Orthotropic mechano-sorptive creep behavior of Chinese fir during moisture desorption process determined in tensile mode

Abstract

Time-dependent creep behavior is one of the most important material characteristics in wood products exposed to various environmental conditions. The creep behaviors, including the general viscoelastic creep (VEC) at constant moisture content and the mechano-sorptive creep (MSC) during moisture content variations, are, however, not sufficiently described. Given the anisotropy of wood, comprehensive data on the creep behavior will require knowledge of its moisture-dependent properties in relation to the three principal axes of anisotropy. The present study examined the tensile orthotropic creep behaviors of Chinese fir (Cunninghamia lanceolata): VEC at constant moisture content and MSC during desorption process, at 0, 20, 40, and 60% relative humidity (30\xa0°C). The creep anisotropy of MSC was more pronounced than that of VEC. The free shrinkage caused the specimens to deform in the opposite direction to load in MSC tests. Based on the databases of free shrinkage, VEC, and MSC, it is obvious that the mechanical stress has a positive effect on MSC, which is presumed to be a result of the double effect composed of the rearrangement of hydrogen bonds and unstable state. The unstable state could be evaluated quantitatively by the mechano-sorptive strain. The mechano-sorptive strain of radial and tangential specimens is affected by the relative humidity to a higher degree than that of longitudinal specimens. Further, the unstable state exerted more influence on tangential specimens and less on longitudinal specimens.