Synergistic toughening mechanisms of macro- and micro-structures in nacre: Effects of T-stresses

Through long-term evolution, biological tissues have optimized their components and structures at multiple length scales to meet the requirements of mechanical properties and biological functions. In this study, we explore how the shell macrostructure of nacre and its brick-mortar microstructure are synergistically designed to adapt to external mechanical conditions. We found that the T-stress effect plays a key role in linking the high toughness of nacre with its hierarchical microstructure. When an abalone shell with a convex morphology is subjected to an impact load from a predator, a compressive T-stress arises with the crack initiating in the macroscale nacreous shell. A hierarchical crack bridging model is presented to investigate how the T-stress affects the microscopic stress transfer mechanism and the macroscopic mechanical properties of nacre. It is found that the negative T-stress enhances the interfacial stress of the mineral platelet with waviness, a typical secondary structural feature of the brick-mortar structure. In turn, the amplified interfacial stress further leads to a significant increase in the macroscopic strength and fracture toughness of nacre. This work not only reports a novel toughening mechanism resulting from the structural hierarchy of nacre, but also provides inspirations for the design of biomimetic composite structures with enhanced mechanical properties.