Effects of gradient distribution and aggregate structure of fibers on the flexibility and flexural toughness of natural moso bamboo (Phyllostachys edulis)

Abstract

Bamboo is a unidirectional fiber-reinforced material with excellent flexibility and flexural toughness arising from its unique cellular hierarchical structure, which creates an urgent demand in the bamboo industry to understand potential mechanisms of those two performances. In this study, a proposed original method was used to quantitatively evaluate the gradient distribution and aggregate structure of fibers on the flexibility and flexural toughness of natural moso (Phyllostachys edulis) bamboo, with a comparison to two other natural wood. Results showed that gradient distribution of fibers across the bamboo culm is the key to bamboo's varying flexibility along the diameter at the tissue level, whereas the asymmetric aggregate structure (vascular bundles) of fibers causes a difference in the flexibility when under different bending directions. Bamboo had possessed high strength and high toughness, but a conflict between the flexibility and flexural toughness existed. Compared to natural wood species of similar density, the flexibility and flexural toughness of bamboo was ∼1.8 and 1.42–4.96 times greater that of wood, respectively. At the cell level, the combination of high-performance hard fibers and soft foamy parenchyma cells act upon the bamboo's flexibility, while the excellent flexural toughness stemmed from the fiber bridging and pull-out caused by high performance and aspect ratio, low microfibrils angle of fibers, and crack deflection along the longitudinal/radial direction. These findings helped contribute critical knowledge surrounding the flexibility and flexural toughness of bamboo, especially for optimizing the process of bamboo-based winding composites pipes (BWCPs).