Weak layers in bamboo, which are prone to the generation of cracks or are the preferred routes for crack growth, govern the machining processes and applications of bamboo. Weak layers are avoided during storing but are utilized during splitting and slicing. Gaining an understanding of the weak layers is a priority that will allow for the determination of whether to avoid or utilize them. In this study, scanning electron microscopy was used to observe the weak layers at the cellular and subcellular levels. A nanoindentation instrument and a Raman microscope were used to quantitatively characterize the mechanical properties and chemical components of these weak layers. The results show that among the three types of bamboo cells, vessel cells were the most vulnerable to damage, while fiber cells were the least susceptible to damage. The weak layers at the subcellular level were compound middle lamella (CML), thin layers of cell walls, and pits. The average storage modulus values were as follows: 13.7 GPa for CML, 17.0 GPa for pits, 20.6 GPa for thin layers, and 25.3 GPa for thick layers. Compared with the thick layers, the maximum decrement of cellulose content was 51% in CML and 41% in thin layers. With the lowest cellulose content, CML was the likeliest subcellular structure in which cracks propagated. The hardness of the pits was lower than that of the adjacent non-pit areas. The mechanical properties of bamboo increased by targeted modification of the weak layers. This work demonstrates a comprehensive investigation into weak layers of bamboo and quantitatively visualizes their mechanical and chemical properties.
Keywords: bamboo; chemical distribution; compound middle lamella; mechanical properties; pits; thin layers; weak layers.