Constitutive equation of wood at variable humidity and temperature

Theoretical analysis of the effects of variations in moisture content and temperature on the creep of wood is presented. Thermodynamics of the processes of diffusion of water in wood microstructure is discussed and distinction is drawn between macrodiffusion and microdiffusion. The constitutive relation for steady states of moisture content and temperature is formulated on the basis of Maxwell chain model whose viscosity coefficients depend on moisture content and temperature. It is shown that the apparent acceleration of creep due to simultaneous drying (or wetting) as well as heating (or cooling) may be modeled as additional, stress-induced shrinkage (or swelling) and stress-induced thermal expansion (or contraction), described by shrinkage and thermal expansion coefficients that depend on the absolute values of the rates of pore humidity and temperature. Certain other sources of irreversibility of creep are also discussed.