>However within a year of getting there new Kevlar contraptions
>some people where noticing a certain amount of sag or flex with their boats
>sitting on trestles or trailers or when lots of power was put into the riggers,
>the reason was that after 6 months in UV Aramid/Kevlar cloths loose
>50% of their rigidity and all the load was being taken by the carbon
>which was now only half the amount as usual. This years boat models are
>all carbon including the seats, riggers and oars! No flex, higher efficiency.
>
>Regards
>Daniel Armstrong Chester UK
Kevlar and carbon have similar tension properties. However,
Kevlar is relatively crummy in compression. When a 50/50
mixed material is put under high compression, the Kevlar in it may
fail to mush, even though the carbon still holds it in one piece.
Maybe that's what was observed in the boats. One could oversize
the part so that the Kevlar in it doesn't fail, but then the carbon
is stressed to only a small fraction of its compression load capability.
The bottom line is that Kevlar and carbon by themselves are great
when used appropriately, but mixing them together in a weave
doesn't seem to make much sense. In a pure tension situation
their strain-to-failure is comparable, so they can share the load
efficiently. America's Cup sails contain a carbon/kevlar mix.
This works well since a sail is always under tension.
Carbon and Kevlar can be effectively used together in
a compression structure if their fiber orientations
are different, e.g. carbon spar caps for bending with
+/-45 Kevlar cloth skin for torsion. The carbon cap
will fail in compression first, since the bias Kevlar
can have a larger bending deformation without failure.
This would not be true for a 0/90 Kevlar skin.
--
- Mark Drela
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