On Mar 11, 2:45 am, wrote:
" wrote:
Amen. But, having been tangentially involved in a completely
disastrous attempt to bond Kevlar (tm) fabric to PVC foam with epoxy I
strongly advise getting advise from the fabric provider before
bonding. ...

I wonder why. Incompatible binder in the cloth?
One issue with both carbon fiber & aramids (you're right, Kevlar is a
trademarked brand name) is that the cloth is much lighter than
conventional fiberglass... duh, that's a big reason to use it... but
it also means that the cloth tends to float up out of the resin. ...

Good points. Kevlar has been used with great success. I can only
speculate on the reasons for the failure. I was in the shop where it
happened re-building my dagger boards and the wing was going together
on the other side of a hull of a big cat that was also building. So,
I didn't see every detail. They were building it using Kelsall's KSS
system with Kevlar twill, epoxy and pvc. They bonded one face on a
flat table with a wet lay-up using modest vacuum to clamp it. Then
they cut darts in the pvc, draped the part into mdf formers and hand
laid the inner skin. That's sop for the KSS system and it generally
works quite well. In this case, though, both the hand laid skin and
the bagged skin could be peeled off the pvc (failing at the glue
line). They did it all a second time on the assumption that the
problem was a bad epoxy mix and it failed exactly the same way. Other
parts that came off the same table using pvc from the same batch
before, during and after were fine. I was using the same epoxy to
build my boards and it was also being used in a couple of other
projects around the shop and it was fine. It's possible that they
could have contaminated the pvc skins (boat shops aren't exactly clean
rooms). That's what the guy building the plane though. But both
sides twice? I think that using twill rather than stitched cloth and
bending the parts around very tight molds (the guy wanted to mold
right around the leading edge for some reason) were contributing
factors. But whatever the reason, the epoxy/Kevlar joint was the weak
one and I think that is cause for concern or at least careful testing
when using Kevlar.

... Carbon
is less of a pain to work with but you can't use it to armor existing
hulls.

It would help add compression srength as an outside layer. I dunno if
it would help with impact resistance. ...

The usual argument against carbon reenforcement is that it is so stiff
that it will fail before the glass takes any load. At which point it
might be more sensible to just do away with the glass. Price and
compatibility wise "S" glass might be a better option than either
Kevlar or carbon. In a crash you want strong but flexible to absorb
energy.

-- Tom.