A straightforward and efficient gradient pressure method for bamboo flattening: Strain and multi-scale deformation

Flattening is an environmentally friendly and efficient processing technique that transforms arc-shaped bamboo strips into regular rectangular ones, greatly expanding bamboo's application potential. However, the current method of bamboo flattening requires softening the strips in a high-temperature and high-pressure environment, followed by continuous roller flattening. This process consumes significant energy and does not allow real-time monitoring of the bamboo strips during the process. This study devised a straightforward and efficient gradient pressure method to flatten naturally arc-shaped bamboo strips into rectangular ones within 30 min, all while preserving the complete culm wall structure without any cracks. The analysis of the deformation characteristics during the process revealed that the outer side of the bamboo strip was subjected to compressive stress, while the inner side was subjected to tensile stress. The parenchyma and vessel cells underwent wrinkling and shrinking due to moisture loss and external pressure. Flattening generated horizontal, vertical, and shear strains in bamboo strips, with horizontal strain being predominant, reaching a maximum negative horizontal strain of approximately −0.131. Besides, significant differences were detected in strain among different parts of the bamboo strip. The overall strain exhibited an obvious left-right symmetrical distribution. For the first time this work analyzed the strain distribution and deformation characteristics during the flattening process of arc-shaped bamboo strips while preserving the intact bamboo culm wall structure. The findings of this research provide a novel option of bamboo culm flattening, enhancing the utilization of raw materials and expanding the application of bamboo as engineering materials.