Effects of nanofiber orientations on the fracture toughness of cellulose nanopaper

Cellulose nanopaper exhibits superior mechanical properties with both high strength and toughness, and the crack bridging mechanism of nanofibers makes the most significant contribution to its fracture toughness. In this paper, we investigate the fracture toughness of a mode-I crack in cellulose nanopaper by using a modified crack-bridging model. Different from previous crack-bridging models, we account for the effect of nanofibers inclined to the crack surfaces. Particular attention is given to the dependence of fracture toughness on the orientation distribution of nanofibers in the cellulose nanopaper. We use a cohesive law to account for the interfacial shear stress of nanofibers, which involve self-healing of hydrogen bonds at their interfaces. Two representative orientation distributions are considered, in which nanofibers are aligned or randomly oriented, respectively. The theoretical results agree well with relevant experiments. This work helps understand the structure-property relationship of cellulose nanopaper and design other fiber-reinforced nanocomposites.