Longitudinal Versus Tangential Stress

      A discussion of how to evaluate and troubleshoot drying stress in kiln-dried lumber. June 13, 2014

Question (WOODWEB Member) :
In 4/4 and 5/4 hardwoods, (we run red/white oaks, maples, ash mostly) it's my understanding that tangential stress is generally easier to relieve than longitudinal stress. However, we have recently had material that passed longitudinal testing with flying colors, but we can't get it to pass tangential.

In another case we had some material that passed all tests, but the storage was too dry, resulting in stress due to the shell being too dry (went down to 4.5 - 5%). We adjusted the climate conditions for several weeks and brought the wood back to equal shell/core, so that it passed longitudinal, but now, the tangential stress is still the same as before we corrected the storage environment. Does anyone have any thoughts?

Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
I would question the procedures you are using for the stress tests. Please tell us the exact procedures, size of the legs, grain direction, etc. It is possible to dry lumber with no longitudinal stress. Hence, you will always get good longitudinal stress tests. If this is the case with you (and it sounds like it), then you have a problem with not getting the correct WB depression or EMC within a few hours after you start conditioning. Do you equalizing for a certain length of time for each load or do you equalize for a variable amount of time as suggested in the drying book? Do you cool the lumber between equalizing and conditioning? If so, how long?

It is indeed true that a piece with a MC gradient will not give an accurate stress test. However, in these cases, as discussed in Drying Hardwood Lumber, 15 seconds (maybe 30 in a water spray conditioning system) in a microwave oven (high power) and then waiting two minutes or so will usually give the same prong test results as the customer will see.

In your case, drying the shell down to 4.5% MC would require under 20% RH. Did you really have such dry storage? How did you measure the shell MC? Moisture meters are inaccurate under 6.5% MC. If you used an oven at 217 F, did you re-weigh the samples after an additional hour of drying? If the pieces were not fully oven dry, then they will read lower than the true MC.



From Contributor S:
1. We are running with flat sawn material

2. We are following the standard USDA Drying Hardwood Lumber p102 tests. I would however like to change my terminology because I may have been misleading:

When I said Longitudinal, I was referring to the tension set stick. It is generally about 14" long. We also do the Reaction wood stick, but haven't had issues with it. We also will cut a 2"-3" wide section from the center of a board about 2'-3' long and cut it in half to replicate our re-saw. When I said Tangential, I was referring to the stress section. It is about 1"- 1 1/2" wide. We do have some disagreement as to the thickness of shell core. Some say that you divide the board thickness into three and your core is the middle third. Others say you divide the board thickness into four and the middle half is the core.

3. We do not have a standard equalizing time, but recently, with production demand being slow, we have gone longer than normal just to be sure.

4. We don't generally cool between equalizing and conditioning except in rare cases when we end up chasing up the dry bulb (not recently).

5. Failing the longitudinal, (as well as the tangential stress test - post microwave) is what tipped us off to the shell core variation, which led us to the storage conditions, which led to us adding humidity back into the storage. When shell/core tests showed less than 1% variance, we re-tested stresses and found that the longitudinal (tension set) stress had subsided, but the tangential (stress section) had not. That is what has perplexed me the most. From my understanding, the stress section should have relaxed before the tension set had.

6. The dry storage was due to a heater that stuck on.

7. All oven testing we do is by microwave.



From Gene Wengert, forum technical advisor:
When you talk about transverse stress, are the prongs touching or are they merely close? Overall, you seem to have the correct procedures. As a result, I do believe that you need to have a knowledgeable consultant (like me or several others) visit your operation and check out the many possibilities. For those not familiar, here are some comments.

Check p. 102 and read the first full paragraph about an alternate test, which is much better. (I asked that the longitudinal stress sticks be dropped from the photo, but was over-ruled. In this picture, the piece labeled "B" will show primarily tension wood and growth stresses. To be accurate, it must come from a region away from the edge, as shown. However, "A" test shows tension wood, growth stress and drying stress. Further, the prongs are so close initially, that the amount of stress is difficult to discern. If this stick is cut, then the variation of the prongs side to side (not pinching) will show tension wood and growth stress. But these sticks, if they show stress, would not represent normal usage as who would rip lumber into such narrow pieces? Further, this test stick should not come from the edge, yet edge stress can indeed be a problem. For all these reasons, the written alternative test is so much better.)

The tension wood tests in the photo are not very accurate, as either of the tests will show tension wood and growth stress. For transverse stress, cut 1/4 of the lumber thickness for each prong thickness (meaning that 1/2 of the center is removed). This has been the USDA test since 1950 and other thicknesses are not that good. In any case, always cut the same thickness. Also, how much movement of the prongs is ok? To be most accurate, as some stress can be in the planer allowance, it might be best to cut the test from planed lumber. However, if the 1/4 prong from rough lumber is straight, we have learned that planed lumber will be ok. Thinner prongs are very poor as the planer allowance will begin to dominate. Check out the second full paragraph on p. 101 for reading hot prongs.



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