Moisture dependence of interfacial bonding and mechanical properties of bamboo as natural fiber reinforced composites

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Bamboo's exceptional mechanical properties stem from its multiscale structure, polymer components, interfaces and their synergistic interactions. While the mechanisms contributing to bamboo’s stiffness and strength through its structural components have been extensively studied, the interfacial bonding behavior between fibers and parenchyma, particularly how moisture influences this interaction, remains elusive. This study endeavored to elucidate the moisture-dependent effects on interfacial bonding performance and failure mechanisms in bamboo by integrating molecular dynamics simulations with vascular bundle pull-out tests. The presence of water molecule significantly reduced the interfacial bonding energy between cellulose and the lignin-hemicellulose composite, from 533 to 279 kcal/mol. The interfacial shear strength between vascular bundles and parenchyma decreased from 19.7 to 13.7 MPa. Concurrently, moisture plasticizes the matrix, swells amorphous regions, and decreases crystallinity by 13.2 %, collectively promoting interfacial slippage. This moisture-induced weakening of interfacial bonding facilitates crack capture and deflection at the interface, thereby enhancing bamboo's toughness while compromising its strength. These findings elucidate a trade-off mechanism: interfacial degradation compromises strength but enhances energy dissipation through controlled failure modes. This work provides fundamental insights into the hygromechanical behavior of natural composites and inspires biomimetic designs for high-performance materials requiring balanced strength and toughness in humid environments.