Checking for Drying Stress in Kiln-Dried Lumber
A kiln owner learns how to properly test his product for drying-induced stresses. June 30, 2007
Question
I am wondering what exactly causes uneven stresses in the kiln. Today I have unloaded a red oak kiln and from each row I took a stress sample from random packs. My 1st row had a slight deviation, my 2nd row had a slight bit more, my 3rd row was perfect, my 4th row had a little reverse and my 5th row was perfect. All the lumber was relatively close to the same dates and entered the kiln with close MC's, within 5 to 10%. Is there something I am missing?
Forum Responses
(Sawing and Drying Forum)
From contributor T:
This sounds normal to me. No two boards are exactly the same, and no two boards will behave exactly the same way, even though they are side by side in the same kiln. We dry 200,000'/week, and I do 3 stress tests on every kiln when it's unloaded. They always vary at least a little bit.
From contributor D:
It will be better when we have standard trees. But it is worth the time to really study the grain in those pieces and see if there is something you can identify that will help in sawing better.
From Professor Gene Wengert, forum technical advisor:
It is virtually impossible to get reverse casehardening if 4/4 and 5/4 lumber is not wetted directly with liquid water. Therefore, I wonder if your results are incorrect. How long since the kiln has been shut off? How did you cut the pieces? Are all legs the same thickness? Moisture gradients in a standard stress test will give incorrect results. What do your samples look like 24 hours later after being in a warm room? When you say that you had some stress, what do you mean? Did the legs touch?
From contributor J:
If you cut the prongs too thin you can almost always display reverse casehardening. I suggest cutting two prong tests from each board -.one with the prongs being 1/4 of the board thickness, the other prong test with the prongs being 1/3 of the board thickness. If either test shows significant bending, then there are problems. If they don't bend significantly, not a problem.
From the original questioner:
I have taken special care to the way I have cut my stress samples (all wrong). I have been cutting the prongs too narrow, which was giving me the reverse case results. Is it safe to say 1/4" from the edge is the magic number? Thanks - it helped a lot! If your sample board is, let's say, 7" wide and you take your stress sample, how far do you cut your prongs (lengthwise)?
From contributor J:
There is no standard. Typically about 3/4" from the lower end. The longer the prongs, the more the prongs will curve. The degree of bending is a function of the prong length squared. So a 6" long prong will curve 4X as much as a 3" long prong will. The grain and growth ring curvature will have some influence on the curvature.
