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Doctoral project: Moisture induced stress and deformation of wood

This doctoral project focuses on the development of a three-dimensional numerical model in the finite-element software Abaqus to be able to analyse moisture transport and stress development in wood, while considering the orthotropic characteristics of the material. The model is used for several applications: from drying of timber boards in green-state to the calibration and validation of the model based on long term four-point bending tests subjected to climate variation.

Image: (a.) Four-point bending test setup inside climate chamber. (b.) Four-point bending test setup in natural climate. (c.) Three-dimensional stress output of numerical model used to simulate bending.

Project information

Doctoral student
Sara Florisson
Supervisors
Sigurdur Ormarsson and Johan Vessby
Project start
Nov 2015
Subject
Building technology (Department of Building Technology, Faculty of Technology)

More about the project

The monitoring of moisture change and deformation of wood is generally done in specific points in and around wood, painting a fragmented picture of its behaviour. With recent developments in three-dimensional modelling, it is now possible to simulate stress and deformation caused by moisture variation, while including heuristic boundary conditions and an accurate description of fibre orientation. The model is used for several applications: from drying of timber boards in green-state, to the calibration and validation of the model based on long term four-point bending tests subjected to climate variation.

Nowadays, with the introduction of glue-laminated and cross-laminated timber, the use of wood as a construction material is increasing in popularity. With that, the demand for an optimal and efficient design of the long-term mechanical behaviour of timber structures grows, and consequently more research is required. Wood is a hygroscopic material that continuously strives to establish an equilibrium moisture content with the surrounding climate. This response influences the integrity of the material and affects the long-term behaviour. From a mechanical or structural perspective, this entails that the wood deforms upon interaction with moisture, shows increased deformations when loaded both with a mechanical and climatic load, and develops surface checks, inner checks or splitting when exposed to severe drying conditions.

The doctoral project focusses on the development of a three-dimensional numerical model in finite-element software Abaqus FEA to be able to analyse moisture transport and stress development in wood, while considering the orthotropic characteristics of the material. To create flexibility and use the computational advantages of the FE-program, user-subroutines are developed. User-subroutines UMATHT and FILM cover the theory for the non-linear transient moisture transport with a nonlinear Neumann boundary condition, respectively. The material routine UMAT describes the moisture induced distortion and the moisture and temperature dependent hygro-mechanical parameters, where ORIENT takes care of the orthotropic material orientation. 

The fully uncoupled numerical analysis is able to describe the moisture transport and viscoelastic hygro-mechanical response of wood on macroscopic level and is employed in several applications. To calibrate or validate the model, experimental work from literature or self-performed tests are applied. The model is used to simulate timber boards dried from green-state to equilibrium moisture content for four different configurations of annual ring orientation in the cross-section, small-clear-wood beams subjected simultaneously with a mechanical load and systematic changes in relative humidity, and timber beams loaded with a mechanical load and exposed to natural climate.