【作者】 巩翠芝；

【导师】 刘一星；

【作者基本信息】 东北林业大学 ， 木材科学与技术， 2009， 博士

【摘要】 木材作为有着生命起源的天然高分子复合材料,有着非常复杂的力学行为。尽管它已有数千年的应用历史,但人们对它的复杂力学行为方面的知识的掌握还是很有限的。木材的微观结构及其细观尺度上的物理机制影响着木材的宏观响应,因而,对木材微观结构及力学性能的研究可以揭示木材宏观性能的本质。虽然在近10年里,木材物理力学领域中越来越多的工作己经转向微观或细观范围的研究,但却较少关注宏观力学行为与微结构演化的相互联系。由于木材的宏观力学性质与其微结构的变化是密切相关的,所以探明木材受荷载后内部微结构的变化,以及它们与宏观力学响应之间的联系,揭示木材的微结构的变形、损伤、破坏机理和演化规律,寻找既能反映宏观变形同时又能显示微结构变化的一些实验方法,并给出相应的一些力学参量和力学特性的定量分析,有着十分重要的理论和实际意义。由于木材的非均质性和各向异性,作为破坏前兆的微观损伤结构远比普通均质材料复杂,所以其损伤萌生与演化过程更是变化莫测。就木材本身的特点及所要面对的问题而言,应用传统的、常规的理论分析研究方法很难把上述问题解决,或者描述清楚。基于上述思想,本论文采用了先进的光测力学实验方法—数字散斑相关方法(DSCM),围绕木材小试样在拉伸载荷作用下的微观力学特性及破坏机理,进行了实验研究。DSCM具有全场、非接触性的特点,在测量中可以完全保留变形过程中的物体表面信息。该方法是一种有希望解决微观力学定量测量的方法,进而可通过计算应力高度集中区域附近的变形分布来研究材料的破坏模式。本论文首先利用配有微力学实验机的环境扫描电子显微镜对木材小试样的拉伸实验过程实行实时在线监测;其次,用数字散斑相关方法(DSCM)软件对环境扫描电子显微镜拍摄的不同载荷阶段的图片进行定量分析。主要研究内容和结论如下:(1)本文分析了数字散斑相关方法(DSCM)中的一些关键技术,并开发了数字散斑相关方法(DSCM)软件测量系统,该系统满足了实际测量要求,为数字散斑相关方法(DSCM)在木材科学研究中的应用提供了基本条件。(2)提出了一种测量较小区域的弹性系数实验方法。在环境扫描电子显微镜内完成木材小试样的径向拉伸实验,用数字散斑相关方法软件计算出试件中部一个完整生长轮在弹性变形阶段某一径向载荷下的应变场;然后再利用Hook定律,以及计算应力的平衡方程式,由应变场反推出径向不同位置的四个弹性系数(径、弦向杨氏模量,剪切模量和泊松比)。为了检验该实验测量方法的准确性,本文又采用了二维六角形蜂窝模型对径向不同位置的四个弹性系数进行了估测,估测结果与实验测量结果基本一致。由于实验方法是对真实材料的测量,因而,结果要比用简化模型估测的值更可靠更精确。(3)采用数字散斑相关方法测量了木材小试件RT系统、TR系统、TR45~。系统以及LR系统在拉伸载荷作用下的弹性变形阶段的位移场及应变场。位移场和应变场等高线的分布,以及它们在连续载荷作用下的变化趋势,反映了载荷在非均质的木材试样内部传递的详细信息,以及应力集中区域。根据弹性阶段不同载荷作用下的应变场的变化趋势可以预测木材将要发生损伤、破坏的区域,以及最终的断裂路径。环境扫描电子显微镜内拍摄的拉伸实验全过程以及最终破坏试件的图片验证了预测的结论。因此,数字散斑相关方法可以用来预测、预报受载材料的损伤和破坏。(4)在生长轮和细胞层次上,分析了木材小试样在不同的加载方向的破坏机理。揭示了裂纹扩展与其微结构变化之间的内在规律性。本文的研究方法与结果,对理解木材的本质和建立木材的微观本构模型,以及木材的设计和使用奠定基础。更多还原

【Abstract】 Timber as a natural polymer composite material with the origin of life,has a very complex mechanical behavior.Even though it has been applied for thousands of years,but the knowledge concerning the complexity of its mechanical behavior is still very limited. Microstructure of wood and its physical mechanism in micro-scale impact on the macro response of wood,and therefore,the research on the micro-structure of wood may reveal the nature of macroscopic properties of wood.Although in the past 10 years,more and more the work of wood physical and mechanical fields has been turned micro-range research,but less concerned about the mutual contact of the macro-mechanical behavior and microstructure evolution.Because of the macro-mechanical properties of wood and its micro-structure changes are closely related,so it has a very important theoretical and practical significance to prove the micro-structure changes and the contact between them and macro-mechanical response after wood loaded,to reveal the deformation of microstructure of the timber,damage, failure mechanism and the law of evolution,to look for some experimental method both reflecting the macro-deformation and showing some of micro-structural changes at the same time,and to give some of the corresponding mechanical parameters and quantitative analysis of mechanical properties.Because of the heterogeneity and anisotropy of the wood,the structure of wood as a precursor in meso-scale is more complicated than that of the general homogeneous material,so the process of its damage initiation and evolution are changing.On wood’s own characteristics, it is very difficult to apply the traditional,conventional methods of theoretical analysis to solving the problem above,or describing them clearly.Based on the above idea,in this dissertation,experimental research focusing on micromechanical properties and damage mechanism of small wood samples is carried out by employing advanced optical measurement mechanics experiment ways—digital speckle correlation method(DSCM).DSCM with the audience,non-contact characteristics at measurement of the deformation process can be completely retained in the surface information. This method is a promising solution meso-mechanics quantitative measurement method,and then by calculating the stress of highly concentrated region near the deformation,the failure mode of wood was studied.First of all,this thesis with the use of micro-mechanical testing machine of the environmental scanning electron microscope on a small sample wood tensile test process implemented real-time online monitoring;Secondly,the use of digital speckle correlation method(DSCM) software,this thesis conducted a quantitative analysis for environmental scanning electron microscope filming different stages of picture load.The main research content and conclusions are as follows:(1) In this paper,some of the key technology of the digital speckle correlation method (DSCM) were analysed and measurement system of the digital speckle correlation method (DSCM) was developed.The measurement system can meet the actual measurement requirements and provides the basic conditions for the application of digital speckle correlation method(DSCM) to wood scientific research.(2) This thesis was first proposed a new measurement of the elasticity coefficient on smaller regional experimental methods,radial tensile test of small wood sample was completed in the environmental scanning electron microscope,using Digital speckle correlation method(DSCM) software,a complete specimen central ring has been calculated at a certain stage of elastic deformation under the radial load-strain field,and re-use Hook’s law,as well as the calculation of the stress balance equation,four different positions of the radial elasticity(Path,String to the Young’s modulus,shear modulus and Poisson’s ratio) were derived with the strain field.In order to examine the accuracy of experimental measurement methods, the paper also uses two-dimensional hexagonal cellular model of radial four different locations to carry out the estimation of elasticity,the results are basically consistent.Because the method are the measurement of real materials,so the results are more reliable and more accurate than the simplified model used to estimate the value.(3) Displacement and strain fields were measuered for RT system,TR system,TR45~o systems,as well as LR system at different load levels under proportion limit from Digital Spackle Correlation method.The result showed that digital speckle correlation method can reveal fine details of the nature of displacements in wood.(4) In the growth ring and cell level,failure mechanisms were analysed on small wood samples loaded at different directions to grain.The result reveals the intrinsic regularity between the crack progress and wood micro-structural.In this paper,the corresponding research methods and results lay a foundation for understanding of the nature of timber properties and setting up of the micro-mechanical model, as well as design and use of materials.更多还原