https://woodgears.ca/wood_strength/

Testing the strength of different wood species

[spreadsheet] If you just want the result spreadsheet, Here it is

[strength02] After I Built my wood strength tester, two fellow
YouTubers offered to send me some samples of different wood species.
These ones are from Peter Collin, already cut to size.

[strength04] John from FarmCraft 101 sent me these samples. Jon
wasn't aware of the high cost of postage to Canada, so he sent some
bigger pieces to cut samples out of. Postage was over $100 USD.

[strength06] I also had some samples of my own to prepare. I used to
have a better selection of different woods, but foolishly I didn't
bring my wood stash with me on the last move in 2018.

[strength08] I planed all the pieces down to 17x17 mm. Peter's pieces
were 20x20 mm, which would have been better for testing with, but the
stepper motor driven screw jack on my machine will only exert about
250 Kg reliably, or 550 lbs. 17x17 mm was about as large as I could
go and still break the pieces.

[strength10] I measured the length of each test piece, then weighed
them all. From that I worked out the density relative to water for
each piece.

My densest samples were Osage orange (0.93), Ironwood, a.k.a.
American hornbeam (0.93), Dogwood (0.83), Hickory (0.80), Beech
(0.76), and Hard maple (0.75).

The least dense samples were Cedar (0.31, 0.36), Pine (0.33), Spruce
(0.33) and Basswood (0.37).

These were all for individual samples. There is considerable
variation in density between pieces of wood of the same species.

[strength12] Test piece loaded into my tester. The screw jack pushes
up in the middle and the test piece is held down by the round pieces
on either side. These are 24.5 cm apart, center to center.

A stepper motor turns the small gear, which turns the large gear
clockwise. This threads the nut downward. The nut pushes against a
bearing below it, pushing the threaded rod up.

The screw jack sits on a platform with four bathroom scale load cells
to measure the force. The load cells are tared with the screw press
already on them so there's no need to adjust for the weight of the
jack and stepper motor on th load cells. I made a video about this
testing machine

A camera module in front of the setup takes a picture at the start of
every test and another after the piece has failed.

[strength14] A python script running on a Raspberry Pi 3 runs
everything. A line is printed with each 0.1 mm increment in jack
displacement, showing the force in Kg, plus a line of '#' symbols to
graph the current force. a '+' is also drawn to show the change in
force from one reading to the next. The current force, in Kg, is also
shown large at the top right.

[strength16] Because the apparatus is fairly stiff and applies
displacement instead of force, once the wood starts to fail, it
typically doesn't snap all the way. A partial crack is enough for the
wood to bend more and relieve some of the force. Once my program sees
a 10% drop in force from peak, it deems the test piece to have
already reached it's ultimate force, even if it hasn't broken
completely. From previous experiments, I knew that large a drop in
force was the end, and pushing further would not increase the force
above the previous peak.

Most hardwoods failed by fibers starting to tear on the tension side,
though well before this, the force graph showed that the wood itself
was starting to yield (plastic defomration) or fail, as the amount of
additional force for each additional 0.1 mm of displacement went
down. During elastic deformation, force should linearly increase with
displacement.

[strength18] But some woods just snapped.

[strength20] Once I got into a routine, each test took about 1.5
minutes. Most test pieces were long enough that I could break it in
two places and get two data points. Harriet (6 years old) came down
to watch for a while, but she found it pretty boring. I took the
opportunity to listen to podcasts while running tests. This apparatus
is a big improvement over watching the numbers on a bathroom scale,
which is how I had previously tested

I started with 42 test pieces, but made a few more using woods I had
lying around, so I had over 50. At two breaks each piece, it was
about 100 breaks.

[strength22] This one is "dogwood", sent to me by Peter Collin. I
hadn't heard about dogwood before, but its a very heavy, hard and
resilient wood!

[strength24] Some of the spruce pieces just snapped half instead of
failing gradually. Though it's more spectacular, I preferred that
samples stay attached to each other after a test. Less messy and
easier to keep track of.

[strength26]

[strength28] This is the computer taking the "final" picture, after
it has deemed the piece to have failed because the force dropped by
more than 10% from peak.


The highest maximum loads were for Osage Orange (293 Kg), Hickory
(275 Kg). Ironwood a.k.a. American Hornbeam (271 Kg), hard Maple (254
Kg), Dogwood (242 Kg), White Ash (200 Kg).

The dogwood sample had it's maximum force at 22 mm of deflection,
whereas the strongest hard maple failed at just 12 mm deflection.
This combination of large deflection and large force means it took
more energy to break the dogwood. So dogwood would make an excellent
wooden spring, such as for making a bow. I figured for a bow, you
want wood that is strong, flexes lots, and is light. So I added a
column to my spreadsheet of numbers multiplying maximum force,
deflection and maximum force, divided by density. The dogwood was the
winner by that criteria, followed by Osage Orange.

[strength30] Then again, looking at the yield curves, dogwood may
have started plastic deformation at around 80 Kg load, whereas Osage
Orange's plastic deformation probably started around 150 Kg. So Osage
Orange may be a better lasting bow. Repeated stressing to the point
of plastic deformation alters the wood, which is an aspect I didn't
test. But both Dogwood and Osage orange failed suddenly and with a
bang, as opposed to gradually starting to splinter with Hickory. I
think a more gradual failure like with Hickory would be more
desirable. It would be interesting to take typical bow wood samples
and design a test procedure to specifically test the wood's ability
to store and recover energy as a spring, with repeated cycling.

White ash also did relatively well on that score. It's no wonder my
dad always used white ash for axe handles. Hickory is even better for
axe handles, but there weren't any hickory trees where we lived.

[strength32] Interestingly, all the dark woods seemed to fail with a
bang, either snapping, or having a large part of the wood fail at
once, while most of the lighter colored hardwoods failed more
gradually.

I think this sudden snapping is an undesirable quality for chairs.
you don't want a chair to break from being knocked over by a kid, so
I wouldn't recommend any dark woods for chairs.

[strength34] [strength36] After that I switched my apparatus to have
a stiff iron bar instead of the workpiece, and mounted a #3 Robertson
screwdriver bit on top of my screw jack. A #3 bit is 3/16" or 4.76 mm
square. I changed the software to raise the jack until it hit 2 Kg of
resistance, then push a further 5 mm and measure the force required
as a measure of hardness. I didn't compensate for flex in my
apparatus, so the indentation depth on the hardest woods, with a
quarter ton of force, might have been as low as 3 mm. But it's a
relative measure anyway.

The common hardness criteria, the "Janka hardness test" involves
measuring the force needed to push a 0.444" (11.3) mm steel ball half
way into a piece of wood. But this would require bigger test pieces
and over a ton of force for the harder woods.

[strength38] I pressed two divots on each workpiece, on different
sides. As expected, the heavier woods scored much better on this than
lighter woods. Top samples were Dogwood (252 Kg), Osage Orange (249
Kg), Ironwood (231 Kg), hard Maple (223 Kg), Cherry (214 Kg), and
white ash (209 Kg, 206 Kg), Hickory (196 Kg), and Beech (189 Kg).

Softest were Cedar (45 Kg), Pine (47 Kg), Spruce (55 Kg), Basswood
(56 Kg)

With only a few samples, I can't say that this 100% representative of
the species, but it confirmed my gut feel

This score is useful for tables. The more it takes to push a divot
into the wood, the less dinged up a table, floor, or any other
furniture will get over time.

[spreadsheet] If you want to play around with the results spreadsheet
yourself, click here to download it
See also:

[joint_sm]Testing different wood joint types (2009)
[shelf_brac] Shelf brackets: Double tenons vs. screwed vs pocket
holes (2020)
[glue_sm] Testing wood glue
strength (2009)
[test_sm] Simple wood hardness test (2009)
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