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

本研究的目的是：建立三维（3D）电子断口（EF）的木材和纸张的断裂表面，通过使用立体对的扫描电镜断口，并评估细胞壁内的微小缺陷，如滑移面，对力学性能的影响，并通过3D-EF的断裂现象。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裂表面的拓扑特征，而且足以测量断裂表面的尺寸和斜率，而无需与它们进行物理接触。为了定量评价滑移面对木材细胞壁材料力学性能和断裂性能的影响，制备了在外力作用下人工诱导细胞壁内产生大量滑移面的预压缩木材试件，测定了其抗拉强度、弹性模量和破坏功用新建立的3D-EF方法对预压缩木材纤维壁的断裂面进行了分析。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，由于单元壁内滑移面的存在，单元壁材料的质量变得脆弱。预压缩木材细胞壁的断裂面均表现为C Ⅱ型断裂模式，断裂面与纤维轴的倾角为60 ~ 65 °，与滑移面角基本一致。从这些实验证据，它的结论是，细胞壁的存在下，滑移面的网站，和细胞壁材料的力学性能的强烈影响，在滑移面网站的局部属性的存在。

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.