Effect of epidermis and pith ring presence on bamboo shrinkage behaviors

Understanding the effects of intact anatomical structures on bamboo shrinkage is crucial for mitigating shrinkage-induced defects in bamboo-based products. However, the effects of bamboo boundary layers (epidermis and pith ring) and their interactive mechanism with vascular bundles (VBs) remain unexplored. This study aims to quantify the effects of these boundaries on shrinkage and clarify how VB content modulates shrinkage under their joint constraints. The results show that the epidermis and the pith ring significantly inhibit radial and tangential shrinkage, while promoting longitudinal shrinkage. This inhibitory effect is further clarified by real-time monitoring, which reveals that the maximum tangential shrinkage (12%) occurred in the mid-VB content areas (37 VB% to 42 VB%). This phenomenon arises because boundary constraints restrict shrinkage in both VB-highest and VB-lowest areas, demonstrating that boundary effects override the influence of VB content gradients. Therefore, the epidermis and pith ring redistributed the shrinkage strain by restricting adjacent zones, thereby localizing maximum strain and crack formation in the mid-VB content areas. This study provides insights into the synergistic effects of anatomical boundaries and VB gradients on bamboo shrinkage, offering a theoretical basis to design low-shrinkage bamboo-based materials.