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

本研究项目的目的是：利用立体扫描电镜断口图对木材和纸张的断口表面建立三维电子断口图，并利用三维电子断口图评价细胞壁内微小缺陷（如滑动面）对其力学性能和断裂现象的影响。新建立的3D-EF方法不仅可以精确地可视化断裂面的拓扑特征，而且可以在不与断裂面进行物理接触的情况下测量断裂面的尺寸和斜率，这是非常有用和强大的。为了定量评价滑移面对细胞壁材料力学性能和断裂性能的影响，制备了受外力人工诱导的细胞壁内存在大量滑移面的预压缩木材试件，并对其抗拉强度、弹性模量和破坏功（韧性）进行了测量。采用新建立的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，随着滑动面数量的增加，木材的强度和韧性急剧下降。因此，由于细胞壁内存在滑移面，细胞壁材料的质量变得脆弱。预压缩木材的细胞壁断裂面始终呈现C II型断裂模式，断裂面与纤维轴的倾斜角在60 ~ 65之间，与滑移面角度基本一致。实验结果表明，存在滑移面的细胞壁会在滑移面位置发生断裂，并且滑移面位置的局部特性会对细胞壁材料的力学性能产生强烈影响。

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.