What affects bending stiffness on a composite stabilizer?

...I just cut two 9" stabs out of spyder foam and bagged them with 2 layers of 1.5 Oz FG layed out on a bias (each layer). The stabs have some flex...Any ideas?

Paul, a big part of the problem is that you laid up both layers on the bias. Great for torsional strength and stiffness, but horrible for bending. You need some spanwise fibers in there to give you some bending stiffness and strength.

Most types of composite cloth have excellent tensile strength and stiffness along the fibers, and excellent shear at 45 degrees to the fibers. The catch is that the shear strength at a 0/90 direction, and the tensile/compressive properties at +/-45 are EXTREMELY low. Glass can be a bit better than some of the other fibers in this regard, but it's still lousy if you try to make it carry loads in the wrong direction relative to the fibers.

What you have is analogous to a spar with an extremely strong shear web, but almost no spar cap. This is not a good way to design for bending!

At the very least, try giving one of those two layers a 0/90 orientation instead of +/-45. Keep the other one at +/-45 for some torsional stiffness. If you expect to see a lot of bending stress on these stabs, you might also want to consider a spanwise doubler (acting as a spar cap) for additional bending strength.

Don Stackhouse @ DJ Aerotech
djarotec@bright.net
http://www.bright.net/~djwerks/