Load transfer characteristics in biocomposites reinforced by periodically graded cellulose microfibrils

Cellulose microfibrils serve as the major load carriers for a variety of biocomposites in nature. The microfibrils possess periodically graded structures, the role of which in load transfer between the fibril and matrix is unclear. In this paper, we revisit the shear-lag theory by taking the periodically graded mechanical properties into consideration. We find that the alternating arrangement of stiff crystalline parts and compliant amorphous parts result in distinct stress distributions compared to ordinary fiber-reinforced composites. The normal stresses in the microfibrils are gradually built up from the amorphous parts to the crystalline parts. Most importantly, the interfacial shear stresses are profoundly reduced by the graded structure, and the interface damages developed due to high external loadings are more evenly distributed over the entire microfibrils instead of concentrating at the localized edge zones. Careful examinations on possible variations in configurations, including the pitch lengths of the periodical structures and the stacking patterns of the microfibrils, confirm that conclusions drawn on the shear-lag analysis are applicable to more complex scenarios. Our study indicates a possible strategy to regulate the interfacial loadings by designing proper graded structures in the reinforcements.