The Effect of Shrinkage Anisotropy on Tangential Rheological Properties of Asian White Birch Disks

The process of wood drying can induce defects caused by drying stress, which limits the processing and utilization of this valuable material. Here, we investigated elastic strain, viscoelastic creep strain, and mechanosorptive (MS) creep strain caused by shrinkage anisotropy using the image analytical method during slow conventional drying of white birch (Betula platyphylla Suk) disks. The rheological properties of wood disks with different moisture contents (MC) were analyzed together with the influences of MC and radial position on each strain. The results showed that relations between stress and strain are complex; below the fiber saturation point (FSP), the wood disk is initially subject to tangential tensile stress; with decreasing MC, the tensile stress turns into a compressive stress. MS creep strain increased with decreasing MC; however, elastic strain and viscoelastic creep strain were positively correlated with MC. Elastic strain decreased after first increasing, and then remained stable while the MS creep strain significantly increased from pith to bark, at 10% MC and 18% MC, respectively. Shrinkage anisotropy was the main reason for strain during the drying processing, and it was one of the main factors causing cracks during drying or application.