Deformation Behavior of Natural Wood Having Hierarchical Structure Under A Compression State
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Deformation Behavior of Natural Wood Having Hierarchical Structure Under A Compression State Tsunehisa MIKI1, Hiroyuki SUGIMOTO1, Kozo KANAYAMA 1 1 National Institute of Advanced Industrial Science and Technology (AIST), Anagahora 2266-98, Shimo-shidami, Moriyama-ku, Nagoya 463-8560, Japan. ABSTRACT A large deformation of bulk wood using slipping between the wood cells has been found just like a plastic deformation generated by slip band in metallic materials. This phenomenon is caused by the hierarchical structure of the wood cell, and the intercellular layer becomes selectively softened in moistened states of wood. In such conditions, bulk wood subject to compression at elevated temperatures can easily be deformed perpendicular to the longitudinal direction of the cells by shear flow stress after being collapsed. INTRODUCTION Wood is a natural polymer composite built up by tubular-shaped cells structured by some layers in which crystalline cellulose microfibril is embedded in matrix substances such as hemicellulose and lignin. In the each layer, there is a specific orientation of the cellulose microfibril and a layer laminates one by one with a different angle of orientation to make a wood cell. Each wood cell is bonded by the intercellular layer which contains relatively a large amount of lignin compared with the other layers [1]. Due to this complicated hierarchical structure as also seen in many biological materials [2], a bulk/solid wood, which is an aggregate of wood cells, generally is hard to deform plastically by applied stress, at most 20% as a longitudinal strain [3] in a wood cell. Therefore, present deformation technique for bulk wood has been performed by collapse of the tubular-shape cells led to buckling of the cell walls [4]. As a conventional wood forming, compression process in an elevated temperature has been carried out, and the products obtained by this kind of method are called as compressed woods or densified woods [5]. In such process, moisture and temperature are very important factors to buckle the cell walls with less damage by a smaller pressure. This is because the matrix in the wood cell wall such as hemicellulose and lignin drastically softens by heating with a rich moisture condition [6, 7]. Many studies on compressed woods have been attempted by means of compression test with a lateral restriction condition to investigate the optimum conditions for obtaining improved mechanical properties. Due to the lateral restriction during compression, bulk density of wood reaches over 1.3 to 1.4 g/cm3, and this leads to increase the strength properties of wood. However, using such restriction state, it is difficult to attain a complicated shape by collapsing the wood cells only. There are, currently, not any techniques by which bulk wood can be deformed into an arbitrary shape using their formability. In this study, a free compression test was performed in which lateral restriction was free to investigate deformation behavior of bulk wood perpendicular to the compression direction. The effects
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