A macro-fiber consist of hundreds of microfibers. Each microfiber has a co-shell multilayer structure. The matrix of the bamboo is not solid. It is formed of microporous structures (Fig. 10.3b). The wall of the porous structure has also layered structure. The macro-fibers are functional gradient distributed along the radius. The fiber density is high at an outside surface and lowers near the inside. The shape and dimension of the macro-fibers vary from inside to outside (Fig. 10.3d). These hierarchical structures result in unique mechanical property of bamboo. Plotted in Fig. 10.4 were the load-displacement curves (continue lines) and the neutral axis-displacement curves (dot lines) of the samples with (sample 3) and without (sample 2) nodes. The two load-displacement curves (continue line) were similar. They both had a linear range and a non-linear range. When loads were below 80N for the sample with the node (88N for the sample without node), the structure responses were linear. The Flexure Modulus was 12.15 GPa for the sample with node and 11.83 GPa for sample without node. The sample with node structures was stiffer. Its load-displacement was above that of without node. The sample with node needed higher load to be deformed. It was indicated that the nodes could reinforce the material and made it stiffer. It was interested to find (a) the neutral axis of the bamboo beam was not at center of the cross-section of the beam, it was offset to the outside surface of the bamboo; (b) the neutral axis continuously shifted toward high fiber density side when load increased. In linear range, the shifting was not obvious for the sample without node but little for the sample with node. The neutral axis tended to stay in its original location. When the load further increased into the non-linear range, the neutral axis started shifting toward the high fiber side (tensile side), the shifting rate was almost same for both sample with/without nodes. Shown in the Fig. 10.4, the initial location of neutral axis of the sample with nodes was more close to the center of the beam than that of the sample without the nodes when the samples subjected same load in linear range. In other words, when the deformations of the samples were the same, larger load was required and the shift of the neutral axis was smaller for the sample with the nodes. Shown in Fig. 10.5 were load-displacement curves and neutral axis-displacement curves of the samples with (sample 3) and without (sample 2) nodes when the samples were reloaded. The responses of the material were different with when the Fig. 10.3 Hierarchical structures of bamboo: (a) macro-structure of bamboo, (b) micro-structure of porous matrix of bamboo, (c) macro-fiber orientation and structure of nodal structure of bamboo; (d) micro-structures on transverse cross section, (e) micro-structures on transverse cross section near the outside surface 10 Self-Shifting Neutral Axis and Negative Poisson’s Ratio in Hierarchical Structured Natural Composites: Bamboo 69
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