Effects of moisture cycling period and cross sectional size of specimens on bending creep of wood under humidity cycling

Bending creep tests of spruce beams 0.5 x 1.5 cm, 1.0 x 3.0 sm and 1.5 x 4. 5 cm in cross section were done under four different conditions of humidity cycling. The relative humidity was alternately held at 10 and 90%, at a co nstant temperature of 20 degreesC. In addition to regular humidity cycling with periods of 1, 4,and 8 days, complex cycles of successive periods of 1, 4, and 8 days were used. Typical mechano-sorptive creep was observed when the cross section was small or the humidity cycling period was long;, Howev er, unusual mecahno-sorptive creep, namely creep increases during adsorptio n and decreases during desorption, was also observed when the cross section was large or the humidity cycling period was short. At the first adsorption stage, all specimens showed Large increases of defl ection. The creep during adsorption was affected by the history of moisture changes. However, the mechano-sorptive creep approached being reversible i n successive desorption and adsorption stages with increasing numbers of hu midity cycles, i.e., the mechano-sorptive creep arrived at a limit cycle st age. The accumulated deflection caused by the mechano-sorptive creep became larger as the number of humidity cycle increased, and finally approached a limit. The limit became larger as the moisture penetrated deeper into the specimen. The accumulated deflection was affected by the history of the longest cycle in the previous humidity cycling.