Evaluation of orthotropic elasticity of gradient-structured bamboo by microtensile testing combined with digital image correlation technique

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Bamboo, a biomaterial with a unique gradient structure, possesses exceptional mechanical properties. The gradient structure impacts the mechanical properties of bamboo in the longitudinal (L), radial (R) and tangential (T) directions. To investigate the elasticity associated with gradient structure, we derived the orthotropic elastic constants: Young’s modulus (E), Poisson’s ratio (ν) and shear modulus (G) at different radial positions (outer, central and inner) through micro-tensile testing and the digital image correlation (DIC) technique. Our results revealed that the Young’s modulus of bamboo exhibited a characteristic orthotropic behavior irrespective of radial position. The ratio of E in the L, T and R directions (EL:ET:ER) was 6.05:1.08:1, 5.12:1.17:1 and 8.02:1.58:1, respectively at the outer, central and inner positions. With increasing fiber fraction, ν (except νRT) demonstrated an upward trend. Under main axis tension, fiber pull-out and brittle fracture were the main failure modes, whereas off-axis tension exposed the vulnerable interface between fibers and parenchyma cells. The enhancement effect of the gradient structure in the L and T directions was more prominent, resulting in a consistent deformation pattern from the inner to outer positions. However, strain localization was found when tensioning through the R direction. Leveraging the derived orthotropic elastic constants, we visualized the three-dimensional elastic behavior of bamboo by body deformation. Bamboo at the outer position exhibited quasi-isotropic behavior in the RT plane due to the high fiber fraction. This study provides critical experimental evidence for the orthotropic behavior of bamboo at different radial positions and valuable data for mechanical modeling and practical applications involving bamboo.