Bonding titanium to carbon tubes does not sound like a homebuilable
technique to me.

You are right the joints will always be the problem. However it is
interesting to study the way in which items such as bicycle frames and yacht
steering wheels are made in one piece from tubular sections in carbon ... I
won't say any more on this. You can find plenty of info on the web.

There are fouth major problems/issues that I can see.

First .... if you layup a tube from carbon you cannot put all of the fibres
along the tube axis. Some will need to wrap around. As carbon fibre is
highly orthotropic (i.e. different material properties in different
directions) then the impressive properties of the carbon fibre along the
fiber axis will not be attained in the completed tube. Don't get me wrong
the carbon will still be lighter and stiffer for a given weight but probably
not by as much as you would expect.

Second ... the properties of carbon composites are strongly dependant on
moisture and temperature conditions. For example the compressive strength of
Fibercotes E-765/T700 24K uni prepreg is 147 ksi at room temperature / dry
conditions. At elevated temperature and equilibrium humidity conditions in a
tropical environment the compressive strength drops to 88 ksi ! Not really
any better than steel. Compressive moduli and any other resin dominated
properties (such as shear) will exhibit similar behavior. Yout need to
design for these low strengths at the extreme environmental conditions in
any composite structure and thus at room temperature you will end up with
larger than required margins and hence more weight.

Third .. composites suffer from microcracking and other issues which will
limit the laminate strains to approx 4500 micro strain at ultimate load (a
rough number ... complicated issue not room to explain here). What this
means in plain english is that you can only use approximately 1/3 of the
potential strength of the material if you want a structure with long life.

Fourth ... composites have no ductility like metals and I would hate to be
sitting in a fuselage made of carbon tubes if it hit anything. The tubes
would fracture and splinter and the pilot would be in all sorts of trouble.
To control this behaviour you would need to add another tougher composite
material such as Kevlar to the laminate (common practise in fwd fuselages
for gliders to improve crashworthiness).







"BobR" wrote in message
oups.com...

Shin Gou wrote:
I am just wondering...if composite materials are stronger and lighter
than steel, why composites tubes aren't glued together to form the
fuselage frame just like the traditional 4103 steel tubes being
welded
together? Any reasons?

Shin Gou
Rans S-9
Warrenton, VA

The better question would be why would you want to use carbon fiber to
build a tubular type framework? The real secret is to utilize the best
strengths of your material and design to that strength. There is no
technical limitation that would prevent using carbon fiber composite
tubes to create a tubular framework. The only limitation I can think
of is the joints for replacing the welded joints of the steel. That
could easily be solved by custom bondable carbon fiber joints. Clamps
could substitute for welded tabs and so on. One thing for certain, you
would never have to be concerned with rust.

One thing though, due to the very limited amount of composite vs steel
involved, you would not be saving a lot on weight.