Proposed destructive testing of mitre joints at the next QLD Forum gtg

[Rocker]

I am hoping to get the help of Forum members at the next QLD gtg, rumoured to be at the Home of the Bitingmidge in late-November, to carry out some destructive testing of mitre joints, in order to determine their approximate ultimate strength in various configurations.

I have built two zigzag frames, a test rig, and various half-Z assemblies for the testing programme.

Photo 1. The test rig, with a half-Z assembly, reinforced by three locking dominoes, clamped to it.

Photo 2. A zigzag frame in rose gum, with jarrah through tenons.

Photo 3. A zigzag frame in blackwood

Photo 4. One of the blackwood frame's mitre joints, reinforced with keys of rock maple

Photo 5. One of the rose gum frame's mitre joints, reinforced by a single jarrah through tenon.

All the assemblies for testing are built from 35 x 40 mm stock, and are glued with Techniglue epoxy.

Rocker

[outback]

If it doesn't get videoed it didn't happen, OK.

[journeyman Mick]

Will the "destructive" testing involve explosives? If so, I might be persuaded to travel down for it.

Mick

[Rocker]

The destructive testing of the 22.5 degree mitre joints yielded some interesting results. The strongest frame, made of blackwood with twin reinforcing keys, finally failed under a load of about 365 kg (about 800 lbs). A surprising result was that a 22.5 degree mitre joint, made in silky oak without any reinforcement of the joint, did not fail along the glue line. Instead, the wood split under a load of about 90 kgs, away from the glue line, which remained intact. Video and still photos were taken, which will be posted in due course.

Rocker

[Fencepost2]

I like your empirical approach Rocker. Could you say more about how the load is applied? What leverage did the loads have on the joints? How far were the loads from the joint line? Was it dead load or was there some impact? And lovely joints may I say.

[Rocker]

Fencepost2,

This was not intended to be a rigorously scientific experiment; it was done mainly to give an approximate indication of the weight that would be likely to cause a cantilevered chair to break. I think what the series of tests showed that joints glued with epoxy normally fail because the wood breaks rather than the glue failing, and that a cantilevered chair built with epoxy should have a generous safety margin in normal use, since it took the weight of four men to break a frame made in a similar configuration to a chair, when it is made from a strong timber such as jarrah or blackwood. Of the woods that we tested, rose gum was somewhat weaker than those two, and silky oak was considerably weaker.

The load in our tests was applied by people climbing gently onto a strong board laid upon the frame being tested. I am afraid my knowledge of mechanics is too sketchy to rigorously define the forces that were applied in these tests.

Rocker

[Rocker]

Pictures of the testing of mitre joints have been posted in this thread: http://www.woodworkforums.ubeaut.com...archid=2016465 , in posts #56 & #70.

Here are some close-up pictures of the broken joints of the blackwood and rose-gume frames:

Photos 1 & 2 show the joints of the blackwood frame reinforced with rock maple keys. The frame ultimately failed under a load of about 365 kg; one of the joints was virtually intact, with the wood itself failing; the other joint failed along the glue lines.

Photo 3 shows how the joint in a rose-gum frame failed. The glue held but the wood broke, with the jarrah reinforcing tenon splitting along the grain. The frame failed at a load of about 300 kg.

Rocker

[DJ’s Timber]

Photos 1 & 2 show the joints of the blackwood frame reinforced with rock maple keys. The frame ultimately failed under a load of about 365 kg; one of the joints was virtually intact, with the wood itself failing; the other joint failed along the glue lines.

All very interesting Rocker. With the Blackwood frame, do you know which side of the frame went first, the glue line or the wood failing.

[Rocker]

Here are some pictures of individual joints (not frames) that were tested with a test rig.

Photo 1 shows a joint made in silky oak with no reinforcement. Surprisingly the wood failed rather than the unreinforced mitre joint. It failed under a load of about 80 kg.

Photo 2 shows a joint made in jarrah, with only a single alignment domino as reinforcement. The joint failed under a load of about 110 kg.

Photo 3 shows a joint made in blackwood with three interlocking dominoes as reinforcement. It also failed under a load af about 110 kg.

Rocker

[Rocker]

All very interesting Rocker. With the Blackwood frame, do you know which side of the frame went first, the glue line or the wood failing.

DJ

I am not sure at this stage which joint failed first. Maybe the video clips may tell us that, when they become available.

Rocker

[Greg_stewy]

Now this made me think.....

Should we use different glues for different woods???
If something were to fail, wouldn't we prefer the join to fail as opposed to the timber??? does that mean that a weaker glue should have been used for the silky oak joint?
If it failed at the join, then we would still be preserving the structural integrety of the timber and have the opportunity do a nice repair at the previous glue line...

just a thought/question that came to mind......

Greg.

[Rocker]

Some people argue that a weak glue, such as hide glue, should be used for chairs, so that when the joint fails, it fails at the glue line, and can be repaired easily. I have to say that I prefer to use a strong glue such as epoxy, which should not fail at all in normal use. This testing demonstrated that weak woods, such as silky oak, should not be used for chair-making. Strong wood, and strong glue, should ensure that the chair remains intact, and should never require repair.

Rocker

[dai sensei]

All very interesting Rocker. With the Blackwood frame, do you know which side of the frame went first, the glue line or the wood failing.

You would have to see the video in slow motion as they did go with a bang scaring the crap out of bunnies balancing on top

[DJ’s Timber]

So who has the video? Get them to check it and see if they can tell

[RufflyRustic]

You would have to see the video in slow motion as they did go with a bang scaring the crap out of bunnies balancing on top

And everyone else in the vicinity as well!!!!

cheers
Wendy