Asymmetric bending characteristics and failure mechanism of bamboo under different loading directions

Bamboo has a special microstructure, its fiber content decreases gradually from the outside to the inside. This kind of gradient distribution results in obviously asymmetric bending characteristics of bamboo, and shows different bending properties and failure modes in different bending directions. In this study, bending properties of moso bamboo (Phyllostachys pubescens) in three different directions (type A: outward bending, type B: tangential bending, type C: inward bending) were tested. Digital image correlation (DIC) and scanning electron microscopy (SEM) were used to analyze the failure process, strain field change and failure morphology of bamboo, and summarized the failure mechanism of bamboo bending in different directions. Results indicated that the tangential bending have the highest modulus of rupture (MOR) and modulus of elasticity (MOE), and the inward bending has the best flexural toughness. The failure mechanism of the three bending directions was different. For type A, the initial crack appeared in the parenchyma tissue in the tensile region where the fiber content was less, so the crack rapidly expanded inside the parenchyma tissue, forming a neat failure section, then led to low toughness of bamboo. For type B, the initial crack appeared in the parenchyma tissue in the tensile region. Due to the moderate fiber content in the tensile region, the fiber bridging phenomenon appeared in the fracture section, led to the higher elastic modulus and strength of the type B. For type C, the initial crack appeared in the parenchyma tissue in the compression region. Because of the more fiber content in the tensile region, the cracks in this part propagated transversely and formed the failure morphology of interlayer separation, which increased the toughness of the specimen. This study is helpful to understand the asymmetric bending characteristics of bamboo, and provide guidance for the use of bamboo as a structural material and the preparation of composite materials.