The biomechanical role of the functionally graded microfibrils in the wood cell wall

The cellulose microfibril in the wood cell wall possesses a functionally graded structure, the biomechanical contribution of which is unclear. At the molecular scale, we analyze the heterogeneous crystallinity of the microfibril and characterize its stiffness gradient with the help of molecular dynamics. The determined stiffness profile is then upscaled to the cell wall level, based on which a shear-lag analytical model is developed to reveal the load transfer mechanism in the cell wall. We find that the presence of the stiffness gradient can significantly alleviate the stress concentration on the interface by reducing the interfacial shear stress up to 70%. The analysis on the debonding procedure shows profoundly enhanced debonding resistance of the cell wall due to the graded structure. Moreover, we identify a novel "dual debonding" mode, meaning that the debonding initiates and propagates in both the edge and inner regions of the interface until the two debonded regions join each other, which is much more complex than the edge debonding mode exhibited by the conventional fiber-reinforced composites. The stress alleviating effect, together with the special debonding mode, profoundly postpones the interfacial debonding of the material, leading to a significantly enhanced debonding resistance. A discussion on the structural profile of the microfibril shows that the special "dual debonding" mode can only be achieved with specific stiffness gradients. In all, we believe our work helps to unravel the biomechanical role of the microfibril in the wood cell wall, and builds the theoretical foundation for tailoring the bio-inspired composites reinforced by functionally graded fibers.