The microstructure and tensile property of the cuticle of turtle shells

Turtle shell is a composite layered structural material consisting of cuticle and bone layers. It possesses superior mechanical properties, such as lightweight, high specific strength and toughness. In the present paper, the cuticle of turtle shell is experimentally studied, whose multi-scale microstructures and tensile properties are mainly focused on, including the characteristics of microstructures, constructing modes of inner interfaces and the influencing factors of the tensile properties. It is shown that the cuticle is constructed by many micro protein platelets, stacking tightly and randomly. In the thickness direction, the upper and lower platelets are connected by discrete protein bridges, while in the in-plane direction, neighboring micro platelets with rough surfaces overlap each other with a certain length. The cuticle has obvious anisotropic tensile properties. The effect of water content on the material properties of cuticle, the tensile strength and fracture toughness is significant. Under tension, it breaks through the fracture of some inner platelets and pull-out of some platelets. Simple finite element simulations are further carried out to analyze the micro-mode of fracture of such layered structural materials, the result of which is consistent with the experimental observation qualitatively. The study of this paper should be useful for further investigation of the multi-scale mechanical property and physical mechanism of turtle shells, and the design of lightweight structural materials with high strength and toughness.