木材の縦引張破断面の走査電子顕微鏡による観察

Abstract

木材の縦引張破壊部を走査電子顕微鏡で観察し, 細胞膜の破壊形態を調べた。試料は縦引張試験した薄片および曲げまたは衝撃曲げ試験した試験体の引張側などいずれも気乾状態で縦引張破壊した材部よりとり出し, カーボンおよび金を真空蒸着して観察した。針葉樹材の破壊部に多くみられるささくれた破断形の仮道管では, 2次膜中層S_2が裂けてとげ状を示した。一方, 軸にほぼ直角に破断した仮道管では, 仮道管膜を横断する破断面が観察され, S_2の破断面に平たんな面とひだ状の面とが認められた。平たんな破断面はS_2のフィブリル傾角の小さい夏材仮道管によくあらわれ, また, 衝撃曲げによる破壊では増加する傾向が認められた。平たんな破断面上には粒状構造が認められたが, これはフィブリルの切断先端にカーボンと金が蒸着されて生じたものと思われる。細胞間層Iの破断面は平滑で, 2次膜外層S_1はフィブリルに沿って裂けるか, または, ほぐれた状況を示した。仮道管の側面にあらわれるせん断破壊面にはS_1の構造が認められた。針葉樹柔細胞の膜の破断面は平滑であり, 一方, 広葉樹道管膜は緩傾斜のフィブリルに沿って裂けた破断面を示した。また, 圧縮あて材の仮道管膜はspiral checkに沿って裂け, 引張あて材繊維のゼラチン層はフィブリルに直角の破断を示した。細胞膜層の破壊形態は引張力の作用方向とその膜層でのフィブリルの方向とのなす角に影響され (Fig. 1), また, ひだ状の破断面はフィブリルの切断が微小な裂けを伴いつつ, こきざみに進行した破壊の伝播経過を示していると考えられる (Fig. 2)。

Wood pieces broken in tension parallel to the grain in the air-dried condition were examined by a scanning electron microscope to study the structural features of ruptured cell walls. Samples to be observed were taken both from the broken end of microtensile specimens and from the tensile side of beams broken in a bending or impact bending test. The samples were coated with carbon and gold evaporated under high vacuum. Softwood samples were mainly used in this study, while additional observations were made on hardwood ones. In the broken tracheids showing the splintering fracture type, the ruptured S2 had sharp splinters (Plate 2). In those tracheids showing the cross fracture type, the fracture crossed the wall nearly perpendicular to the tracheid axis, so that the cross fracture surface of the wall layers could be observed on the broken end of the wall. The cross fracture surface of S2 showed two characteristic features: the flat face and the ridgy face (Plate 3). The flat face was found particularly in S2 of latewood tracheids with the steep helical angles. On the flat face a granular appearance could be seen at high magnification (Plate 5), which was considered to be the broken ends of fibrils coated with carbon and gold. The impact bending test had a tendensy to increase the occurrence of the flat face fracture in latewood tracheids (Plate 8). In earlywood, however, the flat face fracture could be hardly found even in the broken tracheids showing the cross fracture type. In those tracheids the broken end of S2 had the ridgy face of the cross fracture type or the fine splinters of fibrils (Plate 7). The cross fracture surface of the intercellular layer at the cell corner appeared flat and smooth. S1 was split along its fibrillar orientation. The loosened and brushy fibrils of S1 were also found frequently in the split cell walls. Splitting and scission of S3 fibrils appeared to follow the break of the adjacent S2. The intercellular separation also occurred in tension, especially between latewood tracheids. Both the side face of separated tracheids and the inner face of the remainder of a wall showed the fibrillar structure of S1, indicating shear failure within S1 (Plate 9). Axial and ray parenchyma cell walls of softwood broken in axial tension had the brash fractures with flat and smooth face much the same as the intercellular layer (Plate 10, 11). The smooth face of ruptured cell walls would be due to a high lignin contents in them. The vessel wall with the flat helix was split along the fibrillar orientation (Plate 12). The wall of the compression wood tracheid was split along its spiral check (Plate 13), and the gelatinous layer in tension wood fibers was severed perpendicular to its fibrils, resulting in the cross fracture with flat face (Plate 14). The fracture type of a cell wall layer by the tensile load seems to depend on the angle between the direction of the load and the fibrillar orientation of the layer. As explained in a schematic diagram Fig. 1, the tensile load with a larger angle to the fibrils of the layer would make the fibrils separate laterally, resulting in a flat helical fracture (Fig. 1d). The tensile load nearly parallel to the fibrils would cause the scission of fibrils perpendicular to them, leading to the cross fracture with the flat face. With a medium angle between the two the lateral separation and scission of fibrils would be combined in various way, resulting in transitive types of the fracture. A typical one of them is the steep helical fracture i. e. the splintering fracture (Fig. 1c) and another is the cross fracture with the ridgy face (Fig. 1b). It is also considered that the ridgy face may be a trace of the fracture propagation advancing the wall throughout, in which the scission of fibrils would proceed with a short, quick jump accompanying a minute split between fibrils (Fig. 2).