Numerical analysis of Moisture-Related Distortions in Sawn Timber

Doktorsavhandling, 1999

In timber exposed to moisture variations such as in wood drying, shape distortions are often a serious problem since it can make the wood products obtained unsuitable for construction purposes. Two characteristics of wood are that its behaviour is strongly orthotropic and that it is very sensitive to variation in moisture. In addition, wood is characterized by variation in its properties from pith to bark. A further important property of wood affecting its behaviour is its spiral grain. In addition, in timber containing much compression wood, drying distortion is highly dependent upon where the compression wood is located in the board. In the present thesis, a finite element method is used to simulate deformations and stresses in wood during drying. A three-dimensional theory for the numerical simulation of deformations and stresses in wood during moisture variation is described. The constitutive model employed assumes the total strain rate to be the sum of the elastic strain rate, the moisture-induced strain rate, the mechano-sorption strain rate and the creep strain rate. Wood is assumed to be an orthotropic material with large differences in properties between the longitudinal, radial and tangential directions regarding the stiffness parameters as well as the moisture shrinkage and mechano-sorption parameters. The influence of moisture content and temperature on the material parameters is likewise taken into account. In addition, the effect of inhomogeneity in the internal structure of the wood material is considered. The influence of the growth rings, the spiral grain and the conical shape of the log on the orthotropic directions in the wood is also taken account of in the model. Variations in the wood properties with distance from the pith are considered as well. The three-dimensional theory used 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 boards during drying was simulated. These simulations yield information on unfavourable deformations and stresses that can develop during the drying process. Boards without external constraint during drying are analysed, as well as boards from different positions in the timber log, boards with deviations in the pith and boards with external constraints. The finite element model is also used to clarify how the material properties and the internal structure affect stiffness properties in sawn timber. The influence of the stiffness parameters, the spiral grain and the annual ring orientation are of particular interest. 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 examined. Numerical simulations were performed to investigate how the annual ring orientation, the cross sectional size and the drying profiles affect the shape stability of sawn timber. The influence of radial variations in the basic properties, the spiral grain and the conical angle was analysed as well. The study involves an experimental investigation of density, grain angles, shrinkage parameters and the longitudinal elastic modulus of a number of spruce boards containing compression wood. On the basis of the data obtained, numerical simulations are carried out to determine deformations that develop in the boards during changes in moisture. Shape stability of sawn timber can often be improved by gluing pieces of wood together. To study how the internal location and orientation of the pieces influence the drying deformations of glued products, numerical simulations for different products were performed. The knowledge obtained can contribute to more effective use of the raw material through allowing boards with properties resulting in poor shape stability or poor stiffness to be sorted out. Possibilities for improving the shape stability through gluing pieces of wood together are examined as well.