Structure, Scaling, and Performance of Natural Micro- and Nanocomposites

Natural materials boast remarkable mechanical performances in some cases unmatched by their synthetic counterparts, and for this reason, they have become an inspiration for the development of new materials. In highperformance natural materials such as nacre, bone, or teeth, stiffness and toughness are achieved with the staggered microstructure, where stiff inclusions of high aspect ratio are embedded in a softer matrix. While the modulus and strength of the staggered structure is well understood, fracture toughness and scaling remains unclear. In this work, a fracture model based on the fundamental micromechanics of the staggered structure is presented. The model captures crack bridging and process zone toughening, and explicitly shows how these toughening processes are the most efficient with high concentrations of small tablets of high aspect ratio. In particular, a desirable non-steady cracking regime can be achieved with specific requirements for structure and interface properties, which are presented in detail. These attractive toughening mechanisms are only possible if the tablets themselves do not fracture. The benefits of small size have been explored in the past, but here, we show for the first time how the effects of a stress singularity generated by the junctions between the tablets can be alleviated by the softer interfaces, provided that a “soft wrap” condition is met. The models provide new insights into the optimization and scaling of natural and biomimetic composites.