【作者】 王巍；

【导师】 曹军；

【作者基本信息】 东北林业大学 ， 机械设计及理论， 2012， 博士

【摘要】 木材是一种天然的具有纤维增强特性的复合材料,并具有正交各向异性,是人们生活中主要用材,有很多其它材料无法比拟的优势,与水泥、钢材并称为三大建设用材,但人们并未完全掌握木材的复合材料特性,影响木材功效的发挥,木材宏观的力学行为与木材细胞的形态、细胞的力学性质、细胞之间的连接方式以及细胞构成组织的结构密切相关,因此研究木材受力后其内部微细结构的变化与宏观力学响应之间的关系,将提高对木材破坏机理认识的水平,具有很强的现实意义。本文首先进行了木材微观结构特征研究,微观结构决定木材的宏观力学性能,木材是由许多空腔细胞所构成的,木材实体主要构成是细胞壁,所以是木材微观力学主要研究对象,通过分析其弹性常数与木材微观结构的关系反应出细胞壁中的S2层与细胞壁的纵向弹性模量密切相关,在进行木材宏、微观实验时,主要以针叶材落叶松为实验对象,通过薄小试件分析其早晚材细胞的分布特征,测量了早材、过渡材与晚材的纵向弹性模量,及早晚材细胞壁的厚度,得出细胞刚度沿生长轮分布呈非线性关系,同时对木材的密度、干缩与湿胀特性及单层与多层抗拉强度进行测试,建立试件密度与强度的关系、试件刚度与结构变化的关系,获得有限元建模时所需的相关数据。同时考虑了木材曲率对木材的影响,并对木材粘弹性进行了分析,应用了单胞建模理论,进行初步力学性能研究,并通过宏观验证。从复合材料角度出发进行木材微观力学建模时,以木材单胞建模理论为基础,单胞有限元模型为基本单元,构建了木纤维增强单元模型,获得RVE本构关系与边界条件,研究了纤维增强体的相互作用关系,建立了木纤维实体仿真模型,将实验数据输入,根据初始模型刚度参数,计算出各点应力状态求得木纤维的刚度,对模型中现有木纤维的刚度进行修正,获得木纤维模型应力云图,因为RVE的周期性,由此可获得木材的宏观应力应变关系。考虑木材是一种缺陷材料,将带缺陷的木材体分成不同的区,从微观层面考虑缺陷区,将不同的缺陷模型插入每一个区中进行模拟,借助有限元方法,建立了含有缺陷的木纤维单元模型,根据实验数据进行载荷赋值,将木纤维单元模型的非线性参数输入到多层生长轮模型进行验证,从而得到生长轮层间应力分布云图,验证了本文所选用试验方法有较好的适用性和计算精度。本文的研究将复合材料理论与木材微观力学相结合,属于交叉的前沿的研究,对扩展木材使用范围,提高木材利用率,研发能够克服木材缺点、具有特殊强韧性能的新型木质复合材料具有重要指导意义。更多还原

【Abstract】 Wood is an orthotropic natural fiber reinforced characteristics of polymer composites which is the main material in people’s lives, there are many advantages unmatched by other materials, cement, steel and wood are called three construction materials, but the wood there are some natural defect, in the course prone to deformation and other problems affecting the use of wood. Studied the timber forces the relationship between the changes in the internal fine structure of the macroscopic mechanical response will increase the level of awareness on the wood failure mechanism, has a strong practical significance.First, researching a wood micro-structural features and getting the relationship between of macro-mechanical properties of wood, the object of study in wood micromechanics is cell walls because the main components of timber entity is posed by many cavity cell walls, through analysis of its flexibility constant and wood micro-structure of the relationship reflects the S2cell wall layer and the cell wall of the longitudinal elastic modulus is closely related; stiffness properties of cellulose, hemicellulose and lignin, and cell wall stiffness properties in accordance with the relevant literature, consider to the swelling properties of wood, and build the constitutive equation of the cell wall and swelling strain.This article during the timber macro, micro experiments, mainly in softwoods Larch subjects, sooner or later material distribution characteristics of the cell through a thin specimen analysis, measurement of earlywood, transition wood and latewood longitudinal elastic modulus on, and sooner or later, the thickness of the wood cell wall, the nonlinear relationship between cell stiffness distribution along the growth ring wood density, shrinkage and swelling characteristics and single-and multi-layer tensile strength testing, the establishment of the specimen density relationship with the strength of the specimen stiffness and structural changes in the relationship between the finite element modeling of the relevant data required. The figure took into account the wood curvature on the wood, and wood viscoelastic analysis, the application of a single cell modeling theory, the initial mechanical properties of macro validation.Wood micromechanics modeling by means of finite element software to the timber unit cell modeling theory as the basic unit, the unit cell finite element model constructed wood fiber reinforced unit model, build the RVE constitutive relations and boundary conditions, research fiber to enhance the interaction between body, entity simulation model of the wood fiber input, the experimental data, according to the initial model stiffness parameters to calculate the stress state to obtain the stiffness of the wood fiber, the existing wood fiber model The stiffness of an amendment to the wood fiber model stress cloud, RVE cyclical, thus available to the macroscopic stress-strain relationship of wood. Consider that the wood is a defect in materials, the timber body with defects into different area, from the micro level, consider the defect area, the different defect models into simulated in each district, with the finite element method, the establishment of a wood containing defects fiber element model, load assignment based on experimental data, the wood fiber element model of the nonlinear parameters input to the multi-layer growth ring model was validated, in order to get the stress contours in the growth ring layer, the method for solving the timber macro-mechanical properties, verify the accuracy and superiority of this choice of test methods. New wood composite material of this article will be combination of the theory of composite materials and wood micromechanics,the research is a cross-cutting-edge, timber extended use of wood range, timber utilization, R&D to be able to overcome the shortcomings, has a special tough performance and important guidance.更多还原