Initial Trials –
Breaking Soundboard Samples
Rick
Kemper,
A sample of redwood in
deflection, 55.8 lb load
In
this experiment, I broke 18 sample sections of Sitka Spruce, Western Cedar and
Redwood. Each sample was build into
a fixture that approximated a cross section of the soundboard near the bass end
of the harp. With a one inch
pine liner on each side, each 17” sample had an open span (liner to
liner) of 15 inches. Each sample
was made from clear, tight grain stock with minimal run-out.*
The
fixture was suspended from an overhead beam, a dial indicator was attached to
measure the deflection at the soundboard at the center of the sample, and progressive
loads added to the eye bolt to simulate string tension.
This experiment had
four goals:
1. It attempted to
determine how soundboard deflection corresponds to its load
2. It attempted to
measure whether screws through the soundboard and into the liners measurably
improve the sound board’s load bearing capacity
3. It attempted to
measure the relative breaking strengths and failure modes of Spruce, Western
Cedar and Redwood.
4. To Determine whether
soundboard samples broken in this type of fixture fail the same way as real
sound boards do
A few
practical tips for anyone wanting to try this in their own shop:
·
When sound board
sample fails, it can be a sudden explosive bang. Work carefully.
·
Suspend the weights so
they only fall in inch or two to the floor. Wear steel toed shoes and keep your feet
toes out of the way.
·
Make sure the tie- ins
to the overhead beam are secure and rated to carry the loads you are testing
·
You can get a 110 lb
fish scale and a 2” dial indicator on E-bay for ~$35 each
·
You can use plastic
buckets of Sand calibrated to the nearest tenth of a pound for a load
·
Set up the dial
indicator so the tip is pre-loaded.
The sample should break away from the calipers delicate tip
·
Use a secondary
lanyard to tie the dial indicator and holder to the overhead beam so it does
not fall four feet to the floor
DATA:
For
each material, samples #1-3 were simply glued to pine liners. Samples #4-6 were glued and
secured at the edge with a .210" cherry wood batten and number six by
¾ inch steel square drive screw.
Testing stopped when the sample broke. For example, the #1, #2 and #3 Redwood
samples were able to sustain a 25.8 lb load, but failed when they were loaded
with 35.8 lbs of weight. There are
no deflection data points for loads greater than 25.8 lbs for glued redwood
samples.
The
samples all broke at some common failure points.
It
is interesting to note that the glue was never a point of failure in this
trial.** The samples would often
break near the glue line, but they always left a fine “beard” of
wood fibers on top of the glue boundary.
Most Frequent
Left: Redwood
splitting along the grain, just above the liner near
the glue line. Most glued (only)
samples failed in this mode
Right: Rupture across
the grain at the bending point near the string rib. Many of the samples that were glued and
screwed failed in this mode.
Less Frequent
Left: Screwed Cedar
Sample failed when the liner cracked away from oak bass Plate
Right: Screwed
Many
of the glued and screwed samples had an interesting dual failure mode. At 25-45 loads the sound board would
audibly crack just above the liners but the sample would not fall apart as the
screws held the wood being tested to the fixture. After an addition 10-30 lb load had been
added, the sample would then rupture or break off the liner below the tip of
the screw.
Tabular Data:
Deflection
Tables with detail on failure modes.
|
|
|
|
|
|
|
|
|
|
Load |
#1 |
#2 |
#3 |
#4 |
#5 |
#6 |
Average |
|
0 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
|
5.8 |
0.056 |
0.061 |
0.062 |
0.067 |
0.065 |
0.064 |
0.063 |
|
15.8 |
0.151 |
0.153 |
0.158 |
0.166 |
0.160 |
0.153 |
0.157 |
|
25.8 |
0.231 |
0.250 |
0.245 |
0.250 |
0.260 |
0.231 |
0.245 |
|
35.8 |
0.300 |
0.326 |
0.322 |
0.320 |
0.335 |
0.299 |
0.317 |
|
45.8 |
0.365 |
0.400 |
0.388 |
0.394 |
0.405 |
0.348 |
0.383 |
|
55.8 |
0.430 |
|
|
0.460 |
0.467 |
0.419 |
0.444 |
|
65.8 |
0.489 |
|
|
0.520 |
0.535 |
0.494 |
0.510 |
#1
#2 Failed as #1,
but at 55.8 lbs load
#3 The Sitka
failed above the liner on both sides.
#4 Failed as
#1
#5 Failed in
the liner on both sides, liner cracking just below screw tips
#6 Failed as
#5
|
Redwood |
#1 |
#2 |
#3 |
#4 |
#5 |
#6 |
Average |
|
0 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
0.000 |
|
5.8 |
0.088 |
0.133 |
0.193 |
0.128 |
0.089 |
0.115 |
0.124 |
|
15.8 |
0.347 |
0.288 |
0.373 |
0.274 |
0.369 |
0.264 |
0.319 |
|
25.8 |
0.459 |
0.397 |
0.505 |
0.388 |
0.498 |
0.380 |
0.438 |
|
35.8 |
|
|
|
0.503 |
0.600 |
0.472 |
0.525 |
|
45.8 |
|
|
|
0.583 |
0.695 |
0.554 |
0.611 |
|
55.8 |
|
|
|
0.653 |
0.807 |
0.664 |
0.708 |
|
65.8 |
|
|
|
0.778 |
|
0.754 |
0.766 |
|
75.8 |
|
|
|
0.857 |
|
0.854 |
0.856 |
#1 Redwood failed
at liner at both sides.
#2 Failed as
#1
#3 Redwood
ruptured, cracking near center at edge of “string rib”
#4 Failed when
the screw pulled out of liner on one side
#5 Sample
ruptured into three pieces, failing ¼” in from each liner and at
Center near “string rib”
#6 Sample
ruptured into two pieces, failing ¼” in from one liner and at
Center near “string rib”
|
Cedar |
#1 |
#2 |
#3 |
#4 |
#5 |
#6 |