A new approach to quantitative fractography of the cell-wall materials by means of SEM stereoanalysis

本研究的目的是：利用扫描电子显微镜的立体对，建立木材和纸张断裂表面的三维电子断口(EF)，并利用3D-EF评价细胞壁中的细微缺陷(如滑移面)对其力学性能和断裂现象的影响。新建立的3D-EF方法是非常有用和强大的，不仅足以精确地可视化断口表面的拓扑特征，而且足以测量断口表面的大小和斜率，而不需要与它们进行物理接触。为了定量评价滑移面对胞壁材料的力学和断裂性能的影响，制备了在外力作用下在胞壁内含有大量滑移面的预压缩木材试件，测量了试件的抗拉强度、弹性模量和破坏功(韧性)，并用新建立的3D-EF方法对预压缩木材纤维壁的断口进行了分析。结果表明，随着滑移面数目的增加，木材的强度和韧性急剧下降。因此，由于细胞壁内滑移面的存在，细胞壁材料的质量变得脆弱。预压木材细胞壁的断口始终呈CⅡ型断口，断口与纤维轴的倾角为60~65°，与滑移面角度基本一致。这些实验结果表明，存在滑移面的胞壁应在滑移面位置发生断裂，胞壁材料的力学性能受滑移面位置的局域性质的影响较大。

The purposes of this research project are; to establish the three dimensional( 3D ) electron-fractography (EF ) for the fracture surface of wood and paper by using stereo pairs of SEM fractoraphs, and to evaluate effect of minute defects within cell walls, such as slip planes, on the mechanical properties and on the fracture phenomena of them by means of the 3D-EF. The newly established method of the 3D-EF was very useful and powerful not only enough to visualize the topological features of the fracture surface precisely, but also enough to measure the size and the slope of the fracture surfaces without coming into the physical contact with them. In order to evaluate quantitatively the influence of the slip planes on the mechanical and fracture properties of the cell wall materials, the precompressed wood specimens, which had numerous slip planes within the cell walls induced artificially by the external force, were prepared, after which, tensile strength, elastic modulus and work to failure ( the toughness ) were measured, and the fracture surfaces of the fiber walls of the precompressed wood were analysed by using the newly established method of 3D-EF. It was found that the strength and the toughness of wood were drastically decreased as the number of slip planes increased. Therefore, the quality of the cell wall materials became fragile because of the presence of the slip planes within the cell wall. The fracture surfaces of the cell walls of the precompressed wood always showed the C II fracture mode, and the slant angles of the fracture surface were 60 to 65 to the fiber axis, which were closely in agreement with the slip plane angle. From these experimental evidences, it was concluded that the cell walls with the presence of the slip planes should repture at the slip-plane site, and the mechanical properties of the cell wall materials should be strongly influenced by the local properties at the slip-plane site.