Abstract
A three-dimensional numerical model was employed in simulating nonlinear transient moisture flow in wood and the wood’s hygro-mechanical and visco-elastic behaviour under such conditions. The model was developed using the finite element software Abaqus FEA®, while taking account of the fibre orientation of the wood. The purpose of the study was to assess the ability of the model to simulate the response of wood beams to bending and to the climate of northern Europe. Four-point bending tests of small and clear wood specimens exposed to a constant temperature and to systematic changes in relative humidity were conducted to calibrate the numerical model. A validation of the model was then performed on the basis of a four-point bending test of solid timber beams subjected to natural climatic conditions but sheltered from the direct effects of rain, wind and sunlight. The three-dimensional character of the model enabled a full analysis of the effects of changes in moisture content and in fibre orientation on stress developments in the wood. The results obtained showed a clear distinction between the effects of moisture on the stress developments caused by mechanical loads and the stress developments caused solely by changes in climate. The changes in moisture that occurred were found to have the strongest effect on the stress state that developed in areas in which the tangential direction of the material was aligned with the exchange surface of the beams. Such areas were found to be exposed to high-tension stress during drying and to stress reversal brought about by the uneven drying and shrinkage differences that developed between the outer surface and the inner sections of the beams.
Highlights
The renewable character and the low carbon footprint of wood as compared with for example concrete (Dodoo 2011), have made wood increasingly popular for use in structural applications (Malo et al 2016)
The model, implemented in a powerful FE-software, appeared to be ideal for computing and visualizing complicated stress fields characterized by a combination of differing material orientations, as well as marked climate changes, long-term effects, and mechanical loads
The small clear-wood beams analysed as part of the second application show the strong effect that spiral grain and climate have on deflection, calibrated material parameters and normative stress states
Summary
The renewable character and the low carbon footprint of wood as compared with for example concrete (Dodoo 2011), have made wood increasingly popular for use in structural applications (Malo et al 2016). Due to the natural process of sorption that takes place, wood interacts very readily with the surrounding climate, trying to establish an equilibrium moisture content (EMC), even when the direct effects of rain, solar radiation or wind are lacking (Gustafsson et al 1998). When wood is subjected to a combination of change in moisture content (MC) and state of stress brought on by external load (e.g., mechanical load) or internal constraint (e.g., through differential shrinkage or swelling), a continuous change in the level of stress and the occurrence of deformations can be the result. An unfavourable state of stress can lead to an exponential increase in deflection and, in consequence of this, to failure (Armstrong and Kingston 1962; Bodig and Jayne 1982) or to fracture in the longitudinal-radial plane (TL- and TR-oriented cracking) that can weaken the shearing capacity of beams (Larsen 2013; McMillen 1958). With recent developments in three-dimensional modelling, predictions can be made both of moisture flow and of moisture-induced stress and distortion in three-dimensional space
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