Temperature and time dependence of orthotropic viscoelastic properties of moist wood determined in tensile mode

Temperature and time dependence of orthotropic viscoelastic properties for Chinese fir wood (Cunninghamia lanceolate) were investigated in tensile mode. The storage modulus (E') and loss modulus (E '') of the longitudinal (L), radial (R) and tangential (T) specimens with adsorbed water were determined during temperature increasing period ranging from 30 degrees C to 103 degrees C and at a constant temperature of 103 degrees C for 480 min. The desorption of adsorbed water provided space for the rearrangement of hydrogen bonds (RHB) within the polymer networks and caused mechanosorptive (MS) effect. During the temperature increasing period, the MS effect increased first and following decreased for all specimens, except for the T specimens with 7.2% moisture content. In a constant temperature process, elevated stiffness and decreased damping were observed in all orthotropic directions, which suggested that the RHB effect was prominent than the dual effects of the MS and heating. Compared to the L specimens, the multi-effects of the heating, RHB and MS exerted more influence on the R and T specimens. Meanwhile, the unstable state within wood cell walls in the R and T specimens was more pronounced than that in the L specimens. Furthermore, the transverse specimens were affected by temperature to a higher degree than that of the L specimens.