Chapter 10 Self-Shifting Neutral Axis and Negative Poisson’s Ratio in Hierarchical Structured Natural Composites: Bamboo Shaowen Xu, Aniruddha Mitra, Stephen Migues, Jacob Mayfield, Michael Shinall, Bessenbacher Derek, Davis Linley, and Spratlin Russell Abstract Bamboo is a light weight, high strength natural composite. Although it has been wildly used in the world for thousand years, the mechanism of material enhancement and microstructure/property relationships has not been well understood yet. In this study, triple points bending tests were conducted for mechanical behavior characterization of natural composite-bamboo. Digital Image Correlation was utilized to determine the deformation and strain of a bamboo beam. It was found that (a) the material responses to the applied load had both a linear and a non-linear range, (b) the beam was stiffer when it had node at two ends, (c) the neutral axis shifted toward the tensile side of the beam during loading process, (d) when samples were reloaded, the material responses to the load were different, (e) negative Poisson’s ration was found in the high fiber density of the beam. Keywords Neutral axis • Hierarchical structure • Self-shifting • Negative poisson’s ratio • Digital image correlation 10.1 Introduction In nature, biological materials have evolved for over eons to fit their environments, resulting in various multi-scale hierarchical structures with unique properties. For example, bamboo is organized from simple organic building blocks, which are as same as that of a tree, at various length scales to maximize strength, toughness and flexibility using minimal mass. Characterizing the mechanical behavior and understanding structure/property relationships of these hierarchical structures is fundamental for developing new advanced composites with superior properties. Although bamboo has been wildly used for 1000 years, the mechanism of material enhancement and microstructure/property relationships, especially the interaction of reinforced fiber and porous matrix, have not been well studied due to the complexity of the microstructure and lacking of available technologies. In this study, triple point bending tests and microstructure analysis were conducted to characterize the deformation behavior of bamboo. In the test, Digital Image Correlation [1–7] was utilized to determine deformation and strain and understand the mechanism of material enhancement and structure/property relationships of bamboo. The effect of node structure of bamboo on material properties was also analyzed. The goals of this study were to (a) investigate material response of multi-scale hierarchical structures to the applied load, (b) determine the contributions of node structures in bamboo to bulk properties; (c) build knowledge base for research in bio-inspired advanced engineering material development. 10.2 Experiment The bamboos used in this study were harvested from a local bamboo forest in southeastern Georgia. Structure analysis were performed in both transverse and longitudinal cross-sections. The samples used in the tests were cut along a longitudinal axis of bamboo. To investigate the effect of the bamboo node on material behavior, both samples with and without node were studied. All the samples were prepared using a same section of a bamboo so that the samples could be identical (Fig. 10.1). The wall thickness of the bamboo section was 7.10 mm. Shown in Table 10.1 were dimensions of the samples. S. Xu (*) • A. Mitra • S. Migues • J. Mayfield • M. Shinall • B. Derek • D. Linley • S. Russell Department of Mechanical Engineering, Georgia Southern University, Statesboro, GA 30458, USA e-mail: shaowenxu@georgiasouthern.edu #The Society for Experimental Mechanics, Inc. 2017 C.S. Korach et al. (eds.), Mechanics of Biological Systems and Materials, Volume 6, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-41351-8_10 67
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