<論文>木材の繊維方向における比動的ヤング率と内部摩擦

Abstract

木材の繊維方向では,比動的ヤソグ率E'γの対数と内部厚擦tanδの対数との間に勾配が負の直線関係がある。 本研究の目的は,E'/γに及ぼす細胞壁構造の影響を明らかにし,内部摩擦の生じる機構を推定すると同時に,E'/γとtanδの間に刊列関係が存在する理由を明らかにすることである。モデル解析の結果, E'/γの値にはS_2層のミクロフィブリル傾角が最も大きく影響していることが明らかになった。このことを実験的に節かめるために,ヒノキ間伐材について,X線回折によりS_2層の平均ミクロフィブリル傾角φと,両端自由たわみ振動法により,E'/γを求めた結果,E'/γとφとの間の関係は,計算により推定したものと定量的によく一致した。また,tanδとφの間にも,実験的に高い相関関係が存在した。得られた結果から,木材において内部摩擦の生じる主な機構は,S_2層におけるミクロフィブリルの変形とマトリックスのそれとの間の位相差に基づくものと推定した。

In the previous papers, it was reported that specific dynamic Young's modulus and internal friction in the longitudinal direction varied not only among the different wood species but also within the same species and there was a good correlation between these quantities. The purpose of this investigation is to examine the effects of the cell wall structure including crystallinity, microfibril angle in S_2 layer, proportion of S_2 layer, and values of elastic constants of matrix on specific dynamic Young's modulus and to clarify the mechanism of internal friction in wood. According to the results estimated using a cell-wall model, specific dynamic Young's modulus depended remarkably on microfibril angle in S_2 layer, but slightly on the other factors. Therefore, it was considered that the main reason for the variation of specific dynamic Young's modulus resulted from the variation of microfibril angle in S_2 layer. To ascertain this experimentally, average microfibril angle and dynamic mechanical properties such as dynamic Young's modulus and internal friction of Hinoki wood (Chamaecyparis obiusa) from the thinning operation were measured by X-ray diffraction and by a flexural vibration of a free-free beam, respectively, and specific dynamic Young's modulus was plotted against average microfibril angle. The experimental results showed a good agreement with the calculated ones. Furthermore, it was found that there was also a good correlation between internal friction and average microfibril angle in S_2 layer. With reference to these results, it was considered that internal friction in wood resulted from time lag between deformations of framework and matrix in S_2 layer, and the effect of microfibril angle in S_2 layer on internal friction was discussed.