Effect of Subsequent Kiln-Schedule after the High-Temperature Setting Treatment on Internal Stresses of Sugi Boxed-Heart Timber without Back-Splittings
Morihiko TOKUMOTO, Takashi TAKEDA and Takahisa YOSHIDA
Abstract:The high-temperature setting method (HTS method) was @developed in order to prevent surface checking and to reduce drying time for boxed-heart timber without back splitting. @It's principle is based on the sudden formation of tension set at the surface of timber under high temperature and low relative humidity condition at the initial stage of kiln-drying. Using the HTS method, while reduction of surface checks was particularly obvious, honeycombs increased when moisture content of timber decreased. To examine the optimum condition of the HTS treatment and to clarify the mechanism of occurrence of honeycomb checks during subsequent kiln-drying, a more thorough knowledge of the internal stresses in timber kiln-dried is required. The purpose of this paper is to gain the knowledge of the internal stresses in timber just after the HTS treatment as well as after subsequent kiln-drying with various schedules after the HTS treatment. Testing materials were Japanese cedar (Cryptomeria japonica D.Don) green boxed-heart timber (132~132~3000mm) without back-splittings. The procedure used for strain analysis was the modified slicing technique. Main results were shown below. 1. After the HTS treatment, the surface of timber already reversed to compressive stresses in half the number of testing materials. 2. For the timber kiln-dried after the HTS treatment, while compressive strains at the surface significantly developed as drying proceeded, strain distributions tended to show characteristic concave curves which reveal that the shell of timber is stressed in compression and the core in tension. 3. Under flat moisture gradients, strain gradients for the timber kiln-dried with medium-temperature schedule after the HTS treatment were steeper than that for the timber kiln-dried with high-temperature schedule after the HTS treatment. 4. Strain distributions for timber kiln-dried with the conventional medium-temperature schedule without the HTS treatment were almost flat compared to that for timber kiln-dried after the HTS treatment. Key Words:High-temperature setting treatment, Drying set, Stress reversal, Kiln schedule, Conventional medium-temperature kiln-drying, Modified slicing technique, Released strain, Compressive stress