Abstract:

Objective: The purpose of this study was to investigate the crack behavior and mechanism of gradient structure in bamboo under grain splitting and radial compression to provide a theoretical basis for the application in engineering materials. Method: The external force of grain splitting and radial compression were applied on bamboo to induce initial crack. Field emission scanning electron microscope (FESEM) was conducted to observe the fracture appearance and crack propagation in vascular bundles, parenchyma and cells along the transverse and longitudinal direction. Micro-mechanical properties of fibers and parenchymal cells wall were measured to further illustrate the fracture mechanism stemming out from the bamboo’s hierarchical configuration. Result: The FESEM results were as follows: grain splitting-induced cracks appeared and grew in the middle lamellae (ML) of fiber and parenchymal cells along the transverse and longitudinal direction, which resulted in the transverse tearing of vascular bundles and parenchyma. Radial compression-induced cracks also appeared and grew in the middle lamellae (ML) of fiber and parenchymal cells along the transverse and longitudinal direction. In the transverse direction, dendritic cracks occurred on the surface of the fibrous sheath and make the vascular bundles disperse into fibrous bundles. In the longitudinal direction, fibers separated and parenchymal cells exfoliated. The nanoindentation results showed that the indentation modulus and hardness of middle lamellae were lower than that of cell wall layer in fiber and parenchymal cells, which was prone to fracture. Conclusion: In this study, by observing the fracture morphology and crack propagation path of vascular bundles and parenchyma under grain splitting and radial compression, it was found that the vascular bundles split into fiber bundles and interlaminar fracture occurred in the parenchyma. It was found that the fiber and parenchymal cells could be separated by the transverse and longitudinal fracture of the middle lamellae (ML) under the grain splitting and radial compression because of the lower mechanical strength.

Key words: bamboo, fracture, splitting force, radial compression force, micro-mechanical properties