On Wed, 31 Oct 2007 05:19:17 -0700, Lou wrote:

Well Roger, I was really just looking for a couple of simple
answers. Seeing how this is an experimental hobby, I thought
that I would compare the strengths of the plywood that the designer
put on the plans (old design) against some newer materials and idea's.
It may cost me a few bucks but I want to measure strength and breaking
points against each other.

Wellll... for an unscientific and relative comparison that might be
considered a bit crude but it is based on the way it's done in the
labs: We just have better equipment and measurement techniques. it
should even work with doped fabric.

You can get relative numbers this way to compare and these should work
with fiberglass, plywood, and even spruce scarf joints.

You would need to decided on the spacing between supports but take a
pair of pipes or even 2 X 4s and lay the sample over them. Place a
pipe or 2 X 4 over the sample and then start placing weight on the
pipe or 2 X 4. You can measure the deflection per pound of Kg and
that required for failure. This would also work in two axies on a
scarf joint. Of course with a two layer lay-up or thin plywood the
supports would need to be relatively close together. The important
thing is to use the same spacing and equipment for the different types
of material. You could probably come up with some methods a bit more
specific with a web search.

For pull I'd just take two pair of flat metal plates. create some
coupons or wood sections and clamp them in place on one end with a
pair of plates. On the other end use a pair of ViseGrips to clamp the
metal plates on the sample. Then hang a weight from the ViseGrips via
a scale.Keep adding weight. (be careful not to crush the sample when
using ViseGrips) You can measure stretch/elongation, creep, and
failure this way. It is crude, but should give a pretty good
comparison between materials.

For those not familiar with creep it is simply elongation or stretch
measured over time. This one can be surprising when using fiberglass
and resin at elevated temperatures like you'd get out in the sun on a
hot summer day. You can also find why it's not a good idea to clean
the wings with a solvent like acetone. :-))

I saw a beautiful plane that had this done. The wings went from works
of art to having the outline of every rib visible.

Roger (K8RI)
Lou