枝节木材结构与性能研究进展

Research Progress on Wood Structure and Properties of Branch-Trunk Junction

枝节是指枝条、在树干肥大生长过程中枝条基部被卷入树干木质部的节以及树干上枝条去掉后形成的节疤,是树木结构与功能的重要组成部分;但由于枝节木材对产品性能产生的不利影响,其研究与利用受到限制。随着仿生学的发展,枝节木材以其复杂而有序的组织构造、连接方式及高强韧、耐腐朽等特性,作为一种特殊结构材料逐渐引起国内外学者的关注。众多研究表明,枝节木材天然形成的组织构造和连接方式具有许多人工材料无法企及的优势,加强枝节木材结构与性能研究,既有望打破其以往利用受限的境地而开拓全新的应用领域,又可为研发高性能的新型仿生材料提供有益启发和科学依据。本研究从枝节木材的构造、化学抽提物、物理性质和力学性质4方面概述枝节木材结构与性能的研究现状。首先分别阐述枝节木材在宏观观测水平下的构造特征以及枝节木材构造随外界环境变化做出自适应性调整的智能性表现;其次围绕节疤中化学抽提物对树木的保护功效,重点从节疤化学抽提物对树木防腐和树木自然整枝2方面的影响效果进行归纳;然后总结枝节木材与树木干材在密度、干缩性等物理性质及变化规律方面的比较研究;最后阐述枝节木材的力学性质与材料基质、构造排布和功能特性间的相关关系及其应力均布特性的相关研究。同时,本研究还阐述与枝节木材相关的工程材料及结构的仿生实例,以及枝节木材相关研究对仿生设计的一些启示。鉴于枝节部位木材的特殊性及其研究的复杂性,目前人们对枝节木材结构与性能的认识相对薄弱,关于枝节木材细胞水平的精细构造、主要化学成分及各项物理和力学性质等方面的研究报道相对较少且未深入全面。基于此,本研究在前人对枝节木材结构与性能研究现状的基础上提出几点建议:加强对枝节木材宏观和微观各层次构造特性及其在不同生长状态下的调节变化机制研究;探究纤维素、半纤维素、木质素及抽提物等化学成分在枝节木材中的含量和微区分布并探明化学成分配置情况与枝节木材韧性、连接强度等性质的相关关系;开展枝节部位木材密度、水分等物理量的变化规律及其内因的研究;阐明枝节木材精细结构参数对其物理力学性质的影响机制,构建枝节木材精细构造、化学组成等结构参数与物理力学性质的数学模型。

Branch-trunk junctions is an important part of tree’s structure and function, which contains branch, the base of the branch involved in the xylem of trunk in the growth of the trunk, and also the knot formed after the branch was removed from the trunk. However, because of the negative impacts on the performance of the product, the research and utilization of the branch-trunk junctions is limited. With the development of bionics, the branch-trunk junctions have been gradually attracted great attention from scholars at home and abroad and related colleagues because of its complex structure, orderly connection and high toughness, decay resistance and other biological characteristics, and also as a kind of excellent biological composite materials. Many studies show that the branch-trunk junctions contain many artificial materials incomparable functions of intelligent composite material and structure characteristics. The deep research on branch-trunk junctions structure and its performance is expected to break its past use of limited territory and develop new application areas, and may also provide a new type of bionic research and development of high performance material beneficial enlightenment and the scientific basis. This paper summarizes the current research status of the structure and properties of the branch-trunk junctions from four fields,those are, the structure, chemical extraction, physical and mechanical properties. First of all, the structural characteristics at the level of macro observation and the intelligent adaptive performances changing with the external environments were respectivdy expounded. Secondly, the protective efficacy of the chemical extraction of knot was mainly summarized from the effects of chemical extracts on forest antisepticision and forest natural pruning. Then, the comparative studies of the branch-trunk junctions and tree stem in dry density, shrinkage properties and the change rule of physics were summarized. The related research on the correlations between mechanical properties and matrix material, and between structure arrangement and functional properties, and stress distribution characteristics was finally summarized. At the same time, this paper also describes examples of bionic engineering materials and structures and related branch-trunk junctions, as well as the related research providing some inspiration for the bionic design. In view of the complexity and particularity of branch-trunk junctions, the present studies on wood structure and properties of branch-trunk junctions awareness is relatively weak, and the research reports about branch-trunk junctions cell level fine structure and main chemical composition and the physical and mechanical properties is relatively less and not thorough. Based on these, the author research on wood structure and properties of branch-trunk junctions in the former, the paper brings forward several suggestions about further research on the wood material side of bionics, including those: 1) Strengthening the macro and micro level of each branch-trunk junctions wood structural features and adaptive mechanisms of regulation. 2) Exploring the minor cellulose, hemicellulose and lignin and extracts and other chemical components and micro distribution mechanism of synergistic effect on the function of trees. 3) Carrying out branches, knots and parts of the trunk density, moisture variation and internal relationship of connection. 4) Exploreing the fine structure parameters on the branch-trunk junctions of wood physical and mechanical properties of the self adaptive control mechanisms, building the mathematical model of branch-trunk junctions fine structure and chemical composition of the wood structure parameters between physical and mechanical properties.