木材の乾燥過程のクリ-プにおける変形と含水率変化の関係
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Creep of wood during drying was studied in tension and in bending. The specimens of 2 × 10mm2 cross section of Buna (Fagus crenata Blume) wood and Hinoki (Chamaecyparis obtusa ENDL.) wood were tested under condition of decreasing moisture content. Initial moisture content of specimens were about 100 % and 20 %. In tension tests, the specimens of 150 mm length were used and tensile loads were applied perpendicular to the grain (radial-and tangential-directions). Unload spacimens were used as controls and its weight changes were measured with Ugauge. In bending tests, beams of 120 mm length were tested over a span of 85 mm and loads were applied at the center of the span. The central deflection was observed with a cathetometer. After creep was measured in moisture equilibrium state for 3～4 hours, drying was begun at constant temperature of 30℃. The total strain in creep during drying was taken to be made up of an equilibrium component εo + εt (εo : elastic strain, εt: time-dependent strain in moisture equilibrium state) and a non-equilibrium component εm. The main purpose of this report was to clarify the relation between εm and moisture content change M in creep during drying. In the process of drying from about 100 % moisture content, εm begins to increase at 50 % moisture content and 20 % of final value of εm is occured at the fiber saturation point. Then, after the moisture content of specimen reaches to about 20 %, a change of εm is in proportion to moisture content change. The relation between εm and moisture change is similar to the shrinkage-moisture content relationship. Then, creep during drying from about 20 % moisture content were measured and the relation between sm and moisture change M was investigated further. The results obtained are summarised as follows. 1) The average shrinkage coefficients da/dm under tensile stresses are related to the applied stress σ and moisture content m as follows da/dm = -A・σ + dao/dm ......(I) where dao/dm is the average shrinkage coefficient of unload specimen (control) and A is a constant. 2) Non-equilibrium strain component εm in creep during drying is expressed as a function of moisture change M (%) and the apllied stress σ. dεm/dM = φ・σ, ......(II) where φ is a constant. 3) φ in Eq. (II) is defined as a characteristic constant in creep during drying and determined by the next Eq. (III). dJ/dM = φ, ......(III) where J is creep compliance (cm2/kg). 4) φ are dependent on the instantaneous conpliance J in tension and in bending. 5) The relation between φ and J in tension is different from in bending. This fact shows the effect of different loading modes. 6) φ/J are approximately constants independent on J in tension, but are not independent on J in bending. Lastly, Eq. (II) was discussed in relation to the Takemura's and Leicester's models that explain the coupling effect of applied stress and moisture change in specimen in creep of wood during drying.
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