How Big Should Fillets Be?
I think, in general, stitch and glue boat builders oversize the fillets. The epoxy is orders-of-magnitude stronger than the wood and especially stronger than plywood. The weak link in a plywood/epoxy joint is the bond between the plies of the plywood.
I had a few extra sections of a plywood/epoxy joint that were headed for the trash. But, since I love destructive testing, I put them to good use.
This is a very small epoxy fillet in 9mm BS1088 Hydrotek meranti. The legs of the fillet are maybe ¼-inch. This is way too small for a boat hull but plenty big for a cabinet.
The outside has 6oz. fiberglass tape set in epoxy.
I thought I could break them with my hands. I couldn't. I had to whoop 'em with a hammer.
Notice that the fillet is intact.
As expected, the mode of failure is disbonding of the plywood plies.
Larger fillet = More plywood area = Stronger joint
But, this comes at the expense of large amounts of epoxy. And epoxy ain't free!!!
Fiberglass set in epoxy will also spread the load over greater plywood area. Pound for pound, a small fillet covered with heavy fiberglass set in epoxy is probably a more efficient use of epoxy than a gigantic fillet.
Small fillet with heavy fiberglass = More plywood area = Stronger joint
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