Nanostructural mechanism of toughness of crab carapace

Crab carapace is a kind of biomaterial in nature with high fracture strength and fracture toughness. The excellent mechanical property of the carapace is closely related to its fine nanostructures. The observation of scanning electron microscope (SEM) on the carapace of a Cyclodorippoidea crab shows that the carapace is a kind of natural bioceramic composite which consists of calcite crystal layers and collagen protein matrix. It is also found that the calcite crystal layers consist further of long and thin calcite crystal sheets. The thickness of the calcite crystal sheets is within nanometer scale. The nanometer calcite crystal sheets are arranged in a kind of parallel distribution. The maximum pullout energy of the nanometer calcite crystal sheets, which is closely related to the fracture toughness of the carapace, is investigated based on the representative model of the parallel distribution of the nanometer calcite crystal sheets. It shows that the long and thin shape as well as the parallel distribution of the nanometer calcite crystal sheets bring their larger pullout energy and endow the carapace with the high fracture toughness.