【作者】 刘焕荣；

【导师】 江泽慧；

【作者基本信息】 中国林业科学研究院 ， 木材科学与技术， 2010， 博士

【摘要】 竹材的力学性质是竹材研究的主要方向,也是竹材加工利用的重要基础。竹材是一种天然生物质梯度复合材料,具有分级复合结构;其中空、壁薄、离散分布的竹节等外观形态,维管束的梯度分布和细胞壁多层结构造就了竹材强度高、韧性好的优良特性,被广泛应用于结构建筑等领域。断裂是断裂力学的研究范畴,也是构件使用过程中选择和设计的重要依据。本研究以毛竹(Phyllostachys pubescens Mazel ex H.de Lebaie)为研究对象,以杉木(Cunninghamia lanceolata[Lamb.]Hook.)为对照材料,基于线弹性断裂力学理论,研究竹材Ⅰ型(张开型)断裂韧性KIC;运用数字散斑相关法、电子显微镜与加载联用装置和同步辐射μ-CT技术研究竹材断裂破坏的演变过程和破坏模式,揭示竹材的韧性破坏机理。竹材断裂特性的研究不仅是竹材基础性质研究的重要补充,更重要的是为竹材在结构建筑等领域的优化设计、合理、科学、高效和安全使用提供科学指导。本论文的主要研究结果归纳如下:1.预置LR方向裂纹竹材的Ⅰ型断裂韧性KIC明显高于杉木;从竹青到竹黄竹材的KIC呈下降趋势。紧凑拉伸法测得竹材KLRIC平均值为12.211 MPa?m1/2。2.三点弯曲法研究竹材预置LR裂纹的断裂韧性KLRIC表明,由于竹材纤维在竹材内部分布的不均匀性,裂纹预置在竹黄面K LRIC为9.81 MPa?m1/2,裂纹预置在竹青面K LRIC5.476 MPa?m1/2高78.82%。3.三点弯曲法研究预置LT裂纹竹材的K LTIC,竹青、竹肉和竹黄部位竹材K LTIC分别为9.636 MPa?m1/2,6.533 MPa?m1/2和4.361 MPa?m1/2,呈梯度下降,竹材的断裂韧性K LTIC与纤维含量相关。4.沿径向分层竹片预制LT的竹材断裂韧性KLTIC从竹青到竹黄逐渐下降;杉木木片断裂韧性KLTIC介于竹片竹肉2和竹肉3之间,比竹肉2低42.47%,比竹肉3高出5.9%,竹材的I断裂韧性总体上高于杉木。5.数字散斑相关法研究竹材受力过程中应变场演变过程表明,随着拉伸载荷增加,应变量增大,在裂纹顶端区域应变场集中,随着载荷增加应变场集中范围扩大;相同拉伸载荷水平下从竹青到竹黄,应变量应变集中区逐渐增大。三点弯曲加载方式下,预制LR裂纹的竹材,裂纹预制在竹黄面的应变量小于裂纹预制在竹青面应变量;前者应变场集中区域在裂纹顶端沿垂直预置裂纹方向扩展,而后者沿裂纹预置方向扩展,主要受竹材内纤维不均匀分布影响。相同条件下,杉木应变量和裂纹顶端的集中区域都大于竹材,对比表明竹材抗断裂能力高于杉木。6.电子显微镜与加载装置联用,原位观察竹材破坏过程显示,竹材受拉伸和弯曲荷载时其裂纹扩展方式不同。顺纹拉伸时,裂纹先沿着原来方向扩展至纤维束之后,裂纹扩展方向发生改变,沿着纤维束与薄壁组织界面顺纹扩展,主要由于界面强度远小于纤维束的拉伸强度。不同部位竹材拉伸断裂方式不同,竹青为沿界面发生劈裂,竹肉呈台阶式破坏,竹黄断面齐整,破坏模式与竹材的纤维含量相关。三点弯曲加载方式竹材破坏路径呈“Z”字形,纤维束逐层破裂破坏,裂纹扩展需要较多的能量,与木材裂纹扩展过程中会产生很多小裂纹不同,表现出很好的韧性。7. ESEM观察竹材拉伸破坏断面显示,薄壁组织拉伸破坏为整齐破坏,维管束先拔出、拉长,到逐渐破坏。三点弯曲破坏特征为,纤维束被拉出竹材,界面破坏;外侧拉伸纤维束长度较内层长。杉木破坏断面整齐断裂,断面粗糙。竹材的破坏模式为复合材料的断裂特性。8.同步辐射μ-CT技术观察竹材破坏内部结构,竹材拉伸破坏的内部断裂模式为:基质断裂,界面损伤,基质破坏、纤维桥联,纤维断裂。竹青纤维含量高,主要发生界面破坏,纤维断裂;竹肉纤维含量适中,发生基质破坏、纤维桥联;竹黄纤维含量较少发生基质断裂,纤维拉出断裂。断面观察纤维束破坏呈塔状。弦向三点弯曲加载时,竹黄侧破坏比竹青侧快,纤维束逐层剥裂,表现出典型的韧性破坏模式。9.竹材竹青、竹肉和竹黄三个部位的维管束拉伸强度和模量逐渐下降。纤维的拉伸强度和模量分别为1702.98MPa和33.33GPa。维管束拉伸破坏过程为纤维束被拉出,部分纤维先破坏,到最后纤维束完全被拉断,破坏断面有明显纤维束拉出薄壁组织。纤维破坏方式呈阶梯状螺旋式破坏,破坏断面为锯齿状,这与纤维细胞壁的多层结构以及微纤丝的排列方式有关。从细胞水平到宏观水平竹材独特的分级结构与复合材料特性赋予其强度高好、韧性好的特点。更多还原

【Abstract】 The mechanical properties of bamboo are the basis for the use of bamboo and the main research direction in bamboo science. Bamboo is a natural biocomposite material with non-homogeneous and anisotropic characteristics. The shape, structure and composition endow bamboo the characteristics of high strength and good toughness. So bamboo is widely used in structural engineering. Fracture is the major failure performance in engineer. Fracture character is the important criteria for choosing and designing structural parts. In this study, Moso bamboo (Phyllostachys pubescens Mazel ex H.de Lebaie) was taken as the research object, and Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) was taken as the control material. The bamboo fracture toughness KIC in the crack opening manner (I) was studied based on linear-elastic fracture mechanics theory of fracture toughness,. Bamboo failure process and the fracture surface characters were studied by digital speckle correlation method (DSCM) and in situ ESEM with loading setup and synchrotron radiationμ-CT techniques, and the fracture failure mechanism of bamboo was revealed. The study of bamboo fracture properties is not only the supplement for bamboo basic properties, more importantly it is the new idea and new method to predict bamboo damage, which is the scientific instruction for bamboo structure optimizing design and efficient use and scientific processing.The major findings of this study are summarized as follows:1. In LR manner crack condition, bamboo fracture toughness KLRIC is higher than fir. From the outside to the inside of bamboo, KLRIC decreased. The compact tension results showed that the average KLRIC for bamboo was 12.211 MPa?m1/2.2. In LR manner crack condition, bamboo and Chinese fir KLRIC were tested by three-point bending method. The KLRIC was 5.476 MPa?m1/2 and 9.81MPa?m1/2 for the rack in the bamboo outer side and in the inner side. The KLRIC for the inner side was 78.82% higher than that of the outer side. That’s due to the uneven fiber percentage.3. In LT manner crack condition, bamboo and Chinese fir KLTIC were tested by three-point bending method. Bamboo KLTIC decreased along the radius direction from the outer to the inner side. The KLTIC of ouer and intermediary and inner bamboo were 9.636 MPa?m1/2 and 6.533MPa?m1/2 and4.361 MPa?m1/2 respectively. The KLTIC of Chinese fir was 4.617 MPa?m1/2. It was 5.87% higher than that of the inner bamboo. In addition, compared with the intermediary and the outer bamboo, Chinese fir KLTIC decreased 41.5% and 108.71% respectively.4. Along radial direction, bamboo culms were split to 4 slices. From the outer to inner, the slices’KLTIC decreased gradually. Chinese fir slice KLTIC was between the second and the third bamboo slice KLTIC. It was 42.47% lower than the second slice, and 5.9% higher than the third slice. In all, bamboo KLTIC was higher than that of Chinese fir.5. The evolution of bamboo surface strain field during the loading process was studied by digital speckle correlation method (DSCM). With the tensile load increasing, either bamboo or Chinese fir strain increased and the strain concentration region enlarged. Strain concentrated in the tip of crack. At the same tensile load level, from the outer to inner bamboo, the strain value and the concentration region increased gradually. When bamboo with LR crack was loaded in three-point bending mode, the crack at bamboo outer side strain was higher than the crack at bamboo inner side. The first strain concentration region expanded along the crack direction, otherwise the other strain concentration expanded along fiber, which is contributed by the fiber percentage. At the same condition, Chinese fir strain value and strain concentration region were higher than bamboo, which indicated that bamboo had the better crack resistance than Chinese fir.6. Electron microscopy combined with the loading device was a helpful device to study bamboo and wood failure process. It was shown under tension and bending loads bamboo crack propagated in different ways. In tensile loading process, the crack extended in original direction up to meet fibers, the crack propagation direction changed along with the parenchyma and fibers interface extend parallel to the grain, which was mainly because the interface strength was much lower than the fiber bundles strength. Tensile fracture manner were different in different parts of bamboo. The outer bamboo fracture manner was spite along fiber interface,the intermediary bamboo fracture manner was stepped. And the inner bamboo part fractured section was neat. That is because the different fiber content in the three parts of the bamboo. In the loading process of three-point bending, bamboo crack path was "Z" shape. The failure mode was fiber bundle peeled layer by layer. Cracks passed quickly through parenchyma, and after experiencing the axial expanded along fiber bundles. Fiber bundles played an important part in preventing crack expanding.7. In situ ESEM showed that tensile failure is neat parenchyma fracture, and surface was neat but vascular bundles were pulled out first and then stretched gradually to destroy. In 3-Point bending loading mode, bamboo fracture characteristics were fiber bundles being pulled out of bamboo, interface failure, and fiber bundle fracture. In addition, fiber bundle was pulled longer than the inner fibers. However, Chinese fir damaged section was neatly broken and with rough sections. The bamboo failure mode was the typical composite fracture characters.8. Synchrotron radiationμ-CT technique was helpful to study the internal structure of damaged bamboo. Tensile failure bamboo internal fracture characters were: matrix fracture, interface damage, matrix damage and fiber bridging, fiber broke. The outer part of bamboo which with high fiber content mainly damage mode was interface failure and fiber breakage. The intermediary bamboo which fiber content of the meat medium damage mode was matrix damage but fiber bridging. The damage mode of inner part bamboo with less fiber content was matrix completely fractured and fiber pull-out. Fiber bundles fracture surface was like tower. Under three-point bending load in Tangential direction, fibers were peeled and spite layer by layer. And the inner bamboo damage faster than the outer bamboo that showing a typical ductile failure mode.9. Bamboo vascular bundle tensile strength and MOE was decreased gradually along bamboo culms radius direction from outer to inner. Vascular bundle average tensile strength and MOE were 1702.98MPa and 33.33GPa. Vascular bundle tensile failure process was fiber bundles being pulled out with some of the first fiber damage and then fibers were completely pulled off. There is significant damage fiber bundle pull-out section parenchyma. Fiber fracture surface was spiral ladder-like, and the damage section was like jag. That was owing to fiber multi-layered cell wall structure and microfibrils arrangement. From cell level to the macro-level hierarchical structure, bamboo’s unique characteristics and composite materials give high strength and good toughness properties to bamboo.更多还原