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A nonlinear model for analysing heat and moisture flow in wood during drying below the fiber saturation point is presented. The model used considers wood at a macro level without taking the various moisture transports mechanisms at the microscopic level into account. Based on the finite-element method, a coupled system of equations resulting from the adopted heat and moisture transfer equations is established and an iterative scheme is proposed. The numerical procedure is verified by a test example. In a two-dimensional analysis, the influence of the coupling on the combined heat and moisture transfer is studied for a board subjected to a typical kiln-drying condition. How well the results agree with those obtained by introducing a commonly applied simplification is discussed.

The study concerns the question of how the shape stability features of laminated columns of Norway spruce can be improved in terms of twist through optimal orientation of the individual laminates. Both experimental testing and numerical simulations were employed for evaluating twist stability. In all the columns studied, deformations were measured experimentally at different moisture content levels. A number of columns were also selected for numerical analysis in order to obtain a more thorough understanding of the twist behavior involved, their geometries and material properties of interest being determined. The experimental results showed the twist stability of the columns to be highly dependant upon the internal orientation of the individual laminates. It was also found that high quality columns in terms of shape stability could be manufactured, even when the center-core material has a strong twist tendency. The numerical simulations performed were in close agreement with the experimental results.

An experimental study of shape stability in wooden glued boards was performed to verify certain results of particular interest obtained earlier in numerical simulations. Possibilities for achieving products of good shape stability by gluing boards together in an optimal way are discussed. Since twist is often the most serious form of distortion defects for the user, it was the main type of board deformation considered. Three types of glued products were tested. The experiments show clearly that glued boards can be produced that remain stable in shape when exposed to extreme variations in moisture. The results agree well with the numerical simulations performed earlier. It was found that initial twist could be reduced by proper application of pressure during gluing and that through gluing the occurrence of cracking defects on visible surfaces could be substantially reduced.

Numerical simulations were performed to investigate how the annual ring orientation affects the shape stability of sawn timber. The influence of radial variations in the basic properties and the spiral grain is also studied. The knowledge obtained can contribute to more effective use of the raw material through allowing boards having properties that would yield bad shape stability to be sorted out. Possibilities for improving shape stability through gluing pieces of wood together are examined as well.

In timber exposed to moisture variations, drying distortions is a serious problem that might result into sawn timber and other wood products unsuitable for construction purposes. Two characteristics of wood are that its behaviour is strongly orthotropic and that it is very sensitive to variations in moisture content. In addition, wood is characterised by variation in its properties from pith to bark. A further important property of wood, which affects its behaviour, is its spiral grain. For timber containing much compression wood the drying distortions are also highly dependent upon where the compression wood is located in the sawn boards. The present study concerns an experimental investigation of density, grain angles, shrinkage parameters and longitudinal elastic modulus in a number of spruce boards containing much compression wood. On the basis of the data obtained, numerical simulations have been carried out in order to determine the deformations that developed in the boards during changes in moisture content.

A theory for analysing the shape stability of sawn timber was implemented in a finite element program. To illustrate the types of results that can be obtained, the behaviour of a board during drying was simulated. The simulation yields information about unfavourable deformations and stresses during the drying process. To investigate factors that influence drying deformations, a parameter study was performed in which the influence of different constitutive models and different material parameters was studied. In addition, the influence of the spiral grain angle was examined.