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

Differences in the orthotropic viscoelastic properties of Chinese fir wood (Cunninghamia lanceolata) during cooling and heating variations were investigated in the temperature range between -120 degrees C and 25 degrees C. The stiffness and damping of the longitudinal (L), radial (R), and tangential (T) specimens were determined for five equilibrium moisture content (EMC) levels ranging from 0.6% to 25.2%. Results showed that the tensile storage modulus (E') 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 E' (vertical bar Delta E'vertical bar) in the cooling run was more pronounced than that in the heating run. The values of vertical bar Delta E'vertical bar demonstrated an anisotropy at all EMC levels, while the vertical bar Delta E'vertical bar 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 gamma-relaxation was observed in the loss modulus (E '') and loss factor (tan delta) temperature spectrum. The gamma-relaxation peak temperature in the E '' temperature spectrum was lower than that in the tan delta spectrum. The gamma-relaxation peak temperature in the cooling run was lower than that in the heating run, irrespective of the grain orientation. Furthermore, the gamma-relaxation peak temperature shifted to a lower temperature range with an increasing amount of adsorbed water, independent of cooling and heating runs.