Joint strength test

I ran a rough test for my own information and thought I'd share the results. I took a popular board 5-1/2' x 34-1/2" and sawed it in half. I then machined mortises for three 6mm dominoes evenly spaced across the 5-1/2" and centered on the thickness. The mortises on both sides were machined on the middle width setting, can't remember that actual width. I glued the joint with TiteBond II but did not get much if any glue on the dominos. I clamped it up and allowed it 24hrs to cure.
To load the joint I supported the board on 2 x 4s on each end then placed short pieces of 5" wide steel flat bar across joint as in the photo. The joint held 95 lbs and failed when I placed another 12 lb piece (I said it was a rough test). Since an evenly distributed load creates 1/2 the stress of a concentrated load (mine was actually distributed over 5" but close to concentrated) my guess this board could probably have supported 200 evenly distributed lbs and perhaps more if I had carefully applied glue to the mortises and dominos. The second photo shows the failed joint from the bottom side.

Attachment 519051Attachment 519052

Looks like the Dominos pulled out of their mortises. Probably would have held a lot more if those were glued.

How thick was your popular board?

I think in this application using a finger joint router bit would give a much stronger joint. There would be a lot more area being glued and rather than a butt joint it would be almost all face joint. I've only used it once and was a little finnicky getting set up but worked as intended that time. As Cameron mentioned if the Dominos were glued they would have added to the strength.

Yea, almost no glue strength in this case.

What was your logic to not glue the floating tenon? Would you do the same test with a traditional mortise and tenon and not glue them? Joinery with no glue just defies logic.

Have to agree regarding no glue on the dominos. I'm going to repeat the test with glue. One thought is that the domino / mortise fit is tight so not much room for glue.

When glueing end grain to end grain with a PVA glue, it's best to mix up a 50/50 mixture of water and glue and apply that to the end grain. Then give it enough time to soak in, and then follow up with 100% glue before clamping it together. End grain will really soak up the glue, so it's best to give it something to soak up that won't starve the glue joint first, then give it the actual glue to hold the joint together. It's kind of the same principle as putting down a thin, seal coat of shellac, before applying stain to wood to keep it from blotching.

Jimmy thanks for the tip. I followed up with a second test applying glue to the dominoes and the mortises. The board length was reduced by 2" to 32.5" so that reduced the stress on the joint by 6%. The joint failed at 114 lbs, not immediately but I heard a faint cracking sound and it failed shortly. In my previous post I said I added 12 lbs aand the joint failed. That was not right I added 18 lbs not 12. So the joint without glued dominos failed somewhere between 95 and 114 lbs and with gluing at 114 lbs which corrected for the shorter length is equivalent to 107. Gluing the dominos doesn't seem to have added much strength to the joint. I'm thinking of testing your suggestion without dominos to see how much strength the "wash coat" of glue adds. If I do I'll rig up a better loading scheme.

End grain or side grain i double coat and dont thin. washcoat is a different beast for a different reason.

Water could be a good loading scheme.

After looking at it again the joint isn't going to gain much added strength even with the domino's glued. I will presume the board is 3/4" thick and there isn't going to be much strength no matter how you butt the ends in a side load test. Plus using a scale with a jack setting on it would be a much more controlled break. Pushing against it as you slowly increase the pressure. You would have to revise your setup. If you need to join two boards end to end a half lap would be much stronger. Or get a finger joint router bit and use that method. I think what you've established thus far we already knew and that's that this type joint is never going to have much strength in a side load situation.