Managing Drying Stress and Cracking
Shrinkage cracking and stress cracking are tough to control. November 14, 2009
Question
I take it there are two types of cracks: one is a drying crack due to the moisture differences between the inner and outer cells, and the other is a stress crack due to the growth of the tree. How can I relieve the stress crack - drill a hole at the crotch of it?
Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Your question is indeed the million dollar question (even in this poor economy)! The stress is along the entire piece, but is worse (accumulates) at the end. It is not controllable in commercial practice, unless you are willing to put a metal band across the end and up the edges a few inches when first sawn to prevent the end from cracking. (I did see this in a production operation in Australia.) For just a few pieces, you could clamp the ends of the lumber and keep adjusting the clamps as the wood shrinks. In North America, large growth stresses are not common. But in other parts of the world, many species, especially Eucalyptus, are noted for extremely high growth stress. In fact, the stress will crack the end of the log as the tree is falling to the ground. Steaming this lumber before drying has been tested, as well as a bunch of other methods, but I have not heard of any good practical methods. In any case, prevent drying stress with end coating so as not to add to the stress on the end. Be prepared for lumber with high stress to crack even more in the planer or when dropped on the ground. If such lumber is ripped into narrower pieces, the pieces are very likely to warp immediately when sawn due to the growth stress.
From the original questioner:
Then how can you build furniture if you don't know the stress of each stick you cut out of a plank? You can't just cut every piece to size and then let it sit for months so see if it moves.
From Professor Gene Wengert, forum technical advisor:
Good question. If the moisture content does not change substantially, then any movement will be small. Also, with a good finish, we will have uniform and slow moisture changes which discourage cracking and warping. More and more companies are getting very picky about incoming MC. One door company tightened their MC acceptance level and went from over 300 complaints for warp and size change to 3 in a year.
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