Timber is a naturally occurring material, with properties adapted to its local environment. Moisture has a pronounced effect on the strength, stiffness and dimensional stability of timber. One particular concern is moisture induced warping, both during the drying process and during construction and the lifespan of any building. Recent research into sorption kinetics has determined that the rate limiting step in moisture sorption in timber could be due to substrate swelling rather than the process being diffusion limited, or could be a more complex interplay of the various processes. In order to better understand the sorption behaviour and to elucidate the role of the individual processes, we develop a thermodynamically consistent model for the hygro-mechanical response of timber in reaction to a change of humidity. We formulate the sorption model in terms of the conservation of mass, the kinetics of deformation and solvent migration, and the balance of forces. Constitutive assumptions are introduced to resolve the mechanical, chemical and diffusive behaviour, including a Zener model for the viscoelastic behaviour, the Flory-Huggins theory for the heat of mixing, and Fick’s law for the moisture transport. The model is solved numerically using a Finite Difference scheme to obtain sorption curves. The current model will serve as the starting point in an attempt to create a multi-scale model to create an accurate set of hygro-mechanical properties for softwood.
Keywords: Wood, Sorption, Viscoelastic, Finite Difference
Authors
Richardson E.
School of Engineering, University of Glasgow
de Borst K.
School of Engineering, University of Glasgow, Rankine Building
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