STRESS DEPENDENCE OF BENDING CREEP-BEHAVI OR OF WOOD UNDER CHANGING MOISTURE CONDITIONS

This paper describes the stress dependence of bending creep under air- dry, moisture-regain, high moisture, and moisture-loss conditions. The bending creep test was conducted by four-point loading at constant st resses of 20, 35, 50, 65, and 75% of the ultimate short-time strength. Creep strain was more stress-dependent under high moisture condition than under air-dry condition, and all specimens had creep fractures at the 75% stress level under high moisture condition. For moisture-rega in condition, the stress dependence of creep strain was great and was influenced significantly by time. The creeps of all specimens were acc ompanied with fractures at stress level above 65%. The stress level wa s found to increase with gain of moisture in the creep process. On the other hand, for moisture-loss condition, the stress dependence of cre ep strain and creep strain per unit change in moisture content were sm all and hardly affected by time. The stress level was found to decreas e with loss of moisture in the creep process. In the exponential regre ssion equations between the change in moisture content and the creep s train, the average fitted values of the exponent for moisture-regain c ondition ranged from 1.15 to 2.11 and increased with increasing stress level, whereas those for moisture-loss condition had almost constant values of about 1.3, independent of stress level. The difference in st ress dependence between the moisture-regain and moisture-loss conditio ns can be explained in terms of the contribution to creep deformation of the temporary loosening and bonding in the non-crystalline region w ith the sorption and desorption of moisture and the change in stress l evel during the creep process.