Possibly interesting balsa test

From: Bill Gowen <b.gowen_at_earthlink.net>
Date: Thu, 29 Jun 2006 22:27:40 -0000

I've often wondered what the comparison would be between a rectangular
piece of balsa and a round one of equal weight in terms of bending
deflection and torsional deflection. Today I decided to stop wondering
and see if I could find out.

I cut 2 pieces of balsa side by side from a 1/4" sheet of C-grain wood.

One piece was .130 x .245 x 12" and weighed .44g. The C-grain was on
the narrow face. It buckled toward the thin side at 180g and toward
the thick side at something over 500g. (this was a hard number to get)

The other piece was sanded round in my post maker. The diameter was
reduced until a 12" piece weighed the same .44g as the rectangular
piece. The final diameter was .195". The buckling load for this piece
was 360g. This was an unexpected result.

Next I set up a cantilever deflection test. I fixed one end of the
round piece and put enough weight on it to deflect it about 5/8". I
rotated the piece a number of times to see if it was stronger
depending on the grain orientation. The maximum difference in
deflection from the best position to the worst position was about
1/16". This was also unexpected.

The same amount of weight on the rectangular piece with the strong
axis vertical was 7/16". This result was expected.

For the torsional deflection test I put a hook in the end of each
piece and a straight piece of wire through each end so that at rest
the 2 straight wires were in the same plane. With a .5 in-oz load
applied to the hooks, the round piece deflected about 23 degrees and
the rectangular piece deflected about 29 degrees. This was expected
except that I thought the round piece would be even better than it was
in this test.

This was all done in a not very scientific way. If you find the
results interesting for any reason you might want to try it yourself.
Bill Gowen
Decatur, GA USA
Received on Thu Jun 29 2006 - 15:40:08 CEST