Protection mechanisms of the carapace of a box turtle

In this study we explore the materials design principles of the carapace of a Terrapene Carolina box turtle, which possesses a sandwich-like structure consisting of a foam-like interior layer (FIL) enclosed by two dense exterior layers (DEL). A constitutive scheme accounting for the large deformation, plastic yielding and post-yield strain hardening caused by densification of the cells in the foam is developed to model the mechanical properties of the FIL, and a large deformation elastic-plastic model for the DEL is proposed. Computational simulations of the carapace subjected to indentation loading are performed and it is found that the layer sequence plays an essential role in the mechanical properties of the carapace. For the sandwich-like layering, the stiff DEL provides penetration resistance and the FIL contributes to the energy dissipation of the entire structure through plastic deformation, which enables reduction in back-deformations, enhanced penetration resistance and low stresses transmitted to the inner layer. For other layer sequential patterns, the contributions of the DEL and FIL are limited, leading to poorer mechanical performance. Based on these results, we propose that the sandwich-like structure of the carapace of the box turtle is designed to maintain sufficient resistance to penetration deformation, a defeating mechanism, and at the same time to significantly amplify energy dissipation, a defending mechanism. This double function could be used in the development of future human body armor.