Gradient changes in fiber bundle content and mechanical properties lead to asymmetric bending of bamboo

Natural biomaterial bamboo has excellent strength and toughness. The content of fiber bundles and the gradually increasing tensile strength of fiber bundles from the inside to the outside in the radial direction of the bamboo wall are the two main factors leading to significant differences in the fracture behavior and mechanical properties of bamboo under loading from different sides of bamboo strip. In this study, the flexural strength, toughness, and bending fracture energy of the bamboo loaded from the inside of the sample (Mode B) were found as 28%, 191%, and 174%, respectively, higher than those of the bamboo loaded from the outside of the sample (Mode A). The mechanical properties of samples loaded radially (Mode C) were higher than Mode A and lower than Mode B. The fracture process of the sample and the corresponding fracture morphology showed parenchyma fracture (matrix failure), fiber fracture, and interfacial debonding (fiber–fiber, fiber-parenchyma, parenchyma-parenchyma) occurring in all samples. Tearing of the cell wall, fiber debonding, fiber bridging, fiber pull-out, and crack deflection are the major fracture-toughening mechanisms of bamboo. This study reveals the relationship between the gradient changes in the mechanical properties and content of bamboo fiber bundles and the bending and fracturing behavior of bamboo, which provides theoretical basis and ideas for the development of new bamboo engineering materials, the bionic design of bamboo structures, and the structural optimization of fiber-reinforced composite materials.