[Paddlewise] Kevlar/graphite

original message:
>>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.<<

Exactly, when faced with a blow that will break carbon fiber, instead of
getting a hole in the boat (complete with water rushing in) you get a bruise
where the carbon fibers broke enough to let the safety net of Kevlar come
into play but you prevent getting holed. Bruises are not real serious
damage.

>>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
SE MI<<

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.

Matt Broze
http://www.marinerkayaks.com



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