Stiffness Coefficient revisited

From: Bruce McCrory <bruce_at_kbdmcc.net>
Date: Sun, 05 Feb 2006 10:03:22 -0000

Since the latest flurry of postings related to modulus of elasticity
and ways to measure it, I've been building up the motivation to go
through my hord of wood and try the Hunt/Slusarczk program again.
Tonight, after several days of visually sorting my 4#-5# hobby shop
wood for flaws (checks mostly), and bend testing samples from the
better sheets I ran the test on ten sheets that seemed to have promise.

The program translates M of E to a numeric range averaged around 100. I
don't have the math brain to convert this number back to M of E, so I
live with 60 to 155 SC's. When I first used the program several years
ago I tested full 36"x3+" sheets. The first to lean over the good side
of 100 was my 101st sheet. It was then quartered and further sampled.
Since the first decent board, finding stiffer material has been easier.
I've always felt the average sheet was on the high side of elasticity
in this program. Then I compared my own tests of Tru-weight balsa and
found my measurements were 10 to 20-percent lower than the stamp range
(they were in the next lower group). I think I was recording the
fatigue side of the buckle at the time.

The samples were strips from each side of a full sheet then cut in half
for a sample of each quarter sheet. A full sheet test has no connection
to good SC. It only says there is probably a lot of density variation.

Tonight I probably adulterated my testing process by catching numbers
where the fall-off began to slow down rather than at the bottom portion
of the readings. I also averaged the readings of the two bends in each
sample. I think the higher reading is a better representation, since
the batch of possible spars will be final tested by cantilever beam
(deflection gauge).

Anyway, the SC test is very tedious and I've never stopped trying to
figure out a quicker method to weed out culls (under 100 SC). I think
I'm getting closer. The two or three sheets under 100 were tested for
reasons other than potential - they looked nice, felt nice, sounded
good, and/or represented a group of adjoining mill cuts.

What I didn't expect was the variability of density and stiffness
within individual sheets. One that was thin enough to see through was
uniform in color but ranged from 3.4# to 5.5# A couple sheets had good
numbers on one side and junk (70's, but great for A6) on the other. The
rest were pretty consistent in stiffness. I wanted to say that b-grain
was usually better than parallel grain, but some of the higher numbers
were in A and C grain faces. However, good b-grain had good torsion
resistance and I think I found a sleeper for EZB's, finally.

After going through the stock a couple things are glaringly apparent.
Only two of the 10 boards were purchased more recently than 2003, and
the percentage of wood purchased after that date represents 2/3 of the
total. I started buying wood in late 2001. There is far more density
striping in more recent wood, also. My assumption is that most balsa in
the US is grown in Central America.

That's it for now.
Bruce in Seattle


 
Received on Sun Feb 05 2006 - 02:05:57 CET