Homogenisation: a mathematical technique used to predict the shrinkage of oak as a result of the annual ring morphology and the microscopic properties of tissues.
Résumé
As a result of its plant origin, wood is a very complex material which is heterogeneous and highly variable. Homogenization techniques are used in this work to propose a deterministic approach to oak wood properties. Homogenization requires two kinds of information: morphological, which describes the spatial distribution of the components; and the intrinsic properties of each anatomical component. From these data, macroscopic properties are calculated by homogenization. This paper focuses only on wood mechanical and shrinkage properties, measured microscopically with the help of specific experimental devices previously developed. A new numerical X-ray imaging system is presented, which generates numerical images of annual growth ring structure. After the separation of different components by image processing, a Finite Element mesh is built. Finally, a specifically developed computational tool, MorphoPore, solves four homogenization problems. This software calculates the macroscopic properties of the complete annual ring. Two examples are proposed which show how the morphology of the annual ring can be taken into account in the prediction of mechanical and shrinkage properties of wood. The first example proves that the proposed approach is capable of quantifying the influence of morphological variations using virtual structures. The second example deals with two successive actual annual rings: the first one is thin and the second one is large. The influence of morphological changes due to the growth rate of the tree becomes obvious. The most innovative aspect of this tool is that the influences of parameters due to genetic and growth conditions of the tree can be distinguished.
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