Thread: Destructive testing of Rietveld z-joints

I'm still not convinced... you failed to perform the crucial test: a 5yo primed with red cordial.

Still, considering such animals are barred from my corner of the world, I can fairly confidently forecast that a Z-shaped bum-rest will appear here some day soonish. I'm hoping it won't be put to the test though...

Originally Posted by Skew ChiDAMN!!

I'm still not convinced... you failed to perform the crucial test: a 5yo primed with red cordial.

SC,

It is very rare that such creatures enter my abode; but SilentC did manage to introduce a couple some time ago. Delightful as they were, we can still see the red cordial stains on the jarrah picnic table

Rocker

Wow: that is an impressively strong joint. It's clear to me that what "failure" there was, was the same as my original test. The see-sawing of the plank caused the arm of the z-joint to twist about its long axis; this twisting caused the glue line outboard of the keys to separate at the apex where the joint was under tension (the inner part, under compression, stood up without damage). The part of the joint between the keys was undamaged.

Conclusion: these joints, reinforced with keys, are strong.

My z-tables will have keys reinforcing them along the entire width (spaced, say? every inch), since they will be solid along their width, rather than Rocker's 'frame'-style adaptation.

Interestingly, the brass pins weren't effective at stopping the joint opening at the apex (which is where this kind of joint will tend to fail). The long grain to long grain glue area of the keys is the key to this joint.

Only one other observation: I would have been mighty concerned (perhaps this was RR's fear) where the arm of that bessey was going if something snapped:eek:.

Will have to distribute some greenies for this...

Originally Posted by zenwood

.....
Only one other observation: I would have been mighty concerned (perhaps this was RR's fear) where the arm of that bessey was going if something snapped:eek:.

Actually, I was concerned for the fish in the fish pond, to my right in the fourth photo

It was an amazing experiment, one I enjoyed, from a little distance though.

thanks to TeeJay for the gig, Rocker for the z-joint and the guys for the balancing act.

cheers
RufflyRustic

Zenwood,

Close examination of the joint shows that the glue parted only around one of the 6 jarrah fingers in the joint, clearly as a result of the twisting action of the plank on which we stood. The glue around the other five fingers is still intact. The glue parted only in the section of the joint between the pins and the apex, so it is clear that the pins were effective in preventing the joint from opening any further.

It would be interesting to test the joint to destruction in a jig that did not allow any twisting; my guess is that the jarrah would rupture across the grain near the inner angle of the joint. But I suspect it would need a load of 500 kg or more to cause this rupture.

Anyway, I have ordered some more jarrah, and will build a chair using keyed mitres rather than bridle joints for the Z part; I will still use a bridle joint to join the sides of the back to the side rails of the seat.

Rocker

Great work guys, gals, and fish

Just glad no one got hurt, including the fish

Although this experiment has proved that a keyed and pinned mitre joint is very strong, I do have some concerns about its long-term viability. I wonder whether epoxy, like metals, is subject to 'fatigue' from the repeated application of stress, or whether it gradually deteriorates in strength due to some slow chemical process. The pinned bridle joints in the chair that I built would probably still be quite strong even if the glue deteriorated significantly; but the keyed and pinned mitre joint would be much weaker, if the same thing happened.

Perhaps some of the boat-building gurus could comment on how epoxy stands up to prolonged and repeated stress in boats.

Rocker