Tension Set and Lumber Conditioning

Advice on remediating curved lumber after resawing, with a discussion of why "tension set" occurs and how the effect can be reversed. October 19, 2013

(WOODWEB Member) :
I've been resawing a lot of thick (8/4 and 12/4) cocobolo, Honduran rosewood, and other very dense, waxy air-dried woods lately and it is quite common for the outside slices to come off the saw looking like a potato chip. As these woods are quite expensive, I would like to reverse this tension set/casehardening and use these outside slices. I have read about folks using some kind of heat press to accomplish this. I have created a simple setup of two 3-4" thick boards with a heating blanket between them. I hope to put the case hardened board in the middle with the blanket, clamp everything flat, then turn on the heat to condition the board and hopefully remove the tension set.

Will this work? I am a little fuzzy on exactly what will happen inside the wood... Is the goal to plasticize and then re-set the lignin whilst it's clamped flat so that it will stay that way? Do I need to wet the wood to introduce moisture or is that a bad idea? What temperature should I shoot for?

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Tension set is a term for size difference. So heat alone will not be effective. You need moisture on the concave side as well. If you add too much, then it may reverse. Remember that the board will have a little spring to it when you open the press, so you really need to bend the piece a little more than flat. It is hit and miss, so good luck.

From the original questioner:
I do not understand what this means. Would you mind elaborating? I understand why the wood had internal stresses because of moisture gradients during the drying process but do not understand why they weren't relieved when the wood was resawn... nor how they can be relieved. Maybe a better question is: Why does the wood set and how could one reverse that process?

From Professor Gene Wengert, forum technical advisor:

When drying first began, the wood cells on the outside (after they dropped under 28% MC, the fsp) tried to shrink, but were restrained by the wet, non-drying core. Hence, they dried to a larger shape than they would have if they had not been restrained. The most tension set is on the outside, so when you resaw, the outside of the outer piece will have the most set. You can read more in Drying Hardwood Lumber, and it is called Stage I drying. The wood has tension set on the outside that drops off as you go deeper and finally we will see compression set in the core region.

In addition, there are growth stresses in the tree that cause immediate warp, but they tend to be more erratic and not related to the lumber's shape. A complete discussion would take many pages.

The way we remove set is to do the opposite of what caused it - that is, we add moisture, but the cells are restrained from complete swelling. This restraint offsets the previous shrinkage restraint. Incidentally, set results when the proportional limit is exceeded, so a smaller moisture gradient will not cause set.

From the original questioner:
Thanks for your comments; I am struggling to understand this so it is well appreciated.

From Drying Hardwood Lumber in the Stage I section: "As the tension forces in the shell increase as a result of restrained shrinkage, the forces are likely to exceed the proportional limit. If they do, the outer cells will eventually dry to a larger size than they would have if they had been free to shrink without restraint. This process is similar to stretching a rubber band. With a small force, the rubber band can be stretched slightly, and when the force is removed the band will return to its original size. However, if the band is stretched beyond its proportional limit or is stretched and then held stretched for some time, it becomes enlarged. When the force is released, the band returns to a size somewhat larger than its original size. This change in size is called “set” because it occurs with a tension force, but a more accurate term is “tension set.” Another word for tension set is casehardening."

The proportional limit is the piece I didn't understand... At this point the elasticity of the outer shell is lessened and so the shell cells end up somewhat stretched out after EMC is reached. We quickly inject moisture into the outer shell in order to induce stresses beyond the proportional limit in the opposite direction, which effectively shrink the outer cells and relieve the tension set. Do I have that right? If so I can see why it would be hit or miss to get it just right... One must deform only the cells which are already deformed, but in the opposite direction.

From Professor Gene Wengert, forum technical advisor:
You do indeed have it correct. The PL is the point at which semi-permanent deformation occurs. So, with a rubber band, you can stretch it out and then let go and it will return to the original size. This is the elastic range. You can do it again and again, and nothing permanent. (Actually, over time - years - some permanent deformation can occur, called creep, but we are considering short term now.)

Now when you go above the PL with the stress, the rubber band gets stretched out and this will be permanent... Let go and the band is a bit larger. In other words, the PL is the stress level at which point you go from elastic to plastic behavior.

From the original questioner:
Yes, that helps a great deal. Thanks!