Modelling warp and dimensional stability in softwood
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Seasoned softwood board exposed to in-service moisture variations is subject to dimensional changes and distortions. In-service dimensional instability of a board is a critical issue since it causes serious problems in buildings and other engineering structures. This thesis presents research involving the development of numerical models to describe the in-service changes in dimension, at a board scale, for small variations in the moisture content. The models used input data defining the properties of the microstructural components that make up the cellular structure of the wood resulting in the cells having heterogeneous anisotropic properties. The application of full cyclic constraint to the boundaries of the elements that made up the representative volume element was investigated and used to determine the equivalent homogenised properties. A cellular model was developed to describe the behaviour of the wood as a function of density, microfibril angle, spiral grain angle and moisture content. The resultant finite element board models used a database generated from the cellular model to describe the dimensional changes associated with small in-service changes in moisture content. In conjunction with the development of this model the candidate carried out an experimental programme using a specific supplied pinus radiata log. The determination of the cellular shrinkage/swelling coefficients, using an experimental technique developed for this research, verified the accuracy of the homogenization method and its subsequent application to the cellular model. The results from the sawn boards were compared with the results from the finite element board models.