Analysis on moisture-induced stresses in wood cell wall considering periodically graded microstructures

Abstract

The S2 layer of the wood cell wall is essentially a biocomposite reinforced by periodically graded microfibril, the mechanical properties of which are significantly affected by environmental humidity. The function of the periodically graded microstructure of microfibril in regulating moisture-induced stress and strain is still unclear. By developing the shear-lag model, we successfully reveal the stress transfer properties of S2 layers with periodically graded microfibrils at different levels of moisture. We demonstrate that the periodically graded microstructure facilitates the moisture-induced interfacial shear stress to be more evenly distributed over the whole microfibril and significantly reduce the interfacial shear stress at the edges, leading to the postponement of the initiation of interfacial debonding. Besides, we find that the moisture-induced stress is non-monotonic and exists maximum value, due to the fact that the polymers comprising the matrix undergo softening upon water absorption. Meanwhile, the length of the different regions of the microfibril is systematically studied, the results indicate that the interface burden can be reduced by increasing the length of graded regions, and sufficiently long pitch do not affect the magnitude of the interfacial shear stress. Moreover, the microstructure of the periodically graded fibril can effectively reduce the negative effect of debonding. These revealed mechanical mechanisms are conducive to the design of fiber-reinforced composites serving under humid condition.