Mode II fracture properties of laminated bamboo lumber and laminated flattened-bamboo lumber

This study investigates the Mode II fracture behavior of laminated bamboo lumber (LBL) and flattened bamboo lumber (LFBL). ENF (end-notched flexure) specimens with varying initial crack lengths and geometric dimensions were designed to evaluate their fracture properties. The fracture process zone (FPZ) and crack propagation phases were systematically analyzed using fracture testing, digital image correlation (DIC) techniques, and scanning electron microscopy (SEM). The compliance beam method (CBB) and the equivalent crack length approach were employed to determine the strain energy release rate (G_Ⅱ) for both LBL and LFBL specimens. The results revealed that when the initial crack was relatively short, both LBL and LFBL specimens primarily exhibited bending failure. However, as the crack length exceeded a critical threshold, the failure mode transitioned to fracture failure. Increasing the specimen width significantly improved the uniformity of shear stress distribution and enhanced the fiber bridging effect, thereby markedly boosting crack resistance. Additionally, LBL demonstrated higher critical load (P_c) and critical strain energy release rate (G_Ⅱc), indicating superior performance in suppressing crack propagation. These findings provide a scientific foundation for the development of high-performance, environmentally friendly composite materials and contribute valuable insights to the field of materials fracture mechanics.