The University of Science and Technology of China has made new progress in the analysis of multi-stage fiber structure of bamboo knots

With its low weight, excellent mechanical properties and rapid growth, Moso bamboo is becoming a sustainable alternative to wood and chemical synthetics. Compared to bamboo with a unidirectional fiber arrangement, the short bamboo joints appear to have weak mechanical properties, and they are often discarded in the processing of engineered fiber laminates. In the survival and development process of tall and straight moso bamboo, the small proportion of bamboo joints will play a role in fixed-point mechanical support strengthening and fluid multi-directional transportation. This dual-function or multifunctional implementation is necessarily closely related to the internal structure of the bamboo joint, but the spatial fiber structure and structure-activity relationship of the bamboo joint are still vague.

In view of this, the team of academician Yu Shuhong of the University of Science and Technology of China uses the collaborative strategy of multi-scale imaging and multi-modal mechanical properties research to systematically analyze and clarify the spatial multi-stage sub-fiber assembly structure of the bamboo knot, and proposes the design scheme of three fiber-reinforced structures to provide the optimal design scheme for the future creation of bionic fiber composite structural materials. In addition, the integrated design scheme of bamboo structure enhancement and liquid transport is also experimentally verified, and a photo-hot water evaporation device based on bamboo joints is constructed. It was recently published in the National Science Review 2022, doi: 10.1093/nsr/nwac195 under the title “Mechanically robust bamboo node and its hierarchically fibrous structural design”. The first authors of the paper are Chen Siming Special Associate Researcher and Dr. Zhang Sichao, and the corresponding authors are Associate Researcher Gao Huailing and Academician Yu Shuhong.

Using optical microscopy, three-dimensional X-ray computed tomography and reconstruction techniques, scanning electron microscopy and atomic force microscopy, the researchers used imaging characterization methods to explore the structural arrangement of multi-stage subfibers (vascular beams) in the spatial heterogeneity of bamboo nodes (Figure 1a-b), and identified their spatial layout and interoperability. Based on the new structural findings, the researchers further proposed three fiber-reinforced structural design schemes, including space fiber retention tight interlocking (located in the bamboo wall, Figure 1c), space three-axis mutual hanging scaffolding connection (located in the transition area between the bamboo wall and the diaphragm, Figure 1d), and isotropic energy-absorbing interweaving (located in the central area of the diaphragm, Figure 1e).

Figure 1: Structure of slub space heterogeneous multi-stage sub-nano fibers

Under the action of different modal loads such as arch radial compression (Figure 2a-c), unidirectional axial compression (Figure 2d-f), axial splitting (Figure 2g-i), etc., the researchers studied the structural evolution and failure of several key fibers, such as the ring fiber bundle in the bamboo section (located in the transition zone between the bamboo wall and the diaphragm, Figure 2b-c) and the transverse fiber (located in the bamboo wall, Figure 2e, h). Further combined with 3D X-ray computed tomography and micro-crack reconstruction techniques and theoretical simulation analysis (Figures 2b-c, e-f, h-i), the important contribution of several key fiber and fiber-reinforced structural design schemes in maintaining the stability of bamboo joints and bamboo bodies is verified.

Figure 2: Multimodal mechanics of bamboo and the strengthening and toughening of key fibers

The researchers also used three-dimensional X-ray computed tomography to experimentally find that the vascular bundle fibers arranged in multiple directions within the bamboo section can realize the liquid transport exchange, and confirm the integrated design of vascular bundle fiber structure reinforcement and liquid transport. Inspired by this versatile integration, a bamboo-based photohot water evaporation unit was designed, which exhibits good structural stability and evaporation efficiency.

This study uses a combination of experimental and theoretical means to analyze the complex fiber structure in the bamboo section in detail, and extract and verify three fiber reinforced structural schemes. These fiber-reinforced structures are assembled by progressively amplifying cellulose molecules, nanocrystals, nanofibers, microfibers, and vascular bundles. The multi-scale enhanced toughening mechanism plays a cross-scale synergy in maintaining the structural stability of bamboo joints and bamboo bodies, and this research will provide guidance for the optimal design and preparation of high-performance fiber composite structural materials.

Special thanks to Professor Wu Heng’an of the School of Engineering Sciences of the University of Science and Technology of China, Professor Gong Ming of the Engineering Materials Science Experimental Center, Professor Xiang Chengbin and Professor Shen Xiansheng of the School of Life Sciences, and Dr. Xiaohao Sun of Georgia Institute of Technology for their guidance and help. Thanks to units such as Hefei Botanical Garden and Carl Zeiss for their support of this work. The work was supported by the National Key Research and Development Program, the National Natural Science Foundation of China and the China Postdoctoral Science Foundation.

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