Viscoelastic properties of wood from Chinese-fir and poplar plantations

Elastic and strength properties （proportional-limit stress （O＇prop）, Young＇s modulus （E）, breaking stress （O＇max） in static bending parallel to grain in a longitudinal direction）, as well as stress relaxation in air-dried condition and water-saturated conditions at seven different constant temperatures and increasing and decreasing temperatures were investigated for wood from Chinese-fir and poplar plantations. The results show that hygrothermal conditions considerably affect these mechanical properties. The higher the moisture content （MC） or temperature, the lower the strength of wood. Further investigation of the effects of constant temperature on stress relaxation indicates that high temperature specimens have low relaxation moduli and high fluidity. In the case of increasing temperature the range of the modulus of relaxation is larger than in the case of a reduction in temperature, while the residual moduli do not show large differences. This is because the modulus at high temperatures decreases more than that at low temperatures. The fluidity of specimens in a state of water desorption increases slowly at the beginning, increases quickly until the MC reaches an equilibrium moisture content （EMC） and then becomes stable, which is quite different from that in a water-saturated state. Fluidity in a desorption state is much higher than in a water-saturated state. This is probably due to the fact that the former is in an unstable state which can be interpreted as a state with internal strain and has therefore a greater potential to release strain.