Re: [Paddlewise] [PaddleWise] Kevlar vs. Full Carbon Fiber?

Duane writes:

>  I heard a representative say his company is going away
> from the Kevlar lay-up and is going to full carbon fiber instead. He
> explained that doing the full carbon fiber lay-up at the same weight as a
> regular kayak makes the hull very strong and stood on a kayak deck to
> demonstrate.
>
> The thick full carbon fiber lay-up sounds good, but I am wondering about
> impact resistance in expedition and surf settings. I would like to hear
some
> opinions and explanations about which is better, the usual Kelvar lay-up
or
> full carbon fiber?


It sounds like this company does not know that much about the materials they
are dealing with (which is no surprise).  The three most common composite
fibers are glass, kevlar and carbon fiber.  All have very different
properties and costs, the object is to try and optimize the properties to
the particular application in order to save weight.  If there is not any
weight savings then there no point in wasting money on it.

Fiberglass is relatively inexpensive, has good durability and fatigue
resistance, but it is very flexible compared to the others (that is it has a
low elastic modulus).  The draw back with the low elastic modulus is that on
a kayak hull, which is a hollow shell structure, it will usually fail in a
buckling mode, and the only way to prevent that is to make the skin very
thick, adding weight.  Which is why most fiberglass (and polyethylene, also
with a low modulus) weigh so much (too much!).  The only way to prevent
buckling failures with out the extra thick skin, is to use materials that
are stiffer (that is, have a higher elastic modulus).

Both kevlar and carbon have much higher elastic modulus', allowing a lighter
skin for the same hull strength, but at much greater costs.  Kevlar is tough
and about twice as stiff (less flexible) than fiberglass, but costs about
double (or more), so there is potential for weight savings.  It has one very
severe draw back in that it is not temperature stable, that is it has large
changes the fiber length with changes in temperature, allowing moisture to
be drawn into the epoxy matrix by capillary action, this eventually will
cause delimitation.  This is not as much a problem where temperature ranges
are not too extreme (which is how most kayaks are used), but for use such as
on aircraft structures, it is an issue that must be dealt with.

Carbon fiber suffers the same temperature problem but to a much lower
effect, its major drawback is that it has no impact resistance and it is
very brittle.  Though it is about 4 to 5 times stronger than fiberglass for
similar weight lay-ups.  And the cost!--about seven times that of
fiberglass.  Race car frames have to very stiff and light to be a winner,
and long term durability and costs are less of a consideration if you want
to win races, so carbon fiber is the best choice.

The potencial overall weight savings are somewhat less than the relative
strengths would indicate since much of the weight of composite structure is
from the amount of retained resin, the less resin, the lighter the lay-up.
Also the less amount of material, the less resin ends up in the finished
product, so the higher strength material will result in a lighter structure,
IF you actually use less of it.  If you use the same amount, there will be
little weight savings.  It will be stronger, but the question then becomes,
what is strong enough?  Most kayaks do not fail in normal usage, so why
would you carry around all that extra weight unless you actually needed it
for some kind of extreme usage?  A kayak will only be lighter made with
these materials if they are optimized to the same strength as a similar
fiberglass kayak, otherwise it only adds cost.  You have to use a common
basis for comparison or making a statement about the design is meaningless
(except for marketing purposes).  You could make a high strength kayak out
of fiber glass, but it would weight more than one of carbon, but it would
also cost much less.

If there is no weight savings in the finished structure, than it is simply
academic (or marketing), and a waste of money.  Often manufactures will put
a few pieces of carbon fabric in the lay up for marketing purposes and save
a few pounds elsewhere giving you the illusion you are getting something
special for the extra money.  None of these differences in properties mean
anything if the structural design of the hull is not optimized anyway.  A
good optimized design of a simple fiberglass hull could be as much as 30
percent lighter than a typical hull, at much less cost, than using an all
carbon lay-up.  Substantial weight savings could be gained with both an
optimized structural design and the advanced materials, though at the cost
of durability.

It seems to me the best approach, and the better manufacturers will do this
(some better than others), would be to use some high strength and high
priced materials only in the high stress areas where significant weight
savings make it worth while, and then use fiberglass in the other areas that
take a lot of abuse but are not particularly highly stressed, or used for
overlaying the more delicate, higher strength materials.  This keeps the
cost reasonable and allows some weight savings without sacrificing
durability.  If the structural design of the hull is optimized for low
weight (which also saves the excess material costs), and this kind of
selective use of the materials based on their desirable properties, then the
best and lightest design can be accomplished.

Doing a hull in extra think layers of all carbon fiber seems like an extreme
excess, a waste of money, and frankly indicates to me that either the
designers do not know what they are doing when it comes to material
selection, or it is simply a marketing ploy designed to separate you and
your money with little or not real benefit to you.  IMPO.

Peter Chopelas, PE, BSME

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