Contribution of energy dissipation to dynamic fracture resistance of the turtle carapace

The turtle carapace is a biological armor possessing superior damage tolerance. In spite of efforts to characterize the mechanical properties of such biological armor, the protection mechanisms associated with the multilayered structure of turtle carapace are largely unknown. In this study, we carry out the calculations of energy dissipation in dynamic fracture of the turtle carapace. The plastic deformation of keratin-collagen bi-layer, keratin-collagen interfacial debonding, collagenbone interfacial debonding and crack growth in the boney layer are accounted for in the analyses. It is found that the interfacial debonding and plastic deformation of the keratin-collagen bi-layer contribute equally to toughening of the carapace at low impact velocity, while plastic energy dissipation dominates in the case of high impact velocity. As the impact velocity is increased, energy dissipation in the turtle carapace decreases at first and then increases. We reveal that the low energy dissipation of carapace at intermediate level of impact velocity is attributed to small plastic zones in the keratin-collagen bi-layer. Furthermore, we have identified the role of the waviness of the keratin-collagen and collagen-bone interfaces. The presence of wavy interfaces enhances plastic energy dissipation of the keratin-collagen bi-layer and promotes interfacial debonding, thereby suppressing crack propagation in the boney layer. The findings provide mechanistic explanations for incorporation of wavy interfaces in the multilayered structure of turtle carapace.