Composites Basics by A.C. Marshal says that hand laid wet carbon and

glass

have identical tension and compression.

Kevlar is about 60% stronger in compression and 12.5% in tension.

page 10-10
Eric Ruttan

Eric Ruttan,

Good point Eric- why this is so is because carbon needs a very uniform

resin

saturation to achieve it's Maximum strength.
If one area is slightly lean it will then be the weakest point and will

fail

at that point.
Hand lay-up techniques are NOT uniform, no matter how hard you try,
therefore you always have weak points. The weak points would be no

stronger

than e-glass - they could also be weaker or more prone to failure

because of

the catastrophic failure characteristics of Carbon.

You could get an expert to lay-up Carbon (by hand) and you will get a
different result every lay-up - so I have trouble agreeing that Carbon

is

the same as e-glass, however E-glass and S-glass is much more forgiving.

When used in a non-structural area (cowl) this is not a problem.
When used in conjunction with e-glass (as a stiffener) this isn't a

problem

either i.e. flaps, rudder, elevators etc.
George (down under)

I have a great deal of respect for Andy Marshall, and suspect that statement
that hand layups
of carbon and glass are the same must have been taken out of context. Either
one can have
poor properties if not processes properly, but carbon will almost always be
lighter and
stronger in similar applications - but not by any factor approaching 5. The
working
modulus (stiffness) of the composite is incredibly variable with fiber
orientation.
Glass with very axial orientation (as in a pre made material such as
"spartuff" can
have stiffness and strength greater than ordinary carbon composites, but
carbon
with this same orientation has modulus and tensiles comparable to steel. The
statments
about kevlar appear totally off the wall - it is crummy in compression
because the
resin systems do not adhere well, and it is an absolute SOB to work with,
but is
incredibly tough (bullet proof) so it has many specialized applications.
Carbon is
a premium material and can really pay off if you use it properly, but is
almost
wasteful in many aircraft applications. As they used to say about military
tactics
"It all depends on the situation and the nature of the terrain" - Vance

 
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