Re: [Paddlewise] Kevlar/graphite

> >>Date: Tue, 8 Feb 2000 14:59:32 EST
> >From: Phlopz_at_aol.com
> >Subject: Re: [Paddlewise] carbon-Kevlar weave
> >I am suspicious about the structural efficience of a mixed weave. If the
> >fabric is on the surface of a deformed laminate, all of the fibers will have
> >the same strain. The stiffest fibers will carry most of the load. (stress =
> >strain x stiffness).  Thus, the stiffer carbon fibers in the weave will have
> >the highest stress and carry most of the load. The kevlar won't get involved
> >in carrying the load until the carbon fibers break.<<
>

You're assuming that the carbon fibre is stiffer than the Kevlar.  That can be
avoided by choosing the right type.  See:

http://books.nap.edu/books/0309044790/html/36.html#pagetop

for a list of fibres commonly used in composites (plus a few exotics.)  As you
can see, there's a low Young's Modulus CF (P25) that is comparable to Kevlar
49.  It's strength etc is fine.  I have no idea of the cost/availability of this stuff,
but in principle, the problem you describe can be alleviated.

> >>From this assessment, it appears that, to make best use of the carbon
> fiber, the outer surface should be all carbon fiber. Using the more elastic kevlar
> in the weave could prevent catastrophic failure of the structure, but the
> stiff carbon fibers will fail at smaller deflections --before the kevlar is
> called upon to take its share of the load.
> On the other hand, if the kevlar were laminated between skins of carbon
> fiber, the majority of the composite stiffness would come from the stiff
> outer skins. This would be sort of a thin sandwich structure, one with a
> core of kevlar instead of foam or honeycomb paper.
> bob phillips
>
> The problem of this approach is that the differing elasticity of the layers
> will cause delaminating between layers due to the shearing forces created
> when the materials flex differently when the laminate is deformed. This is a
> major problem with cored structures especially in a curved laminate like a
> boat hull. Kevlar is usually used on the inside layers of a mixed material
> laminate because most blows come from the outside putting the inside layer
> under tension. Putting the Kevlar layer between two graphite layers would
> put the brittle graphite on the inside as the tension layer and it would
> fail and delaminate with a hard blow even if the other two layers held up.

If you choose the modulus and relative thicknesses of the layers carefully,
you can minimize shear stresses due to stiffness differences.  My understanding
of the reason for mixing Kevlar and CF was that Kevlar is good in tension, while
CF is better in compression.  Mixing the two balances the strengths and weaknesses.

I had a few crazy ideas -

Is a three dimensional weave a viable alternative?

How about sewing together prepreg layers of CF and Kevlar with tight
stitching?  The thread would have to be something like Kevlar and really
heavy duty (I'm guessing - too lazy to guestimate the shear flow in the
laminate - what kind of stress levels are we talking about here anyway?)
Once done, lay it in a mold and autoclave it.  $$$

Mike

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