving of the fibers, the more the strength and stiffness are lost. It is very important to consider both strength and stiffness. Strength relates to when the fibers or laminates will break, and stiffness relates to how much they stretch under load. Fibers and laminates that stretch a lot are called low-modulus materials, and those that stretch only a little are high-modulus materials. Modulus refers to the "modulus of elasticity" which is a measure of stiffness. It is calculated by dividing the stress in a material under load by the amount that it has strained (stretched) under that load. Stress is measured in units of psi or MPa, whereas strain is measured in inches per inch or mm per mm, so strain is basically dimensionless. Therefore, modulus of elasticity is the measure of psi divided by inches/inch = psi, the same units of strength.

Fiberglass is very stretchy, and speaking only of unidirectional fibers in epoxy resin (to keep this discussion simple), the best fiberglass unidirectional epoxy laminates may have a tensile strength of say, 40,000 psi, and a tensile modulus of say 2,000,000 psi. This is for E-glass fiberglass, that which is known as "electrical" grade, hence the E. E-glass UDR epoxy laminates have a density of about 0.072 lbs/cu.in. The compression strength of E-glass laminates can be maybe 50-60% of the tensile strength, although the compression modulus is about the same.

There is also S-glass, or in non-military applications, S2 glass. It is about 50% stronger than E-glass, and about 25% stiffer. It is also just a bit less dense, about 0.069 lbs./cu.in. Again, these are tension values, and the compression values behave similarly--strength reduced to about 50-60% of tension, say 75,000 psi, modulus about the same as tension.

Kevlar is a different kettle of fish. It is less dense, only about 0.050 lbs./cu.in. As such, it is lighter than the resins it is usually mixed with so it tends to float. This is why vacuum bagging is so important with laminating Kevlar. Kevlar has a tension strength between that of E-glass and S2 glass, about 60,000 psi, but its modulus is about twice that of E-glass, say around 4,000,000 psi. But because it is so light, it gets an added benefit in that it gives higher strength for less weight--that's its advantage. Kevlar's big disadvantage, however, is that its compression strength is lousy. Compression strength is only about half that of E-glass, say 20,000 to 25,000 psi. Compression modulus, however, stays right up there with its tension counterpart, same as the other fibers.

Carbon fiber is the king. Its tension strength is the best at about 90,000 to 100,000 psi, with a modulus of about 10,000,000 psi. Its density is between E-glass and Kevlar, about 0.056 lbs/cu.in., heavy enough to make it sink in resin, although vacuum bagging is still recommended. Compression strength is slightly lower, say 70,000 to 80,000 psi, and it holds onto its modulus at the same level as tension.

By comparison, lets look at aluminum, say 6061-T6. It is an isotropic material meaning that it has the same strength in all directions. This is as opposed to the laminates above which are orthotropic, meaning they have different strengths and stiffnesses in different directions. The numbers I mentioned above are in the direction of the unidirectional fibers. Going across the fibers, those laminates are only as strong as the resin holding them together. Aluminum has a tensile strength of 45,000 psi, and being a ductile material (it stretches plastically under very high load) it also has a yield strength of about 40,000 psi (the point where it goes plastic). Its modulus of elasticity is comparable to carbon fiber, 10,000,000 psi. It's drawback is its weight, being almost twice the weight of Kevlar and carbon fiber at 0.098 lbs./cu.in. Also, aluminum is often welded, and it loses strength in the heat-affected zone around the welds, losing nearly half its strength in those regions. So aluminum structures have to be engineered for their welded strengths.

Now, I have been a little circumspect with these numbers, using words like "about" and "say", indicating estimated values. Strength and stiffness values are dependent on the fiber content (measured by weight or volume) in the laminate, the higher the fiber content, the higher these mechanical properties. Strength and stiffness values also depend on the fabrics used, and the laminate schedule sequence. Obviously, if you have a 0/90 fabric, where half the fibers run at 0-deg and the other half at 90-deg, you'll get strengths and stiffnesses that are lower than the UDR values, but very similar values in the 90-deg direction, instead of just the resin values. And, just as obviously, if you have a preponderance of UDR in a laminate with only a little 0/90 fabric in it, the strength and stiffness will be determined by the proportions between the types of fibers. You can complicate this further by using a mix of fiber materials, such as S2 glass with carbon, or Kevlar with carbon. Because of the wide variety of combinations of fibers, weaves, materials, and orientations, you can just about tailor any kind of laminate to any kind of specific properties that you might be looking for. This is why it takes a composites engineer to sort all these variables out for any application at hand. It should be noted that these fibers and fabrics, when mixed together in various ways, are called hybrids, and they tend to have properties between the values of the bare fibers themselves.

There are other practical aspects that I have not mentioned that also must be taken into account in any given laminate--the floating Kevlar is one. Another related to Kevlar is that, if it gets damaged, say gouged really badly, it is nearly impossible to fix well. The raw ends of the fiber fuzz up, and cannot be wet out again. The fuzzy ends have to be carefully clipped short and laminated over again with good overlapping repair techniques. Kevlar is also very hard to cut--typical scissors don't work well, and you need special cutting tools to cut it.

Another thing I have not mentioned is cost and availability. E-glass can be found and purchased anywhere. I am less familiar with the latest on S2 glass, but I think you can still find UDR, perhaps less so with woven or stitched fabrics. Perhaps others can chime in here. Carbon fiber is fairly common, although there is a great variety of weaves. The market is volatile, and prices jump up and down pretty regularly. Lead times for small amounts are typically very long, so you have to plan ahead.

When engineering a laminate properly, it is absolutely necessary to know precisely which fibers and fabrics are going to be used, right down to the areal weights, the lengths and widths of the fabric broadgoods, and the ply thickness when laminated, coordinated to the achievable fiber content by weight or volume for the laminators at hand. There are so many variables that it can be quite a mistake to try to generate "standard laminates"--there is no such thing. A structure designed for 55% fiber content is no good for a shop that can only achieve 45% fiber content.

As for your specific application on a larger boat, you should first sort out a lot of the practical and cost aspects first, and that may guide you to something simpler and affordable in the final result. As for mixing Kevlar and carbon, personally, I use Kevlar only as a bullet-proof layer (which it truly is) near the core on the outside skin. It prevents impact of hard objects into the core, but buried deep within the outer skin, it is not subject to miscellaneous abrasion damage. The amount is small enough that it need not be balanced on the other side of the core. I steer away from Kevlar otherwise because it is so poor in compression strength. And boat hull panels do experience as much compression as tension as they bend. Laminates either side of the core should also be mirror images of each other with respect to sequence in lay-up and orientation. You maintain those basics, and you can start your engineering of the detailed laminates.

It does take a good computer program or spreadsheet to engineer laminates well. A good guideline for composite engineering is Eric Greene's book "Design Applications for Marine Composites", which you can download for free from his website: http://www.ericgreeneassociates.com/articles.html. Also, the fiber/fabric suppliers VectorPly have a very good composite engineering program called VectorLam which you can download from their website. You can use their built-in properties or program in your own, or vary and save revised properties based on your experience. It is a very powerful laminate engineering program, available for free download here: http://www.vectorply.com/vectorlam.html.

Well, that's a fountain of information. I hope that helps.

Eric

 

Eric, Thank you for the very informative information on fiberglass and other types. It has answered a number of questions I have had on the subject. Really enjoyed the information. Stan Rasor

 

You are welcome, glad I was able to help.

Eric

 

Wow Eric, Great Post! The Links are very helpful!

 

Glad to help.

Eric

 

While we,re asking questions , will kevlar as a bullet-proof layer hand laid next to the core as the first layer with heavy biax placed wet on wet over it reduce floating of the kevlar? Thanks Rick

 

Yes, probably. One has to be careful of one other characteristic, however, which I should have mentioned before. When you look at Kevlar fabrics as dry goods before layup, they look nicely laid out, stitched or woven, and flat, with the fibers well distributed in the plane of the fabric. However, once you wet them out, the tows of fibers tend to "cord", that is, the clump together in chords of fiber, rather like compacted strings, and so you end up with these chords in the laminate with thick, rich resin spaces between the chords. This is not good. But if you use vacuum bagging, there is enough pressure under the vacuum bag to spread these chords back out flat again. That's why we use vacuum bagging techniques. Just a layer of glass on top of the Kevlar will keep it from floating, but it won't be enough weight to prevent the cording.

Eric

 

Thanks Eric, vacuum bagging 101 maybe out of my comfort zone on a 31' hull. Cording verses no puncture resistance which is the better trade out? Thank,s again, Rick

 

Personally, I don't think you absolutely have to have Kevlar in the hull--it is a luxury. The vast majority of boats are built without it and they survive fine. Remember, you are not supposed to hit things with your boat, so the number one defense against puncture is diligent watch at the helm. And even if you do puncture the core and hull, it usually can be fixed. This is done every day. Also, vinylester and epoxy resins are significant factors in puncture resistance--they glue and stick onto the fibers and core so well that you gain much by using them. So, if you are not up to vacuum bagging, at least use good materials and good laminating techniques, keep the fiber stack flat and well squeegeed when laying up. Don't be sloppy; get an extra pair of hands to help when laminating.

Good luck,

Eric

 

Hauled at the shipyard is a very expensise kevlar hulled sailing yacht....bulletproof.

Ha !!!! It sank when a genoa sheet slipped over the side while the yacht was motoring and fouled the propellor. The genoa sheet dislocated the "P" braket , ruptured the stuffing box and water poured in faster than any pump could remove it...

The kevlar hull held up perfectly.

A few years ago a conventional glass motoryacht T Boned the jetty at 8 knots in the fog. Her bow push in and looked terrible...the yacht was repaired and is back in action.

Many times I wonder why people choose exotics at the expense of other detailing on a yacht

 

Thanks for the info Eric. Confirms what I already guessed.

 

On aramide/kevlar/twaron: If used, I prefer to have it in fabrics where it is mixed 50/50 by volume with glass, to ease laminating or even infusion. It also helps keeping ILSS up.