The unique flexibility feature of moso bamboo: Arising implications for biomimetic material design

Bamboo, renowned for its sustainability, renewable nature and extraordinary flexibility, has long been widely used as engineered materials. This coveted attribute can be attributed to the unique gradient microstructure of the bamboo culm, characterized by the volume fraction of reinforcing fibers diminishes exponentially from the epidermis to the endodermis. This inherent structural heterogeneity engenders disparate fracture behaviors and mechanical properties between the compression and tension sides during bending, i.e. the asymmetric bending behavior. Through a qualitative analysis with the shear-lag theory, this study has discerned that a critical range of fiber volume fraction, spanning from 25% to 30%, serves as the critical condition for whether fiber break occurs before or after pull-out. Moreover, it has been elucidated that the significant degree of radially discrepancy in fiber volume fraction manifests as a key determinant of the degree of asymmetry. By parameterizing the variation trend of the fiber volume fraction along the loading direction, it becomes feasible to predict the mechanical properties when subjected to loading from different sides. Such endeavors furnish a pivotal scientific foundation for characterizing the performance of gradient materials with similar densities, while concurrently offering novel insights for the structural design of high-performance biomimetic material.