Comparative investigation on the orthotropic viscoelastic properties of wood during cooling and heating variations

Differences in the orthotropic viscoelastic properties of Chinese fir wood (<i>Cunninghamia lanceolata</i>) during cooling and heating variations were investigated in the temperature range between −120°C and 25°C. The stiffness and damping of the longitudinal (<i>L</i>), radial (<i>R</i>), and tangential (<i>T</i>) specimens were determined for five equilibrium moisture content (EMC) levels ranging from 0.6% to 25.2%. Results showed that the tensile storage modulus (<i>E</i>′) values in the cooling run were lower than those in the heating run at each temperature, regardless of the grain orientation and EMC level. The absolute value of the changing rate of <i>E</i>′ (|Δ<i>E</i>′|) in the cooling run was more pronounced than that in the heating run. The values of |Δ<i>E</i>′| demonstrated an anisotropy at all EMC levels, while the |Δ<i>E</i>′| of L specimens were lower than that of transverse specimens. Compared to per unit temperature, per unit moisture content exerted more influence on the stiffness of specimens. The <i>γ</i>-relaxation was observed in the loss modulus (<i>E</i>′′) and loss factor (tan<i>δ</i>) temperature spectrum. The <i>γ</i>-relaxation peak temperature in the <i>E</i>″ temperature spectrum was lower than that in the tan<i>δ</i> spectrum. The <i>γ</i>-relaxation peak temperature in the cooling run was lower than that in the heating run, irrespective of the grain orientation. Furthermore, the <i>γ</i>-relaxation peak temperature shifted to a lower temperature range with an increasing amount of adsorbed water, independent of cooling and heating runs.