【作者】 邵卓平；

【导师】 吴泽民；

【作者基本信息】 安徽农业大学 ， 木材科学与技术， 2009， 博士

【摘要】 木材和竹材是是植物中可用作结构材料的两种,同时也是具有明显细观结构、可在多尺度下研究的生物复合材料,由于非均匀、各向异性和“天然”存在的微观甚至宏观的缺陷或损伤(裂纹),受荷后这些初始缺陷或损伤的不规则演化行为决定着木、竹材的宏观力学行为。本文以多种木材和毛竹为试材,对木、竹材结构的力学特征和损伤断裂行为、以及材料强韧机制进行了研究,主要工作如下:1、首先在理论上对线弹性断裂力学原理在木材中应用的特殊性作了讨论,证实当裂纹沿纤维方向自相似扩展时,木材的断裂行为可以用线弹性断裂力学方法来描述,其顺纹断裂韧性可以应用各向同性材料断裂韧性实验方法来测试,并且采用紧凑拉伸试件所测得的断裂韧性值,不受该试件几何尺寸和裂纹长度影响。在此基础上,又分析研究了木材顺纹理裂纹断裂的应力强度因子与能量释放率之间的关系。应用分形理论,研究了木材顺纹理断裂表面的分形特征,测量了断口表面的分形维数,首次建立了木材顺纹理断面的分形维数与顺纹断裂韧性之间的关系,该关系有助于揭示出木材抗顺纹理断裂性能与木材细观构造的内在联系。设计制作了测试Ⅲ型断裂韧性的实验装置,首次测试了云杉木材对Ⅲ型裂纹顺纹理扩展的阻力。2、应用有限元法分析了木构件含横纹理裂纹时裂纹尖端附近的应力场特征,采用切向比正应力准则研究了木材横裂纹的启裂方向,并测试了木材启裂时临界应力强度因子。研究结果表明含横纹理裂纹木构件侧向启裂时的临界应力强度因子基本上不受试件尺寸的影响,作为一种名义断裂韧性可以视之为木材的一种基本属性,但由此建立的K准则不宜作为木构件的强度设计准则。木材因其多胞及纤维增强的多层胞壁结构,而具有很强的抗横断韧性,通过对木材横弯断裂试验表明,含Ⅰ型横纹理裂纹木构件在沿顺纹向启裂后仍具有很高的继续承载能力,说明木材具有很强的抗横断韧性,含此类裂纹的木制构件不会因裂尖应力奇异性而发生低工作应力破坏的现象,这一点已通过对多种木材(云杉、落叶松、杨木、红锥)的弯曲、冲击、拉伸的三组对比试验给予充分证明。所以,在对含有Ⅰ型横纹理裂纹的木材构件进行安全评估和强度设计时,若以启裂时的临界载荷并以K准则作为木梁构件的设计准则,必然会造成强度余量上的浪费,故建议采用除去裂纹的净尺寸下的常规强度即可。即:木材的顺纹理断裂,宜采用断裂力学准则(K或G准则);木材的横纹理断裂,宜采用净应力准则。虽然横纹理裂纹不影响木材净面积下的常规强度,但侧向裂纹的扩展仍是一种受到关注的损伤破坏模式,为此本文通过解析法推得了木梁Ⅰ型顺纹层间开裂过程中的应变能释放率计算公式,该式可以用于木胶合板弯曲脱层损伤的分析。最后从界面弱化和多层胞壁细观结构的角度阐述了木材抗横断的强韧机制。3、提出木材细观损伤基本构元的概念,并对木材的损伤基元作了初步界定。在此基础上,首次应用声发射技术对木材构件在承载过程中不同类型损伤、断裂的萌生与发展作了辨识。试验表明木材在载荷作用下,材料内部损伤和断裂的不同机制可引起不同程度的能量释放,从而产生丰富的AE信号。无缺陷试件在加载初期声发射事件发展较为缓慢,且出现的主要是一些低振幅的AE信号,而大量高振幅AE信号出现在峰值载荷附近及断裂阶段。利用声发射监测含裂纹试件在弯曲载荷下的损伤并断裂全过程,可以明显地识别裂纹尖端启裂和扩展的不同阶段。声发射信号的特征与木材损伤模式有关,胞壁断裂对应的AE特征为高幅值、高能量及长持续时间,而胞壁界面损伤与层裂损伤和细胞屈服与压溃损伤对应的AE特征为低幅度、低能量及短持续时间。木材在重复载荷作用下,在低水平载荷下呈现Kaiser效应、在高水平载荷下呈现Feliciy效应,应用声发射的Felicity比能够较好地反映出木结构的损伤程度。4、分析、阐述了竹结构的力学特征,并通过对竹壁节间材建立的简化并联模型,应用复合材料细观力学的混合定律测试分析了竹材的纤维束和基本组织的强、刚度。又通过剥离后的纤维束测试了其强度,并应用剪滞理论测试了该纤维束的弹性模量。5、研究了毛竹材的Ⅰ型(张开型)和Ⅱ型(面内剪切型)层间断裂行为。由于竹材在宏观力学行为上可视之为单轴向的长纤维增强复合材料,并在强度和刚度方面展示了强烈的各向异性性质,因此,由外部施加或因环境条件变化而引起的横向张力与剪切力极易引发起竹材构件沿顺纹方向层裂的起始,随后分层的传播不是由横纹方向的强度控制,而是由竹材的层间断裂韧性控制。为此,基于能量原理,首次采用双悬臂梁(DCB)试样和端部切口弯曲梁(ENF)试样分别测定了毛竹材顺纹向的Ⅰ型和Ⅱ型层间断裂韧性,并对实验方法及影响因素作了探索。借助扫描电镜分析了Ⅰ型和Ⅱ型层间断口形貌,阐述了竹材层间断裂的机制。6、作为一种具有强烈结构特征的纤维增强生物复合材料,竹材在横向断裂的不同阶段或过程中,会因竹纤维束在基本组织中的应力传递模型和断裂机理不同而有多种能量吸收机制发生。为此,通过竹材在横断过程中所表现的不同失效方式面而建立的细观力学模型,测算了各失效模式对竹材横弯断裂的增韧贡献,并通过扫描电镜图片分析了断口形貌,阐述了竹材抗横断的强韧机制。更多还原

【Abstract】 Wood and Bamboo are very only two types of plant which capable of structural materials in all plant kingdom, and is a bio-composites with obvious meso-structure and could be research on multi-scales. Because of the properties of inhomogeneous, anisotropic and some defects or fractures existent in microcosmic even in macroscopic levels, the irregular evolvement behavior of these defects or fractures when under load determined the macroscopic mechanics behavior of wood and bamboo. In this paper, some kinds of wood and moso bamboo were selected as testing materials to research the mechanics character, fracture rupture behavior and mechanism of strong toughness. The main works were as follow:1 .First the particularity of applying the principles of linear-elastic fracture mechanics (LEFM) to wood was expounded in theory, it had confirmed that when the crack self-similar propagate along the direction of fiber, the fracture behavior could be described with LEFM, the fracture toughness parallel to grain could be tested with the same experiment methods in isotropic materials, and the value could be adopted from compact tensile samples, which not influenced by the dimension and the length of crack. Base on this research, the relationship between the stress intensity factor parallel to grain and the rate of energy release were also analyzed.The fractal character of fracture surface parallel to grain were researched by applying fractal theory, then the fractal dimension of fracture surface of 5 kinds of wood were measured, based on these work the relationship between the fractal dimension of fracture surface parallel to grain and fracture toughness parallel to grain were established for the first time, the relationship would help reveal the internal relations between meso-structure and anti-fracture capability parallel to grain of wood.The experiment equipment of fracture toughness in model III were designed, then for the first time we test the resistance to crack expanded parallel to grain in model III of spruce.2. Application of finite element method analyzed the stress field characteristics near the crack tip of the wood components texture with crack perpendicular to grain. "Tangential normal stress ratio criterion" were adopted to research the starting direction of crack perpendicular to grain, the factor of critical stress intensity were also test at the beginning of crack. The study result indicate that the critical stress intensity factor in lateral crack of the wood components texture with crack perpendicular to grain didn’t influenced by the dimension of specimens, as an nominal fracture toughness could be seemed as essential property of wood, so the K-criterion established on these couldn’t become the strength design criterion of the wood components. Because of the structure of cell and the cell wall enhanced by fiber, wood could resist the fracture perpendicular to grain strongly. The testing of fracture in bending strength perpendicular to grain revealed that: after the crack perpendicular to grain crack along the grain, the wood components still presented high carrying capacity , this account for wood could resist the fracture perpendicular to grain strongly, the wood components with model I crack perpendicular to grain couldn’t be fractured by stress singularity the low working stress because of the oddity in crack tip, this conclusion had be fully proved through the contrast test of bending, toughness and tensile in many species of wood(spruce, larch, poplar and prick). So, when the wood components with model I crack perpendicular to grain were carried through safety evaluation and intensity design, it was suggested adopting the remained dimension exclude crack in general intensity. Otherwise if the design criterion followed the critical load and set K criterion as its criterion it could cause waste in intensity margin. Consequently it will bring the waste of so, the exclusion of the general strength of crack under dressed size is suggested, i.e.For model I crack parallel to grain, the LEFM criterion (K criterion or G criterion) is adapted.For model I crack perpendicular to grain, the net section stress criterion is adapted.Although the fracture perpendicular to grain didn’t influence the conventional strength under net section of wood, the crack in the side direction was still an attention destructive model. So, the formula to calculate the strain energy release rate in the process of model I interlaminar crack parallel to grain were derived by analytic method in the research. This formula could used to analyze the delamination damage when bending in plywood.Finally the mechanism of strong toughness of wood resist fracture perpendicular to grain were expatiated through the point of interface weakening and the multilayer cell wall meso-structure.3. The concept of meso-damage basic elements of wood had been proposed. and the basic elements in wood damage were defined initially. Based on these, by using acoustic emission technology the initiation and development of different kinds of damage and fracture of wood components under loading were distinguished for the first time. The results showed that the different mechanism of damage and fracture in the internal of materials under loading could cause different degree of energy release, and then abundant AE signal produced. The occurrence of acoustic emission event under load in non-defect specimens developed very slowly, and the amplitude of AE signals most were low, the abundance AE signals with high amplitude occurred near the peak value of load and fracture process. When the process of fracture under bending load of specimens with crack was monitored by acoustic emission, the different stage of the beginning and the expansion of the crack tip could be obviously distinguished. The character of acoustic emission was related with the scathe of wood, the AE character of cell wall fracture were high amplitude, high energy and long duration, the AE character of cell wall interface damage, interlaminar crack and cell wall yield and collapse damage were low amplitude , low energy and short duration. When wood under repeating load, it presented Kaiser effect under low load, it presented Felicity effect under high load, and by using the acoustic emission the Felicity effect could reflect the damage degree of wood preferably.4. The mechanics character of bamboo structure were analyzed and expatiated, through predigest parallel connection model established on cell wall between internodes, by applying the mixing rule of meso-mechanics of composite material to test the intensity and stiffness of bamboo fasciculi and basic tissue. And the intensity of bamboo fiber bundle was test after peeled of, and modulu of elasticity of the bamboo fiber bundle was test by shear-lag theory.5. The opining and shear interlaminar fracture behaviors of Moso bamboo have been studied in this paper. Because bamboo was seemed as single axial and long fiber compound material, and was intense anisotropism in intensity and stiffness, it was easy to initiated the beginning of interlaminar fracture parallel to grain under the transverse tensile force and cut force which caused by outer force and the change of environment condition, then the spread in divided layer didn’t controlled by the intensity perpendicular to grain, but by interlaminar fracture toughness. Based on the energy concept, double-cantilever beam (BCD) sample and end notched flexure (ENF) sample were applied to measure the Model I and Model II interlaminar fracture toughness of Moso bamboo, and the experiment method and influencing factor were explored. The morphologies of rupture surface of Model I and Model II interlaminar fracture were analyzed by using SEM. then the mechanism of interlaminar fracture of bamboo have been expatiated.6. As an bio-composite materials which were enhanced by fiber, with intense structure character ,there were many energy -absorb mechanism in bamboo in different stage or process of fracture perpendicular to grain, because of the different mechanism of bamboo fiber bundle in the stress transfer and fracture in basic tissue. So the meso-mechanics model were established according to the different disable manner in the processes offracture perpendicular to grain to calculate the contribution of enhancing toughness ofdifferent disable model, and the morphologies of rupture surface were analyzed by SEM toexpatiate the mechanism of strong toughness in resisting fracture perpendicular to grain ofbamboo.更多还原