Abstract
The phenomena and mechanism in wood rheology on changing moisture content are reviewed. Creep and stress relaxation of wood are greatly influenced by changes in moisture content. The creep rate and total creep of wood are found to be considerably greater when the moisture content of loaded specimen decreases than when it remains constant at either green or dry states. During subsequent cycles of moisture changes, the loaded specimen shows partial recovery during each absorption followed by increased deformation during desorption. After extensive moisture cycling, such a deformation may reach more than 20 times the initial one. In unloading the dry specimen, little instantaneous elastic recovery takes place, but on putting the specimen in saturated water vapor, a large recovery quickly occurs. Similarly both the rate of relaxation and the residual deformation are accelerated by reduction of moisture content, and the residual deformation can be largely removed by a subsequent increase in moisture content. The rates of creep and stress relaxation caused by changing moisture content depend on the rate of moisture content change and are little affected by the duration of the process. Several interpretations have been reported for the behavior. In practice, deflection of wooden beam in service, drying stress and set in seasoning of wood, and bent wood process are related to the above-mentioned behavior.
