Load Carrying Mechanisms in Wood at Different Observation Scales: A Combined Random-Periodic Multistep Homogenization Sc
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0975-DD07-09
Load Carrying Mechanisms in Wood at Different Observation Scales: A Combined Random-Periodic Multistep Homogenization Scheme Karin Hofstetter, Christian Hellmich, and Josef Eberhardsteiner Vienna University of Technology (TU Wien), Vienna (Wien), A-1040, Austria
ABSTRACT Wood exhibits a highly diversified microstructure. It appears as a solid-type composite material at a length scale of some micrometers, while it resembles an assembly of plate-like elements arranged in a honeycomb fashion at the length scale of some hundreds of micrometers. These structural features of wood result in different load carrying mechanisms at different observation scales and at different loading conditions. In this paper, we elucidate the main load carrying mechanisms by means of a micromechanical model for wood across different species, based on tissue-independent stiffness properties of cellulose, lignin, and water. The model comprises three homogenization steps, two based on continuum micromechanics and one on the unit cell method. The latter represents plate-like bending and shear of the cell walls, due to transverse shear loading and axial straining in the tangential direction. Accurate representation of these deformation modes results in accurate (orthotropic) stiffness estimates, which deviate, on average, by less than 10 % from corresponding experimental results, across a variety of softwood species. INTRODUCTION Wood exhibits a highly diversified microstructure. It is composed of wood cells, which build up a honeycomb-like structure oriented in the stem direction. The cell wall is a composite material consisting of cellulose microfibrils embedded in an amorphous matrix made up of hemicellulose, lignin, and water. Thus, wood appears as a solid-type composite material at a length scale of some micrometers, while it resembles an assembly of plate-like elements arranged in a honeycomb fashion at the length scale of some hundreds of micrometers. In this paper, we investigate the mechanical relevance of these structural features of wood and elucidate main load carrying mechanisms at different observation scales and at different loading conditions. In order to identify the macroscopic material properties resulting from solidtype behavior and of plate-type behavior of wood microstructures, we employ continuum micromechanics and unit cell theory as homogenization techniques. The composite structure of the wood cell wall motivates application of continuum micromechanics (Mori-Tanaka and selfconsistent schemes) for estimation of its elastic properties. At the cellular scale, the unit cell method appears more appropriate, as it allows for representation of the out-of-plane bending and shear deformations of the cell walls. The model is validated by comparison of tissue-specific elastic constants of softwood, predicted by the model for tissue-specific composition and tissueindependent stiffness properties of cellulose, hemicellulose, lignin, and water, with
corresponding tissue-specific experimental results. The importance of
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