Development of Drying Stresses During High-Temperature Kiln-Drying

High temperature drying at 220?F of yellow birch (Betula alleghaniensis Britt.) was done to gain fundamental information on internal stresses and defects during drying. Tangential shrinkage, surface checking, honeycomb, and set were observed. Surface checking is experienced early in drying when the ultimate strength of the shell is exceeded by the tensile forces in drying. The greatest danger of surface checking was present when maximum tension set developed. Surface checking was reduced when differences of moisture content between the shell and intermediate layers were reduced. Decreasing compressive strain and subsequent tensile strain in all interior slices resulted in decreasing tangential shrinkage. Higher tangential shrinkage of a whole board during drying, in green material, was also found to be influenced by the increased amount of compression set in the core. In determining sets and strains which affect defects and shrinkage, moisture gradient curves were determined to better understand the relationship between moisture content and the above effects. After stress reversal honeycomb was the most common drying defect. This defect is the result of a tension failure where the strength of the wood perpendicular to-grain has been exceeded. Pre-drying is a suggestion to reduce honeycomb during drying.

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