Statistical Analysis Of Microstructure And Fracture Strength Of Bamboo

Bamboo fibers own many distinct characteristics, such as high strength, low elongation and good renewability. Composites reinforced by bamboo fibers tend to possess the high specific strength, high moduli and degradability, which indicate that their mechanic properties are almost equivalent to engineering plastics. The damage behavior of such composites is usually affected by the large scatter on fracture strength of reinforcing fibers. Therefore, a comprehensive investigation on the tensile fracture of bamboo fibers will contribute to better understanding the defonnation and fracture mechanism of bamboo fiber reinforced composites.Firstly, the multi-scale structure of bamboo fiber was investigated. The surface morphology, transverse and longitudinal section, fracture surface of the fibers were observed using the optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). Owning to the preparation process of steam explosion, bamboo fiber surface is covered by a large amount of lignin. Observations of transverse and longitudinal sections indicate that, resulting from the characteristics of the natural growth, bamboo fiber is actually composed of large quantities of elementary fibers with nonuniform sizes, which is bound together along the length by some compositions, such as lignin, lipid and so on. The cross-section of elementary fibers is either hexagonal or hexagonal and they are arranged in a honeycomb pattern. In addition, TEM observations show that elementary fiber has a more complex multi-level structure with more than four layers and is composed of lignin and hemi-cellulose matrix reinforced by spiral-wound cellulose micro-fibrils. Therefore, this complex arrangement may affect the fracture strength scatter of bamboo fibers.Secondly, in order to investigate the effect of micro-structure and size on bamboo fiber strength, large quantities of bamboo fiber were tested in this paper. The third chapter mainly gives a detailed introduction on the parameters of tensile test and the diameter measurement. All bamboo fibers were performed on quasi-static tensile tests. The stress-strain curve shows no obvious yielding point for the tensile process of bamboo fiber, which suggests that its fracture behavior belongs to brittle fracture mechanism. However, the Young’s modulus and failure elongations of bamboo fibers under different gauge sizes show a significant difference. The fourth chapter mainly discussed the size effect of bamboo fiber strength along the length.50 individual samples were prepared for testing at each of five gauge lengths (20,30,40,50, and 60 mm) at the fixed test diameters D=200 ± 20 μm to characterize the strength distribution of bamboo fibers. Under the framework of the Weak-link theory, a modified Weibull model (namely the power-law model) was established for the purpose of analysis. Strength predictions were made using both the classical linear-law model and the power-law model based on length L. Results show that the power-law model performs more accurately than the classical model.Finally, the fifth chapter mainly discussed the diameter dependence of bamboo fibers with contexts including:(1) 50 individual specimens were performed on tensile test at each of five gauge diameters (196.6,317.3,398.4,508.8,584.3μm) at a fixed length for 20mm to enlarge test diameter variation in this study. Based on the One-way analysis of variance (ANOVA), results show that diameter has a significant effect on bamboo fiber strength; (2) in order to characterize the size effect of bamboo fiber along the radial direction, predictions of the traditional linear-law model and power-law model based on the volume V were performed, respectively. The projections for the diameter effect show that predictive level of power-law model is still higher than the linear-law model; (3) this paper aims to explore the quantitative relationship between micro-structure and macro properties. The number of elementary fibers was counted through embedding fibers into resin and SEM observations of fiber cross-sections. Fiber volume was assumed to have a exponential relationship with the quantity of elementary fibers, and for the first time a new Weibull model based on micro-structure (elementary fiber quantity) was obtained, which accurately performs on fiber strength prediction at the macro-scale; (4) an important method on the calculations of related parameters, called Maximum Likelihood Estimation (MLE), was demonstrated. The detailed calculation process on the estimated parameters of linear-law model and power-law model, as well as the MATLAB codes for the nonlinear equations, were also given in this paper.The strength distribution of bamboo fibers can be better characterized by the proposed method, which provides a fundamental research for further developing the accurate strength prediction model for fiber reinforced composites based on the fracture mechanism.