Testing for Case-Hardening and Drying Stress

Tips on detecting drying stress. October 29, 2008

Our company buys in R. oak rough about 6-8% from a number of suppliers. We then rip the boards from anywhere between 2 5/8 and 4 5/8. We just found a customer that wants to buy our rip 4 1/4. They said that anything that goes to them has to be 6-8% and must be stress relieved. Now the problem is we donít know if every supplier stress relieves their lumber and we canít have that.

So my question is, can you put ripped lumber into the kiln to be re-dried (make sure itís between 6-8%) and stress relieved even though itís already been dried?

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
If you put lumber back in the kiln, it is possible to adjust the MC slightly, if needed. This is called equalization. It is more difficult to relieve any stresses. If you dry the wood a few percent MC, it will not add stress, but if there is a moisture gradient, it will look like there is stress.

If you rip the lumber and it stays straight (when you put two adjacent pieces back together), then you are free of longitudinal (lengthwise) casehardening (also called drying stress).

To measure transverse stress (not longitudinal) there is a standard test that has a piece with two legs - it looks like the letter "U" and the legs must stay straight.

Find out which test they want to use and then find out how much deflection is still ok. Being 100% perfect (that is, having no deflection at all) is very hard to do. The best results in the kiln will be achieved when the MC is uniformly low. Therefore, high MC pieces will often not be well stress relieved - longitudinal or transverse. In any case, no moisture gradient when doing the test or you will get false results. For more info, consult Drying Hardwood Lumber in the archives here at WOODWEB.