Comments on: Weird wood stresses stress me

By: Rob

Rob — Tue, 03 Nov 2020 05:59:15 +0000

Thanks, Corey.
“Case hardening” is in fact the term found for the condition I describe here in the U.S Forest Products Laboratory’s Wood Handbook and in the late Professor Bruce Hoadley’s seminal book, Understanding Wood, and generally elsewhere.
Rob

By: correy

correy — Tue, 03 Nov 2020 04:38:28 +0000

Hi, great posts of late. I completely agree on all your points! A boon of info for the uninitiated. My ONLY suggestion might be to change “case hardening” to the correct term, “Tension Set”

By: Rob

Rob — Tue, 04 Aug 2020 02:27:02 +0000

No problem, Antony, but let me clarify. The piece of wood in the photo is a test fork taken from an end cut. The middle is cut out to test for internal stress in the board, especially looking for excessive case hardening, which this board certainly has.
The question here is why there was the delayed, gradual manifestation of the internal stress (from excessive case hardening) after resawing the 8/4 board into two boards. It is usually shows up just about immediately.
But I think Shannon has given us a good explanation.
Rob

By: Antony E Brinlee

Antony E Brinlee — Mon, 03 Aug 2020 19:45:35 +0000

May be way off-base here but it may be because of how this was cut. When you cut the piece from the center down toward the bottom, it leaves the two arms with concentric tree rings going down the length of the board that will curve toward the the center of the tree (assumes riven). It is further exaggerated by the releaf cuts half way up the board allowing a even faster bow/twist to occur. Now the bottom is fixed in place and can’t move. This may well be a result of the mechanical stresses in the wood itself releasing in the direction of spiral left when the center was removed. Of course this is all conjecture because I can’t see any of the grain from the picture.

By: Rob

Rob — Mon, 03 Aug 2020 19:05:20 +0000

Wow, Shannon, great explanation!

OK, so sapele’s high shear strength, high resin and extractive content, and interlocked grain act to create this “slow release” of internal stress. Small fluctuations in moisture content also “loosen” the wood fibers to release the stress.

Thanks for this insight. Once again, knowledgeably watching the wood is so important to woodworking.

Rob

By: Shannon Rogers

Shannon Rogers — Mon, 03 Aug 2020 12:52:51 +0000

Rob, your theory is a good one and I agree with it to some extent. The high strength internal structure of Sapele plus its heavily interlocked grain means that is can withstand a lot of stress. Also African woods and most tropical woods contain a much higher extractive content which acts to ameliorate the moisture changes and leaves the wood more pliable. But eventually that strength gives out without support and actually unravels. The interlocking grain un-spirals and that lead to the twist.
What I suspect is that just the shop climate conditions, even if climate controlled, fluctuated a bit and over time that worked to be the catalyst for deformation. In other words, a little bit of moisture infused into the fibers, then dumped, then infused, then dumped, etc. This acts like bending a credit card back and forth until the plastic weakens and then finally snaps.
Around the lumber yard where I work, Sapele is one of the most common species we stock. It is reknown for remaining super stable after milling but in instances where precision is asks for (like with T&G porch flooring or siding) we will assemble the joints before packaging and shipping because over the course of a week the size of the groove will change. This isn’t just Sapele that exhibits this behavior but several other African species and some South American species where interlocked grain is common and oil and resin content is also high.