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From lignin to spruce: Poromechanical upscaling of woodstrength

Published online by Cambridge University Press:  14 March 2011

Thomas K. Bader ,
K. Hofstetter ,
Ch. Hellmich  and
Josef Eberhardsteiner
Thomas K. Bader
Affiliation:
Institute for Mechanics of Materials and Structures, Vienna University of Technology, A-1040 Vienna, Austria
K. Hofstetter
Affiliation:
Institute for Mechanics of Materials and Structures, Vienna University of Technology, A-1040 Vienna, Austria
Ch. Hellmich
Affiliation:
Institute for Mechanics of Materials and Structures, Vienna University of Technology, A-1040 Vienna, Austria
Josef Eberhardsteiner
Affiliation:
Institute for Mechanics of Materials and Structures, Vienna University of Technology, A-1040 Vienna, Austria
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Abstract

Wood strength is highly anisotropic, due to the inherent structuralhierarchy of the material. In the framework of a combined random-periodicmultiscale poro-micromechanics model, we here translate compositionalinformation throughout this hierarchy into the resulting anisotropicstrength at the softwood level, based on “universal” elastic properties ofcellulose, hemicelluloses, and lignin, and on the shear strength of thelatter elementary constituent. Therefore, derivation of the elastic energyin a piece (representative volume element – RVE) of softwood, stemming fromhomogeneous macroscopic strains prescribed in terms of displacements at theboundary of the RVE and from pressure exerted by water filling thenanoporous space between the hemicelluloses-lignin network within the cellwalls, with respect to the shear stiffness of lignin, yields higher orderstrains in the lignin phase, approximating micro-stress peaks leading tolocal lignin failure. Relating this (quasi-brittle) failure to overallsoftwood failure (or strictly speaking, elastic limit of softwood) resultsin a macroscopic microstructure-dependent failure criterion for softwood.The latter satisfactorily predicts the biaxial strength of spruce at variousloading angles with respect to the grain direction. The model also predictsthe experimentally well-established fact that uniaxial tensile andcompressive strengths, as well as the shear strength of wood, dependquasi-linearly on the cell water content, but highly nonlinearly on thelumen porosity.

Keywords

biologicalmicrostructurestrength

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References

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