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Is Thermal Expansion a Factor in Lumber Stress?3/3
According to theory as I understand it, once lumber is conditioned and passes stress testing, the lumber should more or less remain that way as long as RH remains acceptable. Temperature does not affect the lumber stresses, RH does...at least that's what I thought.
Since we store our lumber is an unheated dirt floor barn, RH limits how long we can store the lumber before the wood takes on moisture and shell/core stresses develop. The wetter shell pushes and the drier core pulls the prongs together. Because of this, we do stress testing on lumber as it enters production to verify that it is still okay.
If we bring frozen lumber from the barn, it fails stress testing (prongs pull together). If we leave the pile inside over-night, it passes testing the next morning. At -20C the air outside does not hold much moisture, so bringing the lumber into a warm building overnight shouldn't dry the outside of the boards (Change shell core variant).
Because of this stress, we are currently warming all of our lumber before processing. Unfortunately, customer orders can change and we are expected to react quickly. With limited heated room available, I am afraid to rip any boards while frozen in fear that they will move and no longer be straight when they thaw. Is this theory founded?
The test for drying stress, or casehardening, requires no moisture gradient. If there is a moisture gradient, then the test will show stress, but actually there is none. Note that the stress we are worried about is a permanent stress, not a transient stress. So, we never rad the prongs of a stress test immediately after cutting, but wait for the gradient to go away. If in a hurry, we can use 15 seconds in a microwave at high power after cutting a prong to move the moisture and then letting the prong cool several minutes before reading the results.
You should warm the wood primarily becuase gluing requires warmed wood.
When you bring cold wood, that is at the correct MC without a gradient of moisture, into a warm, dry room, you can expect that initially the surface will gain some moisture because the surface temperature is below the dew point of the air, much like condensation on a cold glass of ice water in the summer, or eye glasses fogging in the winter. This effect is temporary and the excess water is evaporated when the lumber warms in an hour or so.
After this temporary regain, however, the air in the plant is often drier than the MC of the lumber. That is, the plant's EMC is under the lumber's MC. This causes drying of the ends and surface. This effect is not related to temperature of the wood.
Note that thermal expansion or contraction of wood is impossible to measure without extremely accurate equipment. That is, thermal movement is extremely small. The change of MC by 1/10% is much larger.
When you do process cold wood and have wood movement issues (not gluing issues) it is because of moisture changes and not temperature itself.
My suggestion is that if you are going to store lumber that is at 7.0% MC is a building with a dirt floor and without humidity control, the wood needs to be taken off of stickers and then covered on six sides with plastic (like Saran or a large plastic bag) to avoid moisture changes. The longer the storage time, and the warmer, the greater the importance of doing this. Although a prong stress test with legs that are 1/4 of the lumber's thickness is a good idea when the lumber first leaves the kiln and there is no moisture gradient, this test is of no use subsequently when there is a moisture gradient. Further, once dry, unless exposed to liquid water, it is impossible to create new permanent stress in wood...that is, to create casehardening stress.