Cracks fail to intensify stress in nacreous composites

Linear elastic fracture mechanics (LEFM) implies that crack-like flaws would intensify stress in brittle materials with stress intensity scaling up with the square root of the crack size. Therefore, the apparent strength of materials tends to be much smaller than the theoretical value. In this paper, we examine the stress state in nacreous composites and find that in such materials the crack-induced stress intensification and its dependence on crack size can be suppressed greatly. This feature of nacreous composites can be attributed to the unique "brick-and-mortar" (B-and-M) structure and a synergistic match of the mechanical properties between "brick" (e.g. minerals) and "mortar" (e.g. proteins) phases. Our findings not only provide a fundamental insight into the origin of the excellent mechanical properties of nacreous composites such as high strength, high toughness and flaw tolerance, but also will be of great value to the design and synthesis of new structural materials for superior mechanical properties.