Deformation properties of Finnish spruce and pine wood in tangential and radial directions in association to high temperature drying. Part IV: Modelling

The set of papers (Part I-IV) sums up the results of an extensive project to quantify primarily the creep characteristics but also other deformation properties of Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) wood under conditions relevant in the high temperature drying process. The programme included tension experiments on perpendicular specimens at temperatures 95 °C–125 °C. Based on the experiments, a constitutive model to describe the behaviour was developed for use in numerical simulation of drying stresses and is reported in this paper. The model takes into account all the needed strain terms: hygroexpansion (shrinkage, swelling), hygrothermal, elastic, viscoelastic and mechano-sorptive strains. Elastic and viscoelastic strains are modelled jointly as generalised Kelvin material, using the time–temperature–moisture-content superposition principle. Mechano-sorptive creep is modelled with a series of modified `mechano-sorptive Kelvin elements', in which a part of the strain is irrecoverable unless the direction of the acting stress changes.