This section is mainly culled from a SALGlider thread about pultruded Carbon
Tube splitting easily that ran in March 2004.
The advice comes from Martin Kopplow (MK) and Mark Drela (MD).
The thread started with my saying that I found pultruded tubes split easily
(when use as wing pylon mounts).
1) MD - I don't understand why a pultruded tube would split apart. The only
stress of concern on it is the shear load during launch, but this is still only about 800 psi -- pretty small for any reasonable epoxy. And
the Avia wrapped boom that I used doesn't have any shear plies in it anyway. Maybe you already had some initial crack in the pylon tube?
One way to better resist splitting would be to completely fill the tube with epoxy when potting the threads. This will add about 0.4g for
the two tubes.
2) MK - When you cut the tube with a saw or blade, you'll inevitably cause
almost invisible small cracks starting at the cutting line. These promote
splitting, because they are stress raisers under shear loads (that go with
bending, too).
Cutting pultruded tubes a little longer, then sanding them to size and
applying a little drop of very thin cyano to the open faces helps to make them a
lot more durable.
3) MD - I always cut CF rods and tubes with a Dremel cut-off disk, with very
light pressure. It seems to give a very clean edge. Using a saw not only trashes
the carbon, but also trashes the saw blade.
4) MK - Yepp, your are
absolutely right, but I've seen such 'alternative' methods being practised,
with the known results, of cause. Even when cutting with the Dremel blade, I
always use the cyano: It really reduces the danger of splitting. For parts
that have to take higher loads, I also wrap the end with a thin carbon thread
several times under tension, then secure it by cyano before releasing the
tension. This is useful for pylons, full flying tail feather mounts and
similar stuff where tubes are slid onto rods.
