REFLECTOR: Carbon fiber vs. E-glass

["Al Gietzen" <alventures@email.msn.com>]

Simon says:


>Good and sound tips, Al. I did something similar, but didn't go all the way
>accross over the top. On the other hand I used carbon in lieu of triax.
>Even lighter for the same stiffness.


Generally true.  At the same time, I will use this as a reason to offer
another perspective.

I did a rough calculation and found that I would save about 10 oz in cloth
and epoxy on the beams I put in had I used carbon fiber.  The carbon cloth
would have cost me 4-5 times as much.  The weight difference assumes that I
would get a benefit equal to the difference in tensile strength.  But would
I?

Before assuming that carbon is better, lighter, stronger; one needs to
consider the following.  Unless you use an epoxy with higher modulus, or
post cure; the compressive strength of the material is about the same.  So
the thinner layup of carbon vs a thicker layup of triax is weaker in
compression, not stronger.  Even in tension, for small strain, most of the
stress is carried by the matrix; and comparing a woven carbon cloth (where
the fibers are not straight) with triax (which is knitted so fibers lay
flat) a larger strain is needed in the carbon before the fibers are taking
the tensile load.

Also in deciding to use carbon, be aware that it is not as "tough", i.e.; it
is brittle and more likely to fracture under impact loads.  And consider
what the carbon plies are being bonded to.  Perhaps the bond to the existing
E-glass is the weak point.

There is no doubt the carbon has seriously better tensile strength than
E-glass, I think about a factor of three for comparable weight.  If the
overall structure your dealing with is designed to take advantage of its
properties; there can be significant weight reduction.  But to use a carbon
layup here and there in our airplanes without a pretty careful evaluation of
the loading, is likely to be, in my opinion, mostly a waste of money.

So there you have it; another perspective.

Al Gietzen