Study The Mechanism Of Moisture Affecting The Bamboo Fibre Cell Walls And Macroscopic Mechanical Behavior

Bamboo can be generally viewed as a multiple graded composite material which reinforced by natural fiber. The multi-level structure indicated that only establish a complete macro to micro mechanical testing and research system, then we can be able to deeply understand the nature of the complex mechanical behavior of bamboo. In this paper, at first I have studied the macroscopic mechanical properties of bamboo as the moisture content changes, then studied the longitudinal mechanical properties of bamboo fiber changed with moisture by using Single fiber tensile Technology and Nan indentation, which have abundant content of the Cell-scale bamboo mechanical and provided experimental evidence to the choice of bamboo and bamboo fiber utilization; in the meanwhile, the chemical composition of bamboo and micro-fibril angle with the variation in age have been studied; further, I have studied the failure modes of bamboo timber under different moisture, and analyzed the mechanism of bamboo in macro and micro-mechanical behavior, which provided many important and critical information, and also provided a theoretical basis for truly grasping the mechanism between moisture and mechanical behavior of bamboo. The study's main results are summarized as follows:1,Obtained bamboo and bamboo fiber moisture sorption isotherms and calculated the fiber saturation point:the moisture sorption isotherm of bamboo fiber is significantly higher than bamboo block, and the former fiber saturation point is obviously too large.2,Under air-dried condition, bamboo fiber S2 layer of secondary wall model is about 21Gpa, hardness of about 500MPa; S2 layer of secondary wall of bamboo fiber longitudinal elastic modulus and hardness with the increase of moisture content significantly decreased and negative relationship between. The variation of longitudinal elastic modulus changed with moisture content was less than the hardness in bamboo fiber S2 layer of secondary wall. Moso bamboo fibers S2 layer of secondary wall deformation mechanism dominated by plastic deformation, and that the residual deformation increased with moisture content and the maximum indentation depth increase. In the chemical composition and structure under the same conditions, as well as the moisture content changes within the fiber saturation point, bamboo fiber S2 layer of secondary wall viscoelasticity similar.Bamboo fiber axial tensile elastic modulus is about 35 GPa, tensile strength of about 1400 MPa～1600 MPa, Elongation at break of about 5%. In the same moisture content, the modulus, tensile strength, elongation at break of different age bamboo fiber were not obvious different. Bamboo fiber axial tensile modulus, tensile strength, elongation at break significantly change with moisture content, and at the same age, when moisture content from 5.5% to the saturated state, the mechanical properties of bamboo fibers were significantly lower, for example, the bamboo fiber tensile elastic modulus of four aged down from 35.76 GPa to 24.00 GPa; Tensile strength decreased from 1598 MPa to 1208 MPa; In addition, bamboo elongation at break as the moisture content increased from 4.68% to 5.68%.3,Obtained the influence laws in mechanical properties of bamboo (tensile, compression, bending, shear, etc.) depends on the moisture:the sensitivity on four kinds of mechanical properties of bamboo changes in moisture content were significantly different. Shear strength and compressive strength parallel to grain changes on the most sensitive, which average decreased more than 30% from air-dry state to the saturated state, and bending modulus with 23.54% in the next place, meanwhile, tensile modulus parallel to grain changes with moisture content was on the least sensitive, only 19.79%.4,Proved the laws that moisture effect on the various failure modes of mechanical properties of bamboo:the fracture surface of four mechanical properties was observed through Electron microscopy, and it can be found that the destruction of bamboo under different moisture showed destruction of the different interfaces, dry state was expressed the tear between fibers and parenchyma, while wet was the cell spin-off. While, Air-dry state samples destroyed besides fibers spin-off, also includes the fracture strength of parenchyma, which resulted in increased strength.5,The absolute value of bamboo micro-fibril angle have little difference. In addition to 2.5 years bamboo micro-fibril angle was higher, at 8.81°～12.01°, the other three aged bamboo micro-fibril angle were between 8.14°～9.84°.The main chemical components of bamboo glowed stably a year later, notwithstanding the changes, but not the absolute value of difference. Acid-soluble lignin content significantly, and starting from 0.5, acid-soluble lignin content decreased with age increasing, among that 0.5 bamboo was highest acid Lignin content, average 26.51%.Summarily, both macro-and micro-mechanical behavior of mechanical properties of bamboo depend heavily on the moisture, however, both have their own characteristics:the sensitivity of various indicators of macroscopic mechanical properties have significant differences; meanwhile single fiber and secondary wall S2 layer mechanical properties changed with moisture also obvious, but much smaller than macro indicators, further more microscopic indicators sensitivity significant difference as well. The destruction of bamboo under different moisture showed destruction of the different interfaces, dry state was expressed the tear between fibers and parenchyma, while wet was the cell spin-off. When the moisture increase, the interface between fiber and parenchyma decreased, and finally leaded to changes in mechanical properties. But as the force method, structure, composition differences in bamboo, fiber and cell-wall, there had some diversity on different indicators.