Mechanosorptive Creep of Hemlock Under Conventional Drying: II. Description of Actual Creep Behavior in Drying Lumber

Abstract

The objective of this article was to separate the mechanosorptive creep from the viscoelastic creep deformation of western hemlock (Tsuga heterophylla) lumber, as well as to calculate theoretical values of mechanosorptive creep strain during conventional drying and postdrying conditioning processes. Flat-sawn and quarter-sawn hemlock lumber pieces of 50 mm × 100 mm in cross section were dried in a conventional laboratory dryer at constant temperatures of 40 and 80°C. Width deformation changes along the thickness were measured by a slicing method. Shrinkage, elastic, viscoelastic creep, and mechanosorptive creep strains in both radial and tangential directions were calculated quantitatively. The influence of drying temperature and lumber cross-sectional configuration on its drying rheological properties are discussed qualitatively. The relative magnitude of deformation components, namely, the deformation percentage rates of specified drying strains, were analyzed during the whole drying process. As drying temperature increased from 40 to 80°C, the maximum values of both viscoelastic and mechanosorptive creep strain decreased at the lumber surface. This finding also coincided with the results of free shrinkage of hemlock wood at the same temperature level, highlighting the importance of a free shrinkage function in the mathematical description of drying creep deformation; at drying temperatures of 40 and 80°C, the maximum values of elastic, viscoelastic, and mechanosorptive creep strain for the surface of flat-sawn hemlock lumber were always higher than those for quarter-sawn. Results clearly showed that drying stresses relaxed due to the mechanosorptive creep deformation.