Study On The Properties And Mechanism Of Fracture In Bamboo

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.