Shrinking and swelling of moso bamboo with different gradient variations: Effects of culm height and presence of pith

The hygroscopicity of bamboo leads to shrinking and swelling, which affects the processing and utilization of bamboo and its products. To enhance the understanding of the shrinking and swelling behavior, high-resolution microscopy and the digital image correlation technique were combined to investigate the differences in shrinking and swelling during the desorption and absorption processes at various internodes (No.3, 8 and 13). In addition, the effects of factors such as fiber distribution density and the presence of the pith ring on shrinking and swelling were analyzed. The shrinking and swelling in the radial direction varied with internode position, exhibiting a quasi-linear tangential deformation and a saddle-shaped radial deformation. As the internode position increased, the position of the maximum value of differential shrinking/swelling shifted closer to the interior of the bamboo. Upon removing the pith ring, a reduction in hygro-deformation in the inner part of bamboo was observed, while the outer part experienced an increase. The extent of this influence was directly related to the development of the pith ring. When the moisture content exceeded 10 %, hysteresis was noted in both tangential and radial deformations, as well as in the presence of pith ring. The gradient coefficient of deformation showed a pattern of initially increasing and then decreasing along the height of the culm, peaking at internode No. 8. This study explores the hygro-deformation phenomenon of bamboo and the impact of gradient levels, providing insights for understanding the bamboo-water relations, and optimizing the manufacturing of bamboo-based productions.