scarf joint failed

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scarf joint failed

Postby overdalimit » Fri Sep 09, 2011 1:26 pm

I epoxied my chines and let them set up overnight at 70 plus degrees. The next morning we had them clamped to the frames and at some point during the next night one of the scarf joints partially failed. as of now have re-glued and clamped and will let it cure all weekend. my question is did I not let it cure long enough? can epoxy feel "cured" and yet only be maybe 50% cured? Do I need mechanical fasteners too? This is in white oak. The epoxy adhered to both surfaces and it was definately the epoxy that failed...not epoxy to wood.

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Re: scarf joint failed

Postby gdcarpenter » Fri Sep 09, 2011 1:56 pm

The cure time you allowed was pretty minimal - as I understand you comments the epoxy cured about 12 hours or so before being stressed. Had you not 'stressed' them so soon they would have continued to cure, 24 hours is what I consider a minimum cure time for structural or stressed use. Bending them chines puts one heck of a lot of stress on a scarf joint! I would not recommend mechanical fasteners in a chine scarf joint.
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Re: scarf joint failed

Postby Roberta » Fri Sep 09, 2011 2:05 pm

GD is right on! Let that joint cure at least 24 hrs. Also, try to put scarf joints where the bend is minimal or even non existant. Scarf joints should be 8 to 12 times the material thickness.

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Re: scarf joint failed

Postby BruceDow » Fri Sep 09, 2011 3:32 pm

and.... as much as possible, I position the scarfs where there is not heavy bending/twisting.

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Re: scarf joint failed

Postby Iggy » Fri Sep 09, 2011 4:05 pm

Ya.. if your going to stress the joint... scarf.. butt.. whichever, you need to allow more time for the epoxy to fully cure. It will bond quickly, and the surface will harden to touch, but the epoxy itself is still a bit too flexible for its own good the first day or so and with enough force the epoxy can pull apart.

After it fully hardens the place the joint may fail is the wood itself... as in... the wood tears due to a flaw in it.

And like others have mentioned, locate the scarf a good distance from the bend if possible. The scarf will not be as flexible as the other areas of wood... same goes for butt blocks.. so placement is a big consideration.

It should re-glue just fine.. and fasteners/screws are not necessary unless you have to hold it together temporarily till the epoxy cures.

Give it 2 days... so long as they are warmish days/nights... and give it another shot.
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Re: scarf joint failed

Postby DaveLott » Fri Sep 09, 2011 5:30 pm

No fear overdalimit - I did it too


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Re: scarf joint failed

Postby thudpucker » Sat Sep 10, 2011 1:04 am

I built a Drift boat from plans. They had Scarfing in those plans. I got scared of that. So I used the Butt/Batten method.
Years later I tried scarfing an Axe Handle together.
Long taper, lot's of time in a warm Barn.
It held for a long time but the shocks eventually caused it to fail.
The F/glass pulled some wood from both sides.
I'm not good enough, so I'm sticking with Butt/Batten plywood joinery.

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Re: scarf joint failed

Postby overdalimit » Mon Sep 12, 2011 11:20 am

Well it is now Monday and the joint cured for about 72 hours. The joint is about 16 inches long in the chine log. I bent it and pulled on it and I am satisfied that it will hold this time. note to self let it cure before stressing it. Thanks for being there guys!

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