Three-dimensional hierarchical microstructures of the suture of turtle shell and its effect on the mechanical properties

The suture as a unique and import structural feature of turtle carapace is of great importance not only for the physiology but also for the mechanical performance of turtle shell. However, the microstructures and the constructing architectures of the suture as well as its effect on the fracture mode of turtle carapace are unclear until now. In the present paper, the morphologies and the constructing features of the suture of turtle shell is characterized in detail, and its effect on the mechanical behaviors of turtle shell in both wet and dry conditions is further elucidated by comparing with that of samples without suture. It is found that the suture of turtle shell exhibits three-dimensional hierarchical microstructures, and is constructed by numerous microscale pyramid-like bony teeth interdigitating with each other, forming an interlocking interface in a manner of sinuous course. The bony teeth mainly distribute in the cancellous and ventral regions of turtle carapace, while they are not obvious on the dorsal region. The geometry and size of each bony tooth is also demonstrated. Comparing the mechanical performance of ribs with and without suture with three-point bending experiment, it is shown that although the bending strength of rib with suture is smaller than that of rib without suture, the suture can endow turtle carapace with more flexible, higher toughness and sustaining larger deformation. The underlying mechanical mechanism of the effect of suture on the mechanical properties is further disclosed by comparing the facture modes of wet and dry samples. The wet rib with suture is failed by crack propagation along the suture without any bony tooth fracturing; while the dry rib with suture is failed by crack propagating along the suture combing with some bony teeth fracturing, which leads to different mechanical performance of the wet and dry samples. The obtained results in the present paper can not only deepen understanding of hierarchical microstructures of turtle shell, but also be helpful for the design of novel bio-inspired protective structures.