Moisture-induced stress and distortion of wood : A numerical and experimental study of wood's drying and long-term behaviour

Sammanfattning: With the current advances made in three-dimensional modelling of wood, it is possible to provide an overall picture of moisture flow, and moisture-induced stress and deformations, whereas previously, experiments only provided local measurements. The main aim of the doctoral thesis is to investigate the possibilities of the developed three-dimensional numerical model to predict the behaviour of wood when simultaneously exposed to a mechanical load and a particular climate. Three applications in the fields of wood drying and long-term behaviour of wood are considered: 1) the effect of green-state moisture content on the drying behaviour of timber boards, 2) the calibration of the numerical model based on a long-term four-point bending tests using small wood beams subjected to a constant temperature and systematic relative humidity (RH) changes, and 3) the validation of the numerical model by means of a long-term four-point bending test on solid timber beams subjected to Northern European climate. As part of the second application, an experimental methodology and analytical method were designed. The numerical model was developed in finite element software Abaqus FEA® and consists of several user-subroutines to include material orientation (i.e. annual ring pattern, conical shape and spiral grain), and the selected constitutive behaviour and required boundary conditions. To simulate the moisture flow, a nonlinear single-Fickian approach was combined with a nonlinear Neumann boundary condition, which describes the flux normal to the exchange surface based on a moisture and temperature dependent surface emission coefficient. A strain relation was used that accounts for hygro-expansion, and the elastic, creep and mechano-sorptive behaviour. The analytical method describes the elastic and creep deflection in the constant moment area of the four-point bending setup, and was used to isolate and assess the mechano-sorption deflection in the cumulative moisture content domain. The results show that the three-dimensional character of the numerical model contributed to the analysis and visualisation of the different stress states and deformations that are affected by material properties that vary (i.e. from pith to bark, between heartwood and sapwood, and due to temperature and moisture content), fibre orientation and climate. The simulations made on timber boards clarified phenomena, such as stress reversal and casehardening associated with wood drying, and showed that the green-state moisture content affected the time, size and frequency with which extremes in tangential tensile stress developed inside the timber during drying. The results of the calibration and validation indicated that the numerical model is able to describe moisture change and gradients in the considered temperature and relative humidity ranges (between -2-60℃ and 40-80% RH), as well as the deflection. The experimental methodology and analytical method led to a successful identification of each deflection component and isolation of the mechano-sorptive deflection curves. The experimental methodology benefitted the calibration of the numerical model. In conclusion, the presented three-dimensional numerical model compatible with Abaqus FEA® provides a powerful tool for scientists and timber engineers to study the combined effect of load and climate on stress state and deformations of various timber products in a wide field of applications.