Prongs will remain fixed if the wood is conditioned correctly (Figure 3). In either case, prong movement in a stress test points to a condition that should be corrected to avoid warp during re-sawing or machining. If immediate prong movement occurs, followed by additional prong movement after 12 hours of rest, both casehardening and moisture gradient are present. To determine if casehardening or a moisture gradient is present, the sample will need to rest for 12 hours while it is monitored to detect any changes resulting from moisture loss. In severe casehardening, the prongs may snap together or bypass each other. When the bandsaw removes the core, the prongs move inward. The pinching occurs because of the tension stress in the core. If the two outer prongs pinch together or bypass each other immediately (Figure 2), the board has casehardening or tension set. The time it takes for the prongs to move inward or outward is a good indication of how significant the residual drying stresses are. To conduct a stress test, cut a 1-inch section across the width of the sample board at least 12 inches from the end of the board. The core section is removed with a bandsaw (Figure 1). Also, these samples will help determine when the lumber’s moisture content is equalized and conditioned, which will properly relieve drying stresses. Kiln samples allow the operator to control the kiln better and determine when to adjust the drying time, ultimately improving lumber quality. Other than encountering stress during machining, another way to test lumber at the end of the drying process is by cutting samples for stress testing.Ī helpful way to monitor the moisture content and watch for any drying defects during the drying process is to use kiln samples. Additionally, casehardening stresses can significantly impact yield when re-sawing or re-manufacturing lumber. This can cause the wood to pinch the saw blade and become a safety hazard to the operator. If a woodworker is re-sawing or ripping lumber into smaller cuttings, the stresses are relieved, and unacceptable warping (such as cup and crook) often manifests. How Can You Tell If Lumber Has Stress Or Casehardening?Ĭasehardening is often noticeable during the machining of lumber. Surface checks may close as drying continues, making the lumber look normal however, stress is still present unless it is relieved at the end of the drying process. As drying continues, the core begins to shrink and, as it does, the shell is forced into a state of compression while the core remains in tension. These cracks form as a result of the core in compression and the shell in tension. If dried too quickly, the force of shrinkage can exceed the strength of the lumber, and surface checks (i.e., small cracks) or end checks (i.e., cracks at the ends of the boards) will form.
The initial shrinkage can cause defects if not carefully maintained. As the moisture starts to move out of the board, the wood begins to shrink. This process causes a moisture gradient to form between the low-moisture content in the shell and the high moisture content in the core. It is critically important to relieve this stress as it can lead to warping and twisting when lumber is later re-sawn or machined.Īs lumber dries, the surface first loses moisture causing wetness from the inner portion of the board to move outward toward the surface, where it evaporates. Casehardening is a normal part of the drying process of lumber. However, drying stress-commonly called casehardening or tension set-occurs during the drying process.
Not only does kiln drying allow the wood to equalize to desirable and usable moisture content, it kills fungi and insects that might be present in the lumber. In Wisconsin, this dry-basis moisture content is 6-8%. Lumber manufactured into interior wood products (e.g., furniture, flooring, millwork, cabinets) typically requires kiln drying to reach a targeted moisture content to minimize dimensional changes. By Scott Lyon, WI DNR Forest Products Specialist
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