| Making a deep investigation of the in-tree variation of mechanical properties of wood fibers as well as the main influencing factors of tracheids mechanical properties not only favors the selective utilization of wood fibers in wood fiber/polymer and papermaking areas, but also can supply valuable information for the genetic improvement of wood properties. At present, the most difficult problem encountered in the research of cell wall mechanics of tracheids is the experimental determination of the mechanical properties of tracheids. Microtension of single tracheids, which is a direct method for the determination of mechanical properties of tracheids wall, found limited application in practice because of its complex procedure, rigorous requirements for testing apparatus and especially for experimental skills. In addition, the accuracy of that method is often questioned because the experimental results are easy to be disturbed by many subjective and objective factors. Based on the above considerations, the present research was conducted to realize two goals: 1. Propose novel experimental techniques to estimate quickly the longitudinal mechanical properties of tracheids, and then to explore the in-tree variation of longitudinal mechanical properties of tracheids of Chinese-Fir from plantation. 2. Provides useful information for the genetic improvement of wood properties by studying theoretically and experimentally the main influencing factors of mechanical properties of tracheids.The present research is one part of the national great project of basic research (973)—"Research on the Molecular Basis of Tree Breeding". The paper firstly examined the mechanical characteristics of wood samples at micron scale, which provided some theoretical basis for the application of wood microtome section to study the in-tree variation of longitudinal mechanical properties of tracheids. In chapter 3, microtension of wood microtome section is for the first time used to explore the in-tree variation of longitudinal MOE of tracheids, meanwhile the size effect of mechanical properties of wood at micron scale is taken into count. In chapter 4, As a further study, the continuous stiffness measurement (CSM)technique newly developed in nanoindentaion area is applied for the first time to measure the elastic modulus and hardness of tracheids wall S2 layer. In chapter 5, the concept of zero-span tension known in papermaking area is for the first time introduce to wood science area to explore in-tree variation of longitudinal tensile strength of tracheids and its main influencing factors. In chapter 6, composite micromechanics and classic laminated theory are used to make a comprehensive and detailed analysis of the main influencing factors of mechanical properties of tracheids cell wall. Some experimental results acquired in the foregoing chapters are explained successfully. Results were summarized as follows:Mechanical characteristics of wood at micron scale:1) At micron scale, the longitudinal mechanical behavior of wood microtome section differs greatly from that of wood with normal size. There exists a positive correlation between longitudinal MOE and sample thickness within a certain range, but is much less than that of sample with normal size. For example, the longitudinal MOE of wood sample with normal size is nearly 2.58 times that of microtome section with a thickness of 200?m. The explanation is that complete shear restraint effect existing among cell wall in a solid wood was weakened and even destroyed at micron scale, which will significantly reduce the stiffness of cell wall. In the fifth chapter, theoretical calculation has supported the explanation.2) A three segments mode of stress-strain curve including initial elastic zone, plastic deformation zone and post-damage stiffening zone was found for wood microtome section tensioned in longitudinal direction, which looked different from those obtained on thick specimen. Further study indicates stiffening effect is related to MFA of tracheids,cross-sectio...
|
PDF Full Text Request