Date: 2010-09-24
by Gene Wengert
All stress tests require a uniform moisture content throughout the piece. Pieces with a gradient will give confusing results.
All stress tests require a uniform moisture content throughout the piece. Pieces with a gradient will give confusing results.
As casehardening or drying stress (the case is not harder than the core, so casehardening is a poor descriptive term, but it is commonly used) causes immediate warp (the key is immediate) when machining, it is obvious that conditioning the lumber in the kiln (the technical term for stress relief) is essential. The purchaser of kiln-dried lumber can quickly check for the presence of unrelieved stress by cutting a stress sample.
Transverse or across-the-grain casehardening is measured by cutting a 1-inch-along-the grain, full width and full thickness test section from a piece of lumber (or several pieces). Cut this test piece at least 12 inches from the end of the lumber. Next, on one of the freshly sawn faces, saw two prongs that are connected at the bottom, similar in shape to the letter "U". Note that the thickness of each prong is roughly one-quarter of the thickness of the lumber for 4/4 through 6/4 lumber. If these prongs remain straight, or nearly so, after cutting, then there is no stress. In simple terms, this test is trying to determine if warp will result when the lumber is processed; straight prongs mean no warp when processing the lumber. (Note: When such tests are made during kiln drying, it is necessary to heat the prongs for 15 seconds or so in a microwave oven and then wait several minutes after heating to read the prong position. This heating eliminates any moisture gradients that would give an incorrect reading.)
Longitudinal (or lengthwise) casehardening (which would cause immediate lengthwise warp when ripping or resawing lumber) is measured by cutting a short piece of lumber from a large piece. Ideally, the lumber piece for this test is 8 inches wide. The test piece is typically 8 inches wide, 24 inches long and full thickness. Then, rip this test piece in half, making two 4 inch wide pieces. Put these two pieces back together in the same position that they had before sawing (minus the sawdust). There should be no gaps wider than the thickness of a dollar bill between them if there is no longitudinal stress.
Special note: These are very sensitive tests. Often it is not necessary that you lumber be totally free of stress. Therefore, it may be possible that some bending of the prongs or gaps between the pieces will be acceptable. Each operation must decide what level of stress can be tolerated.
Gene Wengert, "The Wood Doctor," has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
Updated: Sep 21, 2011