Moisture Transport and Moisture-Induced Distortions in Timber - an Experimental and Numerical Study

Doctoral thesis, 2005

Timber products subjected to climatic variations commonly display moisture-induced distortions that can lead to serious problems for end-users. The aim of the thesis, which consists of an extended summary and six appended papers, was to investigate the potential of using the gluing technique suggested here for dealing with problems of moisture-induced distortions and developing the heat and mass transfer models which such distortion analyses require. An extensive investigation of the distortion of individual boards, laminated boards and columns was carried out. An overview of the work together with detailed information on various aspects of wood drying and of the distortion associated with it is presented. The shape stability (particularly the twist) of the timber products studied was investigated using both numerical and experimental approaches. Theories of heat and mass transfer in timber are taken up and the numerical implementation is discussed. The finite element method is utilized to solve the equations concerning the hygro-thermal-mechanical behaviour of wood that were obtained, and a method for determining material parameters needed in wood drying was developed. An adaptive procedure was employed to control the discretization error that the numerical procedure introduced, and a coupled non-linear model for predicting heat and moisture transfer during drying below the fibre saturation point is presented. For drying from an initially green condition, the moisture content predicted was compared with that estimated on the basis of CT-scanning. For the timber products studied, much better shape stability was obtained through proper orientation of the comprising laminates. The results provide support for the deformation predictions obtained through computer simulations. Material parameters of Norway spruce were determined, some of them being a function both of moisture content and of temperature. The theoretical predictions of moisture content agreed reasonably well with those obtained experimentally both above and below the fibre saturation point. The diffusion coefficient determined by optimisation was in accordance with values reported in the literature.