Wood and wood products - linking multiscale analysis and structural numerical simulations
Wood and wood products - linking multiscale analysis and structural numerical simulations
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木材和木制品 - 连接多尺度分析和结构数值模拟
DOI：
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
J. Eberhardsteiner
中科院分区：
文献类型：
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作者：
K. Hofstetter;J. Eberhardsteiner
Summary: Wood is one of the oldest construction materials known to man. Over thousands of years it has been mainly used in a craft framework, so that current design rules are often based on experience and tradition. The scientific knowledge about the material behavior is often surprisingly poor. In order to exploit the extraordinary ecological potential of the material and to enable its structural use also in an industrial framework, improved material models are required. Modern timber construction is characterized by increasing demand of two- and three dimensional bearing components. Dimensioning and design of such sophisticated structures require powerful material models for numerical simulation tools such as the finite element (FE) method. Moreover, the large variability of the macroscopic material properties has to be understood and suitably described to prevent exaggerated safety factors resulting in an uneconomic over-dimensioning of timber members. In order to understand the variability of macroscopic properties of solid wood and the underlying phenomena and to suitably describe them in material models, the hierarchical microstructure of the material has to be considered. At sufficiently small length scales universal constituents common to all wood species and samples as well as universal building principles can be identified. Namely, lignin, hemicellulose, cellulose, and water are such tissue-independent universal constituents with common mechanical properties across the diverse wood species at the molecular level. They build up cell walls resembling fiber-reinforced composites, which are arranged according to a honeycomb pattern. A mathematical formulation of the univeral building principles results in a multiscale micromechanical model for wood which links microstructural characteristics of individual wood samples to macroscopic mechanical characteristics of these samples. Homogenization techniques are employed for this purpose. In particular, the composite structure of the wood cell wall motivates application of continuum micromechanics for estimation of its elastic properties. At the cellular scale, plate-type bending and shear deformations dominate the mechanical behavior, which are more suitably represented by a unit cell approach. Formulation of the localization problem corresponding to the multiscale homogenization scheme allows determination of strain estimates at smaller length scales for given macroscopic loading. Quadratic strain averages (so-called ‘second-order estimates’) over microstructural components turned out to suitably characterize strain peaks in these components. Combination of estimates for such averages with microscale failure criteria delivers predictions for macroscopic elastic limit states. As for solid wood, experimental investigations indicate that wood failure is initiated by shear failure of lignin in the wood cell wall. This can be suitably described mathematically by means of a von Misesfailure criterion. The multiscale models for wood stiffness and elastic limit states are validated by comparison of model predictions for stiffness and strength properties with corresponding experimental results across a multitude of different wood species and different samples. The small errors of the model predictions underline the predictive capabilities of the micromechanical model. For example, the mean prediction errors for the elastic moduli and the shear moduli related to the three principal material directions L, R, and T are each below 10 %.
