Development of a simulation model for drying deformation in radiata pine boards

Drying deformation of radiata pine (Pinus Radiata D. Don) boards results in a great loss of revenue to the wood industry. A computer simulation model, which is capable of predicting the drying quality of radiata pine boards, was developed for pregrading and for setting of drying schedules in sawmills. Before drying, logs must be sawn into slabs. Accordingly, the geometrical configurations of radiata pine boards were generated. The grain of clearwood was determined by the annual growth of radiata pine; and the structures of knot-containing boards were modelled on the basis of the growth features of branches. Scenario simulations of different sawn patterns, knot shapes and knot types were illustrated. During drying, moisture desorption in timber results in deformation. In order to use an appropriate partial differential equation for moisture movement in timber, Fick's second law was expanded by further incorporating the effects of mass velocities and thermal gradient. One of the developed partial differential equations was solved using the finite element method. All drying conditions and the diffusion properties of radiate pine were simulated. Typical heat and mass transport phenomena, including temperature profile, moisture profile, thermal gradient, mass flux and moisture gradient in clearwood and knot-containing boards, were presented using three-dimensional colour graphics. The three-dimensional drying deformation incorporating the effect of moisture profile (i.e. moisture content and gradient) was simulated. The orthotropic mechanical properties of wood were modelled on the basis of density, cambial age (i.e. ring number from the pith), ring width and moisture content. Scenario experiments were performed to illustrate the capability of the developed model. Finally, the predicted drying deformation behaviours were validated to be consistent with experimental studies by comparing deformation patterns. The statistical test further confirmed that the developed model was an accurate representation of the phenomena.