The orthotropic viscoelastic properties of Chinese fir wood during the temperature ramping process

Abstract

In this study, the orthotropic viscoelasticity of Chinese fir wood (Cunninghamia lanceolata [Lamb.] Hook) was investigated during the temperature ramping process. The storage modulus (E′) and loss modulus (E″) of longitudinal (L), radial (R), and tangential (T) specimens with six levels of moisture content (MC) ranging from 0.6 to 22.0% were determined form 20 to 280 °C. The change in E′ was influenced by the dual effects of temperature and MC; the loss of adsorbed water in the wood cell wall caused an increase in wood stiffness. The individual decline in E′ was affected by heating to different degrees and the heating exerted more influence on the transverse specimens and less contribution to the L specimens. Different relaxation processes of E′′ were observed in specimens with different MCs in the three main directions, indicating that the mechanical relaxation of wood was affected by its MC and microstructure. The β and γ-relaxation processes exhibited a conjointly circular arc shape in the transverse directions, which was attributed to the superposition of the transitions of lignin and hemicellulose. A higher α-loss peak temperature (α-TLP) and intensity of the α-relaxation process were observed in the L specimens than the transverse specimens. The β-TLP and γ-TLP decreased with increasing MC in all orthotropic directions, whereas the individual decline in TLP was affected differently by MC. In the β-relaxation process, the TLP was more sensitive to MC changes in the L direction than in the transverse directions. In addition, the individual TLP in the R and T directions was influenced by per unit adsorbed water to a similar degree for the β and γ-relaxation processes. These findings suggest that the difference in the orthotropic viscoelastic performance might cause some drying defects during the water loss process.